Human Reproduction Part 2
Important Tips
• Fate map : Diagram showing presumptive or prospective areas on the surface of blastula. It is done by using certain vital stains like
neutral red, nile blue sulphate, bismarck brown, etc. It was first prepared by W. Vogt (1929).
• Zona pellucida disintegrates just after completion of cleavage.
• Cells of corona radiata disperse just before implantation.
Implantation.
(i) Definition : The process of attachment of the blastocyst on the endometrium of the uterus is called
implantation.
(ii) Period : Though the implantation may occur at any period between 6th and 10th day after the fertilization
but generally it occurs on seventh day after fertilization.
(iii) Mechanism ; First of all, the blastocyst is held closely against the uterine endometrial epithelium. The
uterine capillaries and uterine wall in the immediate vicinity of the embryo become more permeable and a local
stromal oedema is developed. Soon the endometrium around the embryo shows the first sign of a decidual cell
reaction (DCR) which involves :
(a) The epithelium becomes disrupted and the loosely packed fibroblast-like cells of the stroma are
transformed into large rounded glycogen-filled cells.
(b) The area of contact becomes more vascular.
(c) The decidual cells form an “implantation chamber” around the embryo before the formation of a
functional placenta.
(d) The trophoblast is developed from the superficial layer of the morula stage. Later, the trophoblast is lined
by mesoderm to form the chorion which contributes to the placenta formation.
(e) Trophoblast of the chorion penetrates the uterine epithelium by both cytolytic and mechanical activity. The
cphagocytic activity of the trophoblastic cells through the decidual cells continues till it establishes intimate
connection with the uterine blood vessels. The process of implantation is aided by proteolytic enzymes produced by
the trophoblast. After implantation, endometrium undergoes many changes and forms decidua. It is differentiates
into three parts such as : Decidua basalis present between the embryo and uterine myometrium, Decidua capsularis
lies between the embryo and lumen of the uterus and Decidua parietalis is formed by the remaining part of decidua.
The pattern of implantation of the blastocyst varies in different species, which are as follows
(1) Interstitial implantation : The blastocyst get burried into the endometrium e.g. human female,
hedgehog, guinea pig, some bats and ape.
(2) Central implantation : The blastocyst remain the uterine cavity e.g. rabbit, cow, dog and monkey.
(3) Eccentric implantation : The blastocyst comes to lie in a uterine recess e.g. rats, mice.
(iv) Hormonal control of implantation
(a) Role of estrogens : These are a group of steroid hormones mainly secreted by follicular epithelial cells of
Graafian follicle though these are also produced by adrenal cortex and placenta. These include β-estradiol,
esterone, estriol etc. Out of which most important estrogen is β-estradiol. Secretion of estrogens is stimulated by
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FSH of anterior lobe of pituitary glands. These stimulate the uterine endometrial epithelium to enlarge, become
more vascular and more glandular. The uterine glands become tortuous and cork-screw shaped. So the
endometrium prepares itself for implantation. This stimulation by the estrogens on the uterus generally occurs on
the 4th day of pregnancy.
Meridional cleavage
1 Blasto 2 3 4 5
mere begins 30 60 hr after
Zygote Cleavage fertilisation
hr. after fertilisation c
Blastulation Zona pellucida
Inner cell mass
Blastocoel
67 8 Trophoectoderm
(=Trophoblast)
Morula 9
(32 cells) 4 days after Early blastula
fertilisation
Hatching Inner cell mass Syncytial
trophoblast
Trophoectoderm Blastocoel
(=precursor of 10 Invading
extra embryonic Blastula hatches from endometrium
membranes) zona pellucida
11
Blastocyte during
implantation
Endoderm Amnion
12 Amniotic cavity
Epiblast
Zygote Yolk sac
13
Differentiation of foetal
membranes
Cleavage
Fertilisation Morula
Ovulation Blastula
Implantation
14
Fig : Cleavage and stages of implantation of blastocyte
differentiation of foetal membranes can also be seen
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(b) Progesterone : It is also a steriod hormone secreted by yellow-coloured endocrine gland, called corpus
luteum, formed from empty Graafian folicle during the pregnancy. Small amount of progesterone is also secreted
by adrenal cortex and placenta. Secretion of progesterone is stimulated by LH of anterior lobe of pituitary gland.
Progesterone acts on only those uterine cells which have been earlier stimulated by estrogens. Progesterone
further stimulates the proliferation of endometrium of uterus and prepared, placenta formation and normal
development of the foetus in the uterus.
Gastrulation.
(i) Definition : Gastrulation is a dynamic process involving critical changes in the embryo such as
differentiation of cells, establishment of the three primary germ layers and transformation of the single walled
blastula into a double walled gastrula.
(ii) Types of gastrular movement or morphogenetic movement : The movements of cells during
gastrulation is called formative or morphogenetic movements. Following types of gastular movements are found in
different animals
(a) Epiboly : It involves the morphogenetic movement of prospective ectodermal (micromeres) blastomeres
antero-posteriorly to envelop the presumptive endodermal and mesodermal blastomeres. It is found in telolecithal
egg of frog.
(b) Emboly : It involves inward movement of prospective endodermal and chorda-mesodermal blastomeres
from the surface of blastula.
(c) Invagination : It involves insinking of endodermal cells in the blastocoel to form archenteron. It is found
in amphioxus.
(d) Involution : It involves the rolling in of the chorda-mesodermal blastomeres inside the ectodermal cells
over the lips of blastopore. It is also found in the gastrulation of frog.
(e) Convergence : It involves migration of blastomeres from the outer surface towards the blastoporal lips.
c(f) Ingression or polyinvagination : In this, individual blastomeres migrate into the blastocoel either from
only vegetal pole (called unipolar ingression e.g. Obelia; or form all sides called multipolar ingression e.g. Hydra) to
form a solid gastrula called stereogastrula.
(g) Delamination : It involves splitting off the blastoderm into two Epiblast
layers by the appearance of grooves resulting the formation of hypoblast.
It is found in birds. Embryonal knob
(iii) Formation of layers by gastrulation : Gastrulation includes Beginning of
the formation of following structures endoderm
Blastocoel
(a) Formation of endoderm : The blastodermic vesicle enlarges and
cells present on the lower surface of the embryonal knob detach by Trophoblast
delamination from the embryonal knob. The detached cells become flat, (A) (B)
divide increase in number and form the endoderm inside the trophoblast of
the blastodermic vesicle. The embryo at this stage is tubular and encloses a Aminotic cavity
hollow tube (called primitive gut or archenteron) lined by endoderm. The Aminogenic cells
Epiblast
Embryonic
Bleansdtoocdoeerlm
Primary yolk sac
(C) Trophoblast (D)
Endoderm
Fig : Formation of endoderm and ectoderm
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part of endoderm located under the embryonal knob is called embryonic endoderm which later forms embryonic
gut, while the remaining part of endoderm along with trophoblast forms the yolk sac.
(b) Formation of embryonic disc and mesoderm : Meanwhile, the blastocyst continues to grow due to
absorption of more and more uterine milk. The embryonal knob stretches and cells of Rauber start breaking off and
dispersing. So the cells of embryonal knob from a regular layer called embryonic disc which becomes continuous
with the trophoblast. Embryonic disc is differentiated into cephalic, embryonic and caudal regions. Formation of
embryonic mesoderm starts at the caudal region of the embryonic disc where cells undergo rapid proliferation and
form a localized thickening of the embryonic disc and form the mesodermal layer between ectoderm and
endoderm. Embryonic Trophoblast Amnion
Ectoderm disc Chorion
Epidermis and skin derivatives Aminiotic Embryonic
Cutaneous gland (A) disc
Nervous system (Brain + spinal cord) Ectoderm
Mesoderm
cMotor and optic nerve SecoEnnddaordyeyrmolk (B)
Primsaacry yolk
EnEdxotdraerm
embryonic Extra
embryonic
Fig : Formation of extraembryonic mesoderm and coelom
(c) Formation of ectoderm : The remaining cells of blastodisc become columnar and form ectoderm.
(iv) Fate of germ layers : Each of the three germ layers gives rise to definite tissues, organs and systems of
the body. Their fate in embryo and adult has been listed below
Fate of germ layer
Mesoderm Endoderm
Dermis Gut
Muscular tissue Glands of stomach and intestine
Connective tissue Tongue
Endoskeleton Lung, trachea and bronchi
Eye (Retina, lens and cornea) Vascular system (heart and blood vessel) Urinary bladder
Conjuctiva, ciliary and iridial muscle Kidney Primordial germ cells
Nasal epithelium Gonads (Reproductive system) Gills
Internal ear (membranous labyrinth) Urinary and genital ducts Liver
Lateral line sense organ Coelom and coelomic epithelium Pancreas
Stomodaeum (mouth) Choroid and sclerotic coat of eye Thyroid gland
Salivary gland Adrenal cortex Parathyroid gland
Enamel of teeth Spleen Thymus
Proctodaeum Notochord Middle ear
Pituitary gland Parietal and visceral peritoneum Eustachian tube
Pineal body Mesoderm (Mid gut)
Adrenal medulla Lining of vagina and urethra
Hypothalamus Prostate gland
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(v) Significance of gastrulation
(a) Three primary germ, layers are formed.
(b) It marks the beginning of morphogenesis and differentiation.
(c) Metabolic activities of the cells are increased due to great morphogenetic activities of the blastomeres.
Important Tips
• First embryonic membrane to be formed is endoderm.
• Embryonal knob is called precursor of the embryo, while trophoblast forms protective and nutritive extra-embryonic membranes.
• Cells of Rauber : Those cells of trophoblast which are in contact with embryonal knob.
• Yolk sac is also found in some anamniotes like certain cartilage fishes (e.g. scoliodon), bony fishes and few amphibians (e.g. Necturus)
having polylecithal egg.
• Protostomous : When blastopore forms the mouth in development and is found in non-chordates except echinoderms,
hemichordates and chordates.
• Deuterostomous : When blastopore forms the anus in development. Mouth is developed later than anus e.g. echinoderms
hemichordates and chordates.
Neurulation and organogenesis.
Post gastrulation involves two main process. Neurulation is process of laying the neural plate to form the
nervous system. The establishment of the germ layers initiates the final phase of embryonic development, i.e.,
organogenesis. The latter involves differentiation and specialization of groups of cells in the individual germ layers.
The cells of such groups change their form and give rise to morphologically recognizable tissues and organs of the
new individual. The groups of differentiated cells separate from their germ layers in an orderly manner and with
unique precision. Separation of the differentiated cell groups may occur by folding off from the germ layer or by
migration of cells individually and reaggregation at a new place. In this manner, the primordial cells of the germ
layers gradually and accurately give rise to the tissues and organs of the offspring.
cBy four weeks after fertilization, the embryo has a simple heart, limb buds and eye rudiments. It also has a tail
and pharyngeal pouches, the vestiges of its early vertebrate ancestors that disappear later in development. After the
second month, the embryo is recognizable as a primate. From this stage on, the embryo is often called foetus.
Some important events in the human development
Time from fertilization Stage/organs Event
24 hours Cleavage
3 days Morula Embryo is at two-cell stage
7 days Blastocyst
2.5 weeks Notochord The morula reaches uterus
3.5 weeks Organ system Implantaion of blastocyst begins
4 weeks Limb buds Notochord formed, differentiation of tissues that will give rise to heart, blood
2 months Muscles and gonads cells formed in yolk sac and chorion.
Neural tube formed, primordial eye and ear vesicle, pharyngeal pouches
formed, liver bud differentiates, respiratory system and thyroid gland begin to
develop, heart tube bends and begins to beat, blood vessels are formed.
Development and appearance of limb buds, brain forms three primary vesicles.
Muscles differentiate, embryo capable of movement, gonads distinguishable as
testes or ovaries, ossification of bones begins, cerebral cortex is differentiated,
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3 months Sex differentiation blood vessels assume final position.
4 months By external examination sex can be determined, notochord degenerates, lymph
6-9 months (third trimester) Face glands develop.
266 days Face begins to look human, eye, ear and nose look ‘normal’, cerebral lobes
Lanugo (hairs) body differentiate.
growth
Birth (parturition) Lanugo appear but are shed later, tremendous growth of body occurs, neurons
become myelinated
Baby is born, neonate arrives in outer world.
Extra embryonic membrane.
An aquatic embryo is surrounded by water, which protects the embryo, keep it moist, removes wastes and
permits gas exchange. In land vertebrate (reptiles, birds and mammals), these functions are taken over by the
extraembryonic membranes. These membranes are formed outside the embryo from the trophoblast only in
amniotes and perform specific function. Some of these membranes take part in the formation of placenta in
mammals.
(i) Yolk sac : It is formed below the embryo. It contains fluid, not yolk. The yolk sac is a vestigeal organ
inherited from the oviparous reptilian ancestors. Yolk sac encloses by outer mesoderm and inner endodermal layer.
Function : It is mainly digestive in function. It also absorbs the dissolved yolk and passes it to developing
embryo in reptiles, birds and prototherian. In human beings, it is vestigial. In human embryo it act as the site of
blood cell formation until about the 6th week, when the liver takes over this role.
(ii) Amnion : It is formed above the embryo. It consist of outer mesoderm and inner ectoderm. The amnion
and the fluid filled amniotic cavity it encloses, enlarge and nearly surround the embryo. The embryo is suspended in
the amniotic cavity by an umbilical cord. The latter is formed of the stalks of the yolk sac and allantois. The main
blood vessel from the placenta reach the foetus through the umbilical cord. Amniotic fluid secreted by both embryo
cand amnion. The cells of amniotic fluid are the basis of parental test called amniocentesis, for the sex of the foetus
and for checking chromosomal defects in it.
Functions
(1) The amniotic fluid cushions the embryo.
(2) It protecting embryo against bumps and bacterial infections.
(3) It maintains a constant temperature and pressure.
(4) It protects the embryo from jerk, injury and shocks.
(5) It prevents desiccation of the embryo.
(iii) Allantois : It is a fold of splanchnopleur developed from the hind gut of the embryo. It consist of outer
mesoderm and inner endoderm. It is well developed in amniotes with polylecithal egg (e.g. reptiles, birds and
prototherians) and stores the nitrogenous waste of the embryo so act as extra embryonic kidney. In most of
eutherians, it combines with chorion to form allantochorion placenta. But in man it remains small or reduced and
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does not reach the chorion. However, it forms umbilical arteries and veins which grow up to the chorion to
vascularise it.
Functions
(1) The cavity of the allantois serves as a urinary bladder. It stores the protein breakdown product in the form
of water-insoluble crystals of uric acid and inside the egg upto the time of hatching. But with the acquisition of
viviparity in the marsupials and the placental mammals, the original function of the allantois as a urinary bladder
becomes altogether lost.
(2) The vascular “chorioallantoic membrane” lies in a close proximity to the inner surface of the porous shell.
It acts as an extraembryonic lung by supplying the embryo with oxygen.
cAllantois
(3) Together with the chorion, the allantois also surrounds the albumen to form the albumen sac and thus
assists in the absorption of nutritionally rich albumen.
(4) In mammals, allantois supply oxygen and nutrient to the embryo.
(iv) Chorion : It is outermost fold of somatopleur (outer ectoderm and somatic mesoderm) and surrounds the
embryo. In reptiles birds and prototherians, allantochorion act as extra embryonic lungs help in exchange of gases.
But in primates including human beings, only chorion forms the placenta (chorionic placenta).
Function : It protects the embryo and forms placenta for metabolic exchange between the foetus and the
mother. Ectoderm Amniotic cavity
Endoderm Amnion
Mesoderm
Ectoderm Embryo
Mesoderm Embryonic gut
Chorion
Endoderm Extra-embryonic
Mesoderm coelom
Mesoderm Yolk sac
Endoderm
Placental chorionic
villi
Fig : Foetal membranes and placenta (early stage)
Types and functions of extra embryonic membrane
S.No. Name of membrane Characteristics and functions Remarks
(1) Yolk sac (1) Formed by inner endoderm and outer mesoderm (= splanchnopleura) Vestigeal in humans.
(2) Digestive function (= extra embryonic duct)
(2) Amnion (3) Absorbs dissolved yolk and supplicate it to developing embryo. Well developed in reptiles, bird
and prototherians.
(1) Formed by inner ectoderm and outer mesoderm (somatopleur)
above the embryo. Well developed in all amniotes
(2) Between the embryo and amnion there is a cavity called amniotic
cavity filled with amniotic fluid secreted by amnion and embryo.
(3) Amniotic fluid act as shock absorber and prevent desiccation of
embryo.
(4) Ex-foliated embryonic (= foetal) cells are used for (a) pre-natal
sex determination (b) congenital defects (c) inborn metabolic
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disorders. This technique is called amniocentesis.
(3) Allantois Develops as fold of splanchnopleur; developed from gut of embryo. (1) In placentals, it combines
with chorion to form allanto-
chorion placenta. (reduced in
human)
(2) Acts as extra-embryonic
kidney in reptiles, birds and
prototherians.
(4) Chorion Outermost fold of somatopleur and surrounds the embryo (1) Takes part in forming the
true chorio allantoic placenta
(2) Acts as extra-embryonic
lung in reptiles, birds and
prototherians.
placenta. The chorionic
villi receive blood from thePlacenta.
embryo by umbilical artery
cand return it by umbilical
(i) Definition : Placenta is defined as a temporary intimate mechanical and physiological connection between
foetal and maternal tissues for the nutrition, respiration and excretion of the foetus.
(ii) Structure : Human placenta consist of chorion only. Hence, it is called a chorionic placenta. Allantois
remains small. The allantoic blood vessels, however, extend to vascularize it. A large number of branching villi from
the vascular chorion penetrate the corresponding pits, the crypts, formed in the uterine wall. The latter becomes
very thick and highly vascular to receive the villi. The intimate connection established between the foetal membrane
and the uterine wall is known as the placenta. The placenta has two parts : the part contributed by the foetus, i.e.,
chorionic villi, is called the Chorion Placenta Endometrium
foetal placenta; and the Lacuna
part shared by the mother,
i.e., part of uterine wall, is
termed the maternal
Umbilical Lacuna
UmvbeaiilnrictearlyUmcobrildical Lacuna
vein. These blood vessels Chorionic villus
are derived from the Lacuna
allantois and run between
the foetus and the uterine Lacuna Uterine arteriole
wall in the tough umbilical
cord covered with cells Foetal artery Chorionic
derived from the amnion Foetal vein
Uterine venule
and chorion. The choroinic Lacuna
villi come to lie in uterine
lacunae that receive blood Maternal
from the uterine arteriole blood space
and return it by uterine
Fig : Diagram showing relationship between foetal and maternal blood
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venule. The cells forming the wall of chorionic villi bear microvilli which increase their surface area for quick and
adequate exchange of materials by diffusion, active transport and pinocytosis.
The placenta is fully formed by the end of the third month and it lasts throughout pregnancy. When complete,
it is a reddish – brown disc. In the placenta, the foetal blood comes very close to the maternal blood, and this
permits the exchange of materials between the two. Food (glucose, amino acids, simple proteins, lipids), water,
mineral salts, vitamins, hormones, antibodies and oxygen pass from the maternal blood into the foetal blood, and
foetal metabolic wastes, such as carbon dioxide and urea, also water and hormones, pass into the maternal blood.
The placenta, thus, serves as the nutritive, respiratory and excretory organ of the foetus. The continuous uptake of
oxygen by foetal blood is ensured by the difference in affinity for oxygen between foetal and maternal
haemoglobin.
The maternal and foetal blood are not in direct contact in the placenta, because (i) the two may be
incompatible; (ii) the pressure of maternal blood is far too high for the foetal blood vessels; and (iii) there must be a
check on the passage of harmful materials (blood proteins, germs) into the foetal blood.
(iii) Functions
(1) Placenta helps in the nutrition of the embryo as the nutrients like amino acids, monosugars, vitamins, etc.
pass from the maternal blood into foetal blood through placenta.
(2) It also helps in respiration of the embryo as O2 of the maternal blood and CO2 of the foetal blood diffuse
through placenta into the foetal blood and maternal blood respectively.
(3) It also helps excretion of the embryo as nitrogenous wastes of foetal blood like urea pass into maternal
blood through placenta.
(4) It also acts as an endocrine gland as it secretes certain hormones like estrogens, progesterone and
human chorionic gonadorophin (HCG). HCG maintains the corpus luteum for the continued secretion of
progesterone to maintain the pregnancy. At the end of gestation period, it also secretes relaxin which helps in
softening of pubic symphysis and child birth. It also secrete small amounts of chorionic thyrotrophin, chorionic
ccorticotropin and chorionic somatomammotropin.
(5) Antibodies against diphtheria, smallpox, scarlet fever, measles, etc. pass from maternal blood to foetal
blood through placenta and provide passive immunity.
(6) It stores the glycogen till the formation of liver.
(7) Though the placenta acts as an effective barrier for certain toxic chemicals like histamine but certain
germs like AIDS virus, syphilis bacteria, viruses of German measles, etc, intoxicants like nicotine of cigarette smoke;
and addictive drugs like heroin and cocaine can pass through the placenta and cause the developmental defects.
(iv) Placenta and disease
(1) Viral and bacterial infections of placenta are known as placentitis.
(2) If the mother suffer from certain diseases like syphilis, smallpox, chickenpox, AIDS and measles; their
pathogen enter into the foetus through placenta.
(3) Many drugs used medicinally may penetrate the placental barriers.
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(4) Drugs such as thalidomide taken as a sedative by woman in early pregnancy (25 to 44 days) cause
extensive deficiencies in the development of limbs, the alimentary canal (non perforation of the anus) and the heart
in children.
(5) Nicotine from cigarette smoke crosses the placenta and stunts foetal growth.
(6) Addictive drugs such as heroin and cocaine reach the foetus, causing addiction to the new born.
(v) Classification of placenta
(a) According to the foetal membrane involved in the formation of placenta.
(1) Yolk sac placenta : In metatheria or marsupials, such as kangaroo (macropus) and opossum
(Didelphys), placenta is derived from yolk sac and chorion. In metatheria, yolk sac placenta is only weakly
developed so that embryonic nutrition and growth remain limited and the young is born very small and immature.
To compensate the deficiency of intrauterine development, it is transferred to the abdominal pouch or marsupium
and fed on milk until fully formed. In higher mammals (Eutheria) a yolk sac placenta is usually not found but in
some mammals (Hedgehogs and rabbit) it may be temporarily develop in early stages.
c(2) Chorionic connective tissue
(2) Allantoic placenta : In the majority of Eutherian, the chief organ of embryonic nutrition is the allantoic
placenta consist of allantois and chorion and also called allantochorionic placenta. Outside Eutharia, a primitive
allantoic placenta occurs only in perameles (bandicoot) which is a metatherian.
(3) Chorionic placenta : It occurs in primates (man and apes) and is formed only by chorion. Allantois
remains small, burrows into body stalk (umbilical cord) and does not reach chorion. However, its mesoderm and
blood vessels grow upto chorion whose villi enter the uterine crypts forming chorionic placenta.
(b) According to the intimacy between the foetal and maternal part.
Histologically there are six barriers are found in placenta which are as follows.
(1) Endothelium of foetal blood vessels
Foetus
(3) Chorionic epithelium
(4) Uterine epithelium Mother
(5) Uterine connective tissue
(6) Endothelium of maternal blood vessel
On the presence or absence of above barriers histologically placenta is divided into following types
(1) Epithelio-chorial : Most primitive and simplest type with all six placental barriers.
Examples : Odd hoofed mammals such as horse, ass, pig and lemurs.
(2) Syndesmo-chorial : Uterine epithelium absent, with five placental barriers.
Examples : Even hoofed mammals such as cow, sheep, goat, camel etc.
(3) Endothelio-chorial : Uterine epithelium and uterine connective tissues are absent, with four placental
barriers.
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Examples : Carnivores (dog, cat, lion, tiger etc.), Tree shrew and mole.
(4) Haemo-chorial : Uterine epithelium, uterine connective tissue and endothelium of maternal blood vessel
absent, with 3 foetal layers.
Examples : Primates (man, apes and monkey).
(5) Haemo-endothelial : Foetal capillaries indirect contact with maternal blood, only one placental barrier.
Examples : Rat, guinea pig and rabbit.
(A) Foetal side (B) (C) (D) (E)
1 Foetal blood
(1) Non deciduous placenta : In most1 1
mammals villi are simple, unbranched and merely22 21
3 3 3
copposed without intimate contact between foetus and
46 Maternal blood
55
66
Maternal side
Fig : Histological types of placenta
(A) Epithelio-chorial, (B) Syndesmo-chorial, (C) Endothelio-chorial, (D) Haemo-chorial, (E) Haemo-endothelial,
(1) Endothelium of foetal blood vessel, (2) Chorionic connective tissue (3) Chorionic epithelium, (4) Uterine epithelium,
(5) Uterine connective tissue (6) Endothelium of maternal blood vessel
(c) According to shape and distribution of villi : Depending on the shape of placenta, manner of
distribution of villi, degree of connection between foetal and maternal tissues and behaviour of placenta at the time
of birth, the following types and subtypes of allantoic placenta can be recognized.
Arealae of villi Cotyledon or
areala of villi
uterine wall. At the time of birth or parturition, villi Chorionic folds (B) Cotyledonary
are easily withdrawn from maternal crypts without (A) Diffuse Compound villi
causing any tissue damage. Thus no part of uterine
tissue comes out and no bleeding occurs. Non
deciduous or non-deciduate placenta has following
subtypes according to the manner of distribution of villi.
(i) Diffuse : Villi remain scattered all over the (C) Incomplete (D) Zonary (E) Discoidal
surface of allantochorion. e.g. pig, horse, lemur. zonary
Primary placenta
(ii) Cotyledonary : Villi are arranged in with compound
separate tufts or patches called cotyledons. e.g. goat,
sheep, cow, deer. villi
(iii) Intermediate : Villi are arranged in Secondary placenta
cotyledons as well as scattered. e.g. camel, giraffe. (F) Metadiscoidal (podiscoidal)
Fig : Types of placentae according to the distribution of villi
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(2) Deciduous placenta : Villi are complicated, branched and intimately connected. At birth, a variable
amount of maternal tissue is pulled out with the shedding of blood. Deciduous or deciduate placenta is also
differentiated in the following subtypes
(i) Zonary : Villi form an incomplete (e.g. racoon) or complete girdle encircling the blastocyst. e.g. cat, dog,
seal.
(ii) Discoidal : Villi are restricted to a circular disc or plate on the dorsal surface of blastocyst. e.g.
insectivores, bats, rodents (rat, mouse), rabbit, bear.
(iii) Metadiscoidal : Villi are at first scattered but later become restricted to one or two discs. It is
monodiscoidal in man and bidiscoidal in monkeys and apes.
(3) Contra-deciduous : Foetal villi and uterine crypts are so intimately connected that even most of foetal
placenta is left behind at birth to be broken and absorbed by maternal leucocytes e.g. bandicoot (perameles), mole
(Talpa).
Important Tips
• Yolk sac is also found in some anamniotes like certain cartilage fishes (e.g. Scoliodon), bony fishes and a few amphibians (e.g.
Necturus) having polylecithal egg.
• Amnion, chorion and allantois are formed only in amniotes embryo as their development occurs on land either inside egg or in the
uterus of mother. These are not formed in anamniotes as their development occurs in water so there is no problem of dessication, supply
of oxygen and removal of oxygen.
• Splanchnopleur : A fold formed of endoderm and splanchnic mesoderm.
• Somatopleur : A fold formed of ectoderm and somatic mesoderm.
• Amniocentesis : Prenatal diagnostic technique in which amniotic fluid is withdrawn to know sex of developing foetus congential
chromosome defects and inborn errors of metabolism.
• Human placenta is fully formed in about 10 weeks and lasts throughout pregnancy.
c• Presence of HCG in the urine sample of a female confirm the pregnancy.
Gestation period and parturition.
(i) Gestation period : Gestation period is the duration between fertilization and parturition.
Gestation period
S.No. Animal Days
(1) Mouse (Minimum) 19-20
(2) Rat 20-22
(3) Rabbit 28-32
(4) Cat 52-65
(5) Dog 60-65
(6) Pig 112-120
(7) Goat 145-155
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(8) Man 270-290
(9) Cow 275-290
(10) Horse 330-345
(11) Elephant (Maximum) 607-641
(ii) Parturition : It is the expelling of the fully formed young from the mother’s uterus after the gestation
period (about 280 days in human female).
(a) Mechanism : A developing foetus secretes hormones from its adrenal glands. These hormones diffuse
into the maternal blood and accumulate to cAmnionpresses
stimulate the release of oxytocin (birth
hormone) from the mother’s posterior pituitary. against cervix
Oxytocin causes the forceful contraction of
smooth muscles of myometrium, called labour 1. Cervix is beginning to dilate in response
pains which pushes the young gradually out to pressure of the head of the baby and
through the dilated cervix (caused by relaxin) contracts under the dual influence of
and vagina, with the head foremost. It is aided oxytocin and prostaglandins. Relaxin
by a reflex (whose centre lies in the lumbar softens lower part of uterus and relaxes
region of spinal cord) and voluntary contraction pubic symphsis.
of abdominal muscles. In the beginning, the
labour pains occur once every half or quarter of (A)
an hour but soon become more frequent. The
foetal membranes burst and amniotic fluid is Contraction of
released but foetal membranes remain behind. uterine myometrium
This expulsion stage lasts about 20 minutes
to one hour. It is followed by placental stage Dilation of cervix
of 10-45 minutes during which the umbilical 2. Passage of the body (head foremost)
cord, placenta and foetal membranes are through the birth canal.
(B)
Fig : Two stages in parturition. The placental hormones play a critical role in
tandem with oxytocin
expelled as decidua or after birth. It is
because after the child birth, the uterus reduces in size causing detachment of placenta. Umbilical cord is tied and
then cut which finally shrinks into a depressed scar called umblicus or navel. Sometimes, the foetus fails to come
out then the baby is delivered by a surgical procedure. Such a baby is called cesarean.
(b) Control : Parturition is controlled by hormones :
(1) Oxytocin : Causes powerful contractions of myometrium during parturition.
(2) Relaxin : Causes widening of pelvis by relaxing the pubic symphysis of the pelvic girdles.
Important Tips
• Teratogens are those physical, chemical and biological agents, which may cause malformations in the developing embryo.
• Teratology : Branch of developmental biology which deals with the study of abnormal development during embryogenesis.
• Morning sickness : A desire for unusual food during pregnancy.
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• Gestosis : Any disorder of pregnancy.
• Postpartum care : Care after childbirth.
• Lanugo : Most of the body of foetus is covered with downy hairs called lanugo which are generally shed before birth
• Uterine milk : Nutritive endometrial secretion.
• Miscarriage or Abortion : Loss of embryo due to breakdown of endometrium due to lowering of progesteron secretion from corpus
luteum.
• Nidiculous or Altricial young : Underdeveloped and helpless young born e.g. cats, dogs, rats, etc.
• Phocomelia : Abnormal baby born with flipper-like limbs due to continued use of Thalidomise (sleeping pills) by the pregnant female.
This drug passes through placental care.
Development in frog.
(i) Breeding
• Frog breeds in the rainy season, June to September.
• Male frogs produce crocking sound (mating call) by their vocal sacs.
• The sexual embrace of the male and female frogs is called amplexus (false copulation).
(ii) Ovulation
• Ovulation is the release of eggs from ovary in the body cavity.
• The eggs in the stage of secondary oocytes are released into the body cavity by rupture of ovary during ovulation.
(iii) Spawning
• Spawning is the act of laying of eggs by the female frog stimulated by the male during amplexus.
• Spawn is a cluster or mass of eggs laid by a female.
• A spawn of Rana tigrina contains about 3000-4000 eggs.
• The diameter of frog’s egg varies from about 0.75 to 2.5 mm.
• The egg is surrounded by a thin vitelline membrane and three layers of jelly coats made of gelatin.
• Gelatin protects the egg from predators and also acts as an insulator keeping the egg warm.
c(iv) Fertilization
• Fertilization in frog is external taking place in water.
• The sperms are released on the egg mass before it reaches water.
• When a sperm enters into the egg of frog, second meiotic division occurs.
• A sperm enters into the ovum at some point in animal hemisphere.
• A gray crescent appears in the equitorial zone geometrically opposite to the sperm entrance.
• Gray crescent marks the dorsal side of the future embryo.
• Sperm entrance point marks the anterior side of the future embryo.
• The bilateral organization is established at the time of sperm penetration.
• The region where sperm enter the egg cell is called ‘reception cone’.
• Entry of sperm induces following changes in the ovum
Formation of fertilization membrane around the ovum.
Completion of second maturation division of egg nucleus.
Activation of the egg for development.
Fusion of male and female pronuclei, i.e., amphimixis.
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Formation of gray crescent.
(v) Structure of egg
• Frog’s egg is mesolecithal (based on distribution of yolk).
• Upper black of darkly pigmented part is animal hemisphere. Lower unpigmented or white part is vegetal
hemisphere.
• Cytoplasm is concentrated in animal pole. It is directed dorsally and pigmented animal pole is related with
camouflage, to escape notice of predators.
• Vegetal hemisphere of frog’s egg contain yolk. It remains directed downwards.
• The correct sequence in the development of frog is fertilization, cleavage, morula, blastula and gastrula.
(vi) Cleavage
• Cleavage is a term used for the early cell divisions of the zygote upto the completion of blastula stage.
• First cleavage of frog is meridional passing through median longitudinal axis, holoblastic and equal.
• The first cleavage furrow appears at animal pole and results in two blastomeres, right and left.
• The second cleavage is right angle to first one, again meridional results in four blastomeres, identical with
respect to cytoplasm, pigment and yolk gradient.
• Second cleavage is again holoblastic and equal.
• Gray crescent is present only in two blastomeres of future dorsal side.
• All divisions from third cleavage are unequal holoblastic.
• Holoblastic equal cleavage in frog ends after second cleavage division.
• Third cleavage plane is horizontal, but above the level of equator (latitudinal).
• Third cleavage results in the formation of embryo with eight blastomeres, four upper one are smaller and
pigmented and four lower most are larger and yolk laden.
• Smaller cells are micromeres and larger ones are called macromeres. The micromeres contain no yolk
and macromeres contain large amount of yolk.
c• Fourth cleavage involves two synchronous meridional divisions resulting in the formation of 16 blastomeres.
• Fifth cleavage involves two simultaneous latitudinal divisions resulting in the formation of 32 blastomeres.
• The divisions after the fifth cleavage becomes very irregular and asynchronous. The micromeres divide
faster than macromeres.
• After sixth or seventh cleavage division, the embryo looks like a mulberry-shaped ball of cells. This called
morula stage.
• Morula is a solid ball of cells. A cavity called blastocoel appears towards animal hemisphere. The blastula of
frog is called coeloblastula.
• Blastocoel is filled with an albuminous fluid secreted by surrounding cells.
• The two cells of frog’s egg formed by the first cleavage represent the right and left half of the embryo.
• In frog, there is a regulative development. The cleavage is indeterminate. If one of the two blastomeres
of frog is damaged, the development will be normal.
(vii) Gastrulation
• Gastrulaion is the process by which a blastula is converted into gastrula.
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• Blastula is a hollow ball of cells. By the end of gastrulation, it is converted into a three-layered embryo made
of ectoderm, mesoderm and endoderm often enclosing an archenteron.
• Gastrulation includes three kinds of morphogenetic movements of cells namely
Epiboly of ectoderm
Invagination of endoderm
Involution of chordamesoderm
• Epiboly is migration and spreading of micromeres over the embryo is known as epiboly.
• Invagination of prospective endoderm cells occurs below equator, exactly below the midpoint of gray
crescent of blastula. It results in the formation of a slit later giving rise to blastopore.
• Involution is the insinking and movement of chordamesoderm cells towards anterior side along the roof of
blastocoel.
• Gastrulation results in the formation of a new cavity, archenteron which opens outside through blastopore.
• Archenteron is present the lumen of future gut.
• Blastopore occurs in gastrula and opens into archenteron.
• Blastopore will give rise to future anus in frog.
• If involution does not takes place (chordamesoderm cells evaginates instead of involution) no gastrulation
takes place. The development will be stopped.
• Ingression is the migration of individual vegetal cells to the interior of the embryo.
• By the end of gastrulation, blastocoel will be reduced. A yolk plug of endodermal origin closes the
blastopore.
• Posterior side of future tadpole is represented by the side of frog’s embryo bearing the yolk plug.
(viii) Organizer
• The dorsal lip of blastopore in the amphibian gastrula is called primary organizer.
• The theory of organizer (inductor) in amphibian was introduced by Spemann in 1938. He was awarded
cNobel prize for this work.
(ix) Neurulation
• Neurulation takes place after gastrulation. During this stage a neural tube is formed.
• The embryo lengthens along its anterior-posterior axis, neural plate (ectodermal) become thickened and
raised above the general level as ridges called neural folds.
• Neural folds meet and fuse at the mid dorsal line.
• Neurulation includes the formation of neural tube, notochord and gut.
• Formation of notochord is known as notogenesis.
(a) Post neurular development
The development takes place inside egg membrane upto tail bud larval stage.
Hatching occurs in 6th day of embryonic life.
During hatching, the young frog is called tadpole larva.
Newly hatched tadpole larva remain attached to aquatic plants by its oral sucker.
After 24 hours of hatching, mouth and anus are perforated.
The larval body is elongated forming head, trunk and tail.
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(b) External gill stage of tadpole
Just above one day after hatching, the external gill stage starts.
Eyes become fully developed and functional.
Horny jaws with teeth appear along the rim of mouth.
Tail elongates and becomes a powerful swimming organ.
Pronephric kidneys become fully developed. Frog’s tadpoles are ammonotelic. Nitrogenous waste matter
excreted by frog’s tadpole is ammonia.
The branchial clefts are perforated and finger-like external gills project from the sides of the head in branchial
region.
External gills are three pairs in number; tadpole respires mainly by gills using oxygen dissolved in water.
Gut is differentiated into pharynx, oesophagus, lung rudiments, stomach, liver, gall bladder and intestine.
Tadpole is herbivorous (phytophagus), feeds on aquatic plants.
Tadpole has a long coiled intestine because digestion takes place relatively long time.
(c) Internal gill stage of tadpole
Tadpoles grow older, the hind limb buds and internal gills develop.
External gills are replaced by four pairs of internal gills covered with a fold of skin called operculum.
Operculum encloses a chamber, opercular chamber, opens to exterior by spiracle.
Spiracle is present only on the left side of the tadpole.
During respiration, the water currents enter the mouth, bath the gills of pharynx and exit through the
spiracle.
Oral sucker disappear and lateral line receptor develop. This serves to perceive stimuli of movements,
currents and vibrations of water.
A tadpole of frog resembles a fish in many features except that the tadpole does not possess paired fins and
scales on the body.
The tadpole cannot survive when exposed to air because its skin is thin and delicate.
c Exposure of tadpole to land leads to dehydration and death.
First sign of metamorphosis is the appearance of hind limbs.
End of tadpole in the life history is marked by appearance of forelimbs.
(x) Metamorphosis
• Metamorphosis is the abrupt transition from larval to adult form.
• Metamorphosis includes morphological, anatomical, physiological and behavioural changes.
• Two or three weeks after breathing with gills, the tadpole larva undergoes drastic changes called
metamorphosis.
• Two types of changes during metamorphosis are regressive and progressive.
• Some of the regressive morphological changes are :
Disappearance of larval sucker, long tail, gill clefts, internal gills and lateral line sense organ.
• Some of the progressive morphological changes are :
Formation of forlimbs breaking through operculum.
Replacement of pronephros with mesonephric kidney.
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Enlargement and development of hindlimbs.
Development of tongue and teeth.
Thickening of skin and development of mucous glands.
Two chambered heart becomes three chambered.
Development of lungs.
• During metamorphosis, the disappearance of larval organs is by histolysis and formation of adult organs is
by histogenesis.
• The tail is shortened by reabsorption with the help of lysosomal enzyme cathepsin. This process is also
known as autolysis.
• When there is no iodine in pond water or
thyroxine is not secreted in the body,
metamorphosis fail to begin. The tadpole
continues growing and becomes abnormally
large.
• If thyroxine is given, the tadpoles
cmetamorphose too rapidly giving rise to vary small
• Nervous system undergoes no special changes (least changes) during metamorphosis.
• Respiratory system undergoes maximum changes during metamorphosis.
• During metamorphosis, skin gets cornified and mucous and poison glands develop.
• The feeding habit changes from herbivorous habit of tadpole to carnivorous habit of adult.
(xi) Hormonal control of metamorphosis
• Hormonal control of metamorphosis in amphibian was discovered by Gudernatsch (1912).
• Metamosphosis occurs only when adequate amount of thyroxine is secreted by thyroid of tadpole.
• Since iodine is the main constituent of thyroxine, it is found that deficiency or abundance of iodine in pond
water also affect metamorphosis.
Normal 5-week tadpole
If thyroxine is given the tadpoles
metamorphose too rapidly giving
rise to very small black frogs
which soon die
If thyroids are removed giant
black frogs which soon die. tadpoles can be reared which are
unable to metamorphose
• If thyroids are removed, giant tadpoles
can be reared which are unable to
metamorphose.
• On removing the thyroid from the Normal-size frog after
tadpole of frog, it will remain tadpole metamorphosis
throughout life.
• If a small quantity of thyroid extract is
added to water in which frog tadpoles are
present, it will hasten the metamorphosis.
• The endocrine gland which initiates
metamorphosis in frog is thyroid.
• Thiourea is antithyroid drug, it inhibits Fig : The effect of thyroxine on metamorphosis of tadpoles
metamorphosis of frog.
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• Neoteny refers to the retention of a larval or embryonic trait in the adult body, e.g., Cartilaginous skeleton in
adult chondrichthyes and larval gills in some adult salamanders.
• Paedogenesis or paedomorphosis refers to development of gonads and production of young ones by larval
or pre adult animal e.g. liverfluke and ambystoma.
• Deficiency of iodine in the soil results in the failure of metamorphosis in ambystoma.
• Axolotl is the larva of ambystoma, it shows paedogenesis.
Important Tips
• Metamorphosis : Transformation of young into a morphologically and physiologically different adult is called metamorphosis.
It is of 2 types :
• Retrogressive metamorphosis : When an advanced larva changes into a degenerate adult e.g. Herdmania, Sacculina.
• Progressive metamorphosis : When a simplified larva changes into an advanced adult e.g. Frog.
• Embryonic induction : Morphogenetic effect of one embryonic part on another embryonic part. Inducing embryonic part is called
inductor or organizer, while responding embryonic part is called responding tissue. This induction is through certain chemicals
called evocators.
• Primary organizers include dorsal lip of blastopore; grey crescent (neural inductor) and chorda-mesoderm (induces forebrain).
• Secondary organizer e.g. optic area induces formation of lens.
c• Tertiary organizer e.g. Lens induces formation of cornea.
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Methods of Birth Control.
Meaning : The regulation of conception by preventive methods or devices to limit the number of offspring is
called birth control.
Methods : A variety of methods are known for birth control. The birth control methods which deliberately
prevent fertilization are referred to as contraception. These methods are of 2 main types : temporary and
permanent.
(i) Temporary Methods : These are further of many types –
(a) Safe Period (Rhythm Methods) : A week before and a week after menses is considered the safe period
for sexual intercourse. The idea is based on the following facts-
(1) Ovulation occurs on about the 14th day (may be 13 th to 16th day) of menstruation.
(2) Ovum survives for about 1-2 days.
(3) Sperms remain alive for about 3 days.
This method may reduce the chances of pregnancy by about 80 percent. However, a great care is needed in its
use. Rhythm method is also called natural family planning. It is also termed temporary abstinence because it requires
refraining from sexual intercourse when conception is most likely, i.e., a few days before and a few days after
ovulation. Changes in cervical mucus and body temperature during the menstrual cycle mark the ovulation time.
Thus, the natural family planning requires adequate knowledge of these physiological signs. Some couples use the
natural family planning method of increase the chances of conception so that unplanned pregnancies are avoided.
(b) Coitus Interruptus : This is the oldest method of birth control. It was in use over 2,000 years ago. It
involves withdrawal of the penis from the vagina by the male before ejaculation so that semen is not deposited in
the vagina and there is no fertilization. This method also has some drawbacks. Male produces some lubricating fluid
cfrom his Cowper's glands before ejaculation. This fluid contains many sperms. A lapse in timing or willpower may
result in late withdrawal and hence pregnancy.
(c) Spermicides : Foam, tablets, jellies, pastes and creams, if introduced into the vagina before sexual
intercourse, adhere to the mucous membrane and immobilise and kill the sperms. These contain seprmicides such
as lactic acid, citric acid, boric acid, potassium permanganate and Zinc sulphate.
(d) Mechanical Means : These are of 3 types :
(1) Condom (Nirodh) is a thin sheath, usually made of rubber, to cover the erect penis. It is the most widely
used contraceptive by males in India as it is cheap and easily available. It is given free also by government. It checks
pregnancy by preventing deposition of semen in the vagina. Condom should be used regularly and put on before
starting coital activity, otherwise sperm-containing lubricating fluid may be left in the vagina. Condom should be
discarded after a single use. Condom is also a safeguard against infection of AIDS and sexual diseases.
(2) Diaphragm and cervical cap are dome-shaped rubber plastic covers that are fitted on the cervix in the
female's vagina, and check the entry of sperms into the uterus. These must be kept fitted for at least six hours after
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sexual intercourse. They are smeared with a spermicidal jelly or cream each time they are used. The diapharm and
cervical cap are the counterparts of condoms in the female
(3) Intrauterine devices (IUDs) are plastic or metal object placed in the uterus by a doctor. These include loop,
copper-T, spiral, ring, bow, shield, etc. They prevent the fertilization of the egg or implantation of the embryo. Their
presence perhaps acts as a minor irritant and this makes the egg to move down the Fallopian tubes and uterus rather
quickly before fertilization or implantation. Drawbacks of IUDs include their spontaneous expulsion, even without the
woman's knowledge; occasional haemorrhage; perforation of uterus; tubal pregnancy (implantation of the embryo) in
the oviduct; and chance of infection. Use of mechanical contraceptives have pregnancy rates of less than 10%.
(e) Physiological (Oral) Devices : Birth control pills (oral contraceptives) check ovulation by inhibiting the
secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) that are necessary for ovulation.
Hence, no eggs are released in a woman on the pill and conception cannot occur. The birth control pills have side
effects such as nausea, breast tenderness, weight gain and break-through bleeding (slight blood loss between
menstrual periods) and high blood pressure. On the other hand, the oral contraceptives reduce the chances of
certain types of cancer to occur in their users. A combined pill is the most commonly used birth control pill. It
contains synthetic progesterone and estrogen in doses high enough to check ovulation. Pill Mala D is taken daily,
and the pill Saheli is taken weekly. Oral contraceptives have pregnancy rates less than 1%. Birth control pills are
likely to cause cardiovascular problems.
(f) Other Contraceptives : Certain contraceptives, such as progestin minipill, are implanted under the skin
of the upper arm. They prevent pregnancy for 3 to 4 years. They steadily release a tiny amount of progestin into the
blood. Injectable one- month contraceptives are made in Germany, Maxico and China. These are marketed to
many countries.
(g) Abortion : Abortion is the medical termination of pregnancy (MTP) before the foetus becomes viable. It is
one of the most widely used methods of fertility control in world. Certain pills act as abortants. They function by
inducing menstruation which checks the implantation of the zygote or detaches the implanted egg. There are
movements against abortion practically all over the world. A drug named RU-486, an analogue of progesterone,
cdeveloped in France terminates pregnancy within the first few weeks, It blocks the progesterone receptors in the
uterus, thereby preventing progesterone from maintaining pregnancy.
(h) Abstinence : The best and 100% reliable way to avoid conception is to abstain from sexual intercourse. It is an
unnatural mode of birth control, and seems impracticable. Some couples practice abstinence at certain times with success.
(ii) Permanent Method : Sterilization provides a permanent and sure birth control. It is called vasectomy in
man and tubectomy in woman. It involves the removal of a short segment of each vas deferens or oviduct and
tying up of the remaining ends tightly with surgical thread. The operations are minor, usually performed under local
anesthesia, give very little discomfort, and do not affect the sexual life. Contrary to common belief, man with
vasectomy is still capable of ejaculation, but the latter consists only of secretions of various glands, and has no
sperms. sperms are still produced but reabsorbed into the body. Production of testosterone continues and its
distribution does not need the ducts.
(a) Laparoscopy : Now a telescopic instrument called laparoscope is used in tubal ligation. This blocks the
fallopian tubes. Eggs continues to be produced because the ovaries are intact, but they fail to pass into the uterus
and sperms fail to reach the eggs for fertilization.
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(b) Most Effective Birth Control : Sterilization is at present the most effective means of birth control. It is
difficult to reverse.
(c) Extent of Contraceptive Use : According to a UN report in the Tribune dated 24.8.87, half of the world
couples use contraception and one in three chooses, sterilization.
(d) Medical Advice : The birth control measures should be used with the guidance of qualified doctors. The
government provides these facilities free at the family planning centres. Contraceptives are given free or at nominal
prices at these centres to the couples of reproductive age desirous of preventing conception.
(e) Advice for Fertility : The couples who are not getting children can also seek advice and remedy at the
family planning centres.
(11) Foams, creams, jellies, etc. Chemical spermicides inserted in vagina before intercourse, prevent sperm from entering
cuterus.
(f) Abortion or Medical Termination Pregnancy (MTP) : has now been legalised in India.
S. No Method Action
(1) Rhythm method No intercourse during woman's fertile period (day 12-20).
(2) With drawl Penis is withdrawn before ejaculation.
(3) Tubectomy / Tubal ligation Woman's fallopian tubes are cut and tied, permanently blocking sperm release.
(4) Vasectomy Man's vasa deferentia are cut and tied permanently blocking sperm passage.
(5) Intrauterine device (IUD) Small plastic or metal device placed in the uterus, prevents implantation. Some contain
copper, other release hormones
(6) Oral contraceptive Synthetic estrogens and progesterones prevent normal menstrural cycle; primarily prevent
ovulation.
(7) Male condom Thin rubber sheath on erect penis collects ejaculated semen.
(8) Female condom Plastic pouch inserted into vagina catches semen.
(9) Diaphargm Soft rubber cup covers entrance to uterus, prevents sperm from reaching egg and holds
spermicide.
(10) Cervical cap Miniature diaphragm covers cervix closely, prevents sperm from reaching egg and holds
spermicide.
(12) Implant ( Norplant) Capsules surgically implanted under skin, slowly release hormone that blocks ovulation.
(13) Injectable contraceptive Injection every 3 months of a hormone that is slowly released and prevents ovulation.
(Depo-Provera)
Amniocentesis.
(i) Aim : It is a technique to determine :
(a) Sex of the developing baby.
(b) Genetically controlled congenital diseases.
(c) Metabolic disorders in foetus.
So amniocentesis is a pre-natal diagnostic technique.
(ii) Procedure : It involves following steps :
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(a) Location of the foetus is determined by a technique called sonography (using high frequency ultrasound
waves) to prevent accidental damage to the foetus.
(b) A fine hollow needle is passed through the abdominal and uterine wall of a pregnant female (about 14th to
15th week after conception) into the amniotic cavity.
(c) A small amount of amniotic fluid is withdrawn. It contains foetal skin cells and a number of proteins,
especially enzymes. The cells can be cultured in vitro for further examination.
Amniotic fluid
withdrawn
Placenta
Uterine wall
Amniotic cavity
Fig : Amniocentesis
(iii) Significance
(a) Sex determination : The somatic cells of foetal skin drawn with the amniotic fluid are stained to
determine the presence of sex chromatin (barr body). Presence of barr body indicates that the developing foetus in
female as female is with 2 X-chromosomes out of which one X-chromosome is active, while other X-chromosome is
heterochromatised into a darkly stained barr body.
(b) Congential disease : By Karyotypic studies of somatic cells, abnormalities due to changes in
cchromosome number like Down's syndrome, Turner's syndrome, Klinefelter's syndrome etc. can be determined.
(c) Metabolic disorder : By the enzyme analysis of amniotic fluid, different types of inborn metabolic
disorders like phenylketonuria, alcaptonuria etc. can be detected. These inborn errors are caused by the absence or
inactivity of specific enzymes due to gene mutations. So with the help of amniocentesis, if it is confirmed that the
child is likely to suffer from some incurable, congenital defect, the mother can go for abortion.
(iv) Drawback : However, these days, the amniocentesis is being misused also. Mothers even get their normal
foetus aborted if it is a female. This is just equivalent to killing of a normal child. So Govt. of India enforced the Pre-
natal Diagnostic Techniques (Regulation and Prevention of Misuse) Act, 1994, since January 1, 1994
under which all genetic counselling centres and laboratories are required to apply for registration. The violation of
this Act can bring a fine of Rs. 50,000 and imprisonment for two years. The doctor's registration is also cancelled till
the complaint is disposed of.
(v) Chorionic Villus Sampling (CVS) : Amniocentesis is possible without a chance of injuring the foetus
with the needle only after the sixteenth week of pregnancy. At this time, abortion is not safe. A new technique,
named Chorionic Villus Sampling (CVS), can be done during the eighth to tenth week of pregnancy when abortion
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is safe for the woman. For CVS, cells are sucked into a catheter passed through the cervix. CVS technique provides
a mass of rapidly dividing foetal cells, thus facilitating the examination of chromosomal disorders.
Test Tube Babies.
The technique of in-vitro fertilization and in-vitro development followed by the embryo-transfer in the uterus of
the normal female to start the development and finally leading to normal birth, is called test tube baby.
(i) History : First attempt to produce a test tube baby was made by a Italian scientist, Dr. Petrucci (1959
A.D.). Although, this human embryo survived for only 29 days, but his experiment opened a new filed of biological
science. The first test tube baby was born to Lesley and Gilbert Brown on July 25, 1978, in Oldham, England. Mrs.
Brown had obstructed Fallopian tubes. Dr.Patiricke Steptoe and Dr. Robert Edward both from England
experimented on Mrs. Brown successfully. the world's first test tube baby (a baby girl) was named as Louise Joy
Brown. Later, test tube babies were also born in Australia, United States and some other countries. India's first test
tube baby was born on 3rd October, 1978 in Kolkata. Her name was Kanupriya Aggarwal and was created
by Dr. Subash Mukherjee.
(ii) Procedure : It involves the following steps :
(a) Removal of unfertilized ovum from reproductive tract of a female.
(b) Ovum is kept under aseptic conditions.
(c) Fusion of sperm and ovum in a culture medium, outside the female body, to form the zygote.
(d) Zygote is stimulated to develop in vitro upto 32-celled stage.
(e) Developing embryo is implanted on the endometrium of the uterus at 32-celled stage. So the pregnancy in
the woman starts and further development of the child continues in the womb till it is born. Such a baby called a
test tube baby.
(iii) Significance
(a) It is boon to infertile mothers.
c(b) It can be used for men with Oligospermia (low sperm count).
(c) Old superior cows can donate oocytes.
Embryos can be frozen and preserved in an embryo tank for 10 years for future use.
In very rare cases, a surrogate mother may have to be used to bring up in vitro fertilized ovum to maturity.
Though biological realization of a test baby is a remarkable achievement, it has raised several ethical and legal
problems like the right over the child.
Important Tips
• China (1234 million) and India (953 million) are two most populous countries; USA (265.8 million) and Indonesia (200.6) come
next.
• India's population growth rate is about 2% a year and China's 1.4%.
• Maximum population growth rate in the world is in Kenya (5.5%).
• Austria has shown a negative growth rate.
• The most thickly populated country of the world is Bangladesh.
• Greenland is the most thinly populated country (45/Km2) followed by Australia.
• The International Conference of Population and Development (ICPD) was held at Cairo in September 1994.
• Mumbai will become the second largest megapolis in the world by the turn of the century with a population of 18.9 million.
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• Tokyo is the largest city with 26.5 million people.
• The first census in India was carried out in 1891.
• In last census Nagaland registered the highest growth rate of 56.86% while Kerala the lowest, 13.98%.
• The most thickly populated state of India is West Bengal (766/Km2); the most sparsely populated area of the country is Arunachal
Pradesh (10 Km2).
• The first district to become 100% literate is Ernakulam in Kerala.
• The first state to become 100% literate is also Kerala.
• Chandigarh has the lowest number of females per 1000 males with 790.
• Bihar stood at the bottom with a literacy rate of 38.48% with Rajasthan being close to it having 38.55 literacy percentage.
• National average of infant mortality rate is 72, where as U.P., It was 86, in Bihar 73, In Rajasthan 85 and In M.P. 89 while that of
Kerala is only 15.
• Indian population is a young population, while the population of USA, England, Germany, etc. is ageing population.
• In china, more than 8,000 live for more than 100 years, about 8 million above 80 years, about 20 million above 70 years and about
120 millions people above the age of 60 years.
• Population explosion : Also called Population holocaust. It is high growth rate of human population.
• Nearly half of the world population is distributed in four countries : China (1120 million), India (844 million-1991 census),USSR(291
million) and USA (251 million).
• World Population Day : 11 July. (since 1987 when on 11 July on that year, the world population crossed five billion).
• In July, 1997, Indian population was growing at the rate of 16 million per year which means 45,000 per day and 31 per
minute.
• Minimal decennial growth is reported from Kerala (13.98%) while maximal decennial growth is reported from Nagaland (58.86%). It
was due to better fertility awareness among rural women folk.
• Indian Population Project-VI (IPP-6) ended in March, 1997.
• According to revised estimates, if the present trend continues, India will surpass China in 2050 A.D. With population of 2160
million.
• China's birth rate is 18 births per 1,000 population and its TFR is 2.2 children per woman. Conversely, India's birth rate and TFR are
30.5 per 1,000 persons and 3.5 per women respectively (1991 census).
c• After 40 years of age, contraceptive pills increase the chances of cardiovascular diseases.
• Matez Gasper : World's fifth billion child, a male infant born in Zagreb (Yugoslavia) on July 11,1987.
• Fatima : World's 6th billion child, a female infant born in Sarajevo (Kosovo) on October 14, 1999.
• India till 1920 was in the first stage (high stationary) of demographic cycle but now India has entered third stage (late expanding) of the
cycle.
• In China, person is considered as of "one year age" at the time of birth.
• Most natural population is an open population in which individuals undergo immigration or emigration, while yeast population in a
test tube is a closed population in which individual do not undergo immigration or emigration.
• Couple protection rate : Bringing eligible couples under the umbrella of various modes of contraception.
• Family Planning Programme was adopted as a National Programme in 1995 A.D. Now it has been renamed as Family Welfare
Programme.
• Government-sponsored Family Planning Programme was started in India in 1951.
• Mirena : A new contraceptive plastic coil coated in copper and is fitted in the uterus. Copper acts as a spermicide while coil releases
small amounts of an artificial hormone, gastrogen, which further increase the spermicidal effect of copper. It combines both mechanical
and hormonal contraceptive methods and has been found to have more than 99% efficary. It is introduced by a German company.
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• Central Drug Research Institute (Lucknow) has developed a plant-based spermicidal cream “Consap” from Reetha (Sapindus
mukrosii).
• NIm-76 : It is neem-based, pre-coital vaginal contraceptive cream having spermicidal properties. It has been developed by Defence
Institute of Physiology and Allied Sciences (DIPAS), New Delhi. It has been found to be safe, non-hormonal and non-toxic
with no side effects. Nim-76 punctures the sperm skin causing leakage of cellular contents so preventing the capacitation of sperms. It
also has both anti-bacterial and anti-fungal properties.
• Contraception : Method of birth control to check fertilization.
• Sterilization is the most effective method of birth control.
• Laparoscope : Instrument to aspirate the oocytes from the ovary for IVF ( In Vitro Fertilization)
• UN Population Award, 1992 was awarded to an Indian Industrialist, J.R.D. Tata, for his efforts to stabilize Indian population.
• UN Population Award, 1998 : It was jointly awarded to a group of Ugandan Elders (credited to reduce the practice of female
genital mutilation) and head of Jamacia's Family Planning Board
• India’s first test baby was “kanpuriya Aggarwal”. India’s second test tube baby was “Kumari Harsha” She was born on August 6,
1986.
• Vas aplasia : Absence of both vasa deferentia.
• Ogino (1930) : Described “Safe period” or “Calander period” to control pregnancy .
• Success rate of test tube baby is less than 20%.
• Literacy rate : 2001 = 65.38%; 1991 = 52.11%; 1997 =62% (+10%).
• G.I.F.T. – Gametic Intra- Fallopian Transfer is the latest technique to produce the child.
• Gamete-Intra Fallopian tube transfer : Sperm (obtained by masturbation / electro ejaculation) and ovum obtained by laproscopy
care injected into the mid part of the oviduct by a separate catheter in a cycling female (in proliferative stage).
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Origin of Universe and Earth.
Philosophers and scientists have been busy to solve the riddle as to how the universe and our earth were
formed and how and when 'life' originated on earth. The branch of life science for the study of 'Origin of life' and
evolution of different forms of life on earth was called Bioevolution or Evolutionary Biology by Mayer, (1970).
The study of universe or cosmos is called Cosmology. Our earth belongs to the Solar system having nine stars
called planets constantly rotating around a common Sun. On the basis of the order of the distance from the sun
these planets include Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto while moon is a
satellite of earth. The universe is made up of matter and energy and it was formed about 10 to 13 billion years ago
as a red hot, dense, rotating gaseous cloud of cosmic dust called Ylem or primaeval matter. The Ylem consisted of
particles of matter (like neutron, proton and electron) and antiparticles of antimatter. Scientists like Lemaitre
(1931), Gamow (1948), Dicke (1964) etc. supported the Big-Bang Hypothesis which explains that collision
between these particles and antiparticles caused a tremendous explosion to form atoms of hydrogen. Cosmic
evolution began with the fusion of hydrogen atoms with progressively heavy atoms of different elements. Stellar
systems and stars were formed by spreading of original gaseous cloud into the space and divided into smaller and
larger masses. Most of the stars are masses of red hot gases even today.
Kant (1755) and Laplace (1796) supported Nebular Hypothesis which explains our solar system to have
evolved about 4.5 to 5 billion years ago from a rotating red-hot gaseous cloud containing millions of free atoms of
different varieties. First of all sun was formed when this cloud condensed, next the planets were thrown off from the
sun and later on, in turn satellites were thrown off by planets. However, Nebular hypothesis was rejected by
astronomers of present century like Weizsacker (1944), Alfen (1950) and Hoyle (1955).
According to them the sun was originally surrounded by a disk of rotating gas. Later many rotating concentric
whorls were formed by break up of the disc and by gravitation and rotation particles of each whorl collected and
condensed to form a planet. Our earth was formed about 4.6 billion years ago as a red hot gaseous cloud of free
atoms with temperature of 5000o to 6000oC.
Structure of present earth. The earth is orange like in shape. Its pole to pole diameter is 12640 km and
equatorial diameter is 12783 km. It is about 15
crore km away from sun and about 484000
km away from moon. Due to tremendous
ctemperature gases existed in atomic form but
gradually they cooled down in hundreds and
millions of years into molten core. According to
density other elements got stratified. Earth
contains the central solid core, the middle
mantle and shell and outer crust. The earth
rotates at its polar axis in one day and it rotates
around the sun in 365 1 days or one year.
2
Origin of Life.
Life is the part and parcel of the universe and both are very intimately associated with each other. We know
that “Life is the most unique, complex organisation of molecules, expressing itself through chemical reactions which
lead to growth, development, responsiveness, adaptation and reproduction” that matter has achieved in our
universe. Origin of life is a unique event in the history of universe.
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(i) Ancient theories of origin of life : Various theories have been put forward to explain the phenomenon
of origin of life. A few of them were only speculations while others were based on scientific grounds. These theories
are –
(a) Theory of special creation.
(b) Theory of spontaneous generation or Abiogenesis.
(c) Biogenesis
(d) Cosmozoic theory.
(e) Theory of sudden creation from inorganic material.
(f) Naturalistic theory.
(a) Theory of special creation : According to a Spanish Priest Father Suarez (1548 – 1617 B.C.), the
whole universe was created in six days by the God. First day Earth and heaven, second day sky, third day dry land
and vegetation, fourth day Sun, Moon and other planets, fifth day fishes and birds, and sixth day human beings
other animals were created by God. This theory was based on some supernatural power.
(b) Theory of spontaneous generation or Abiogenesis :
This theory postulates that life originated from non-living matter
spontaneously from time to time. This theory was supported by Plato,
Aristotle, Anaximander, John Ray, Needham, Von Helmont, etc., upto
the end of seventeen century. Huxley (1870) criticised this theory and
propounded the theory “life originated from preexisting life only.”
(c) Biogenesis : Scientists like Redi (1668) Spallanzani (1767),
Louis Pasteur (1866–1862) provided experimental support for the
Biogenesis concept of Huxley.
Francesco Redi (1668) showed that maggots could not be
created from meat. Actually, the smell of meat attracts flies which
clay eggs on the flesh. These eggs hatched into flies.
Abiogenesis means origin of life from non-living organisms.
Ridi’s Experiments
Spallanzani (1767) showed that even primitive, unicellular
organisms cannot arise from non-living matter.
Louis Pasteur (1860-62) obtained air samples in the flasks
of broth (yeast and sugar solution) whose drawn-out necks were
sealed cooling these contained a partial vaccum. Where a sample
was required, the flask was opened. Air was drawn in and the
flask was resealed. Flasks were incubated. These flasks which
were opened in the streets became turbid while those exposed to Pasteur’s experiment with swan-necked flasks
dust-free air rarely contained bacteria.
Louis Pasteur also, used swan-necked flasks whose long, curved necks permitted exchange of air between
outside and inside of the flask, but dust and bacteria were trapped along the wall of the neck. On tilting the flask, the
bacteria got washed down into the broth, so that the latter became cloudy due to bacterial growth.
(d) Cosmozoic Theory : Richeter (1865), Preyer (1880), Arrhenius (1908), Hoyle (1950) and Bondi (1952)
believed in eternity of life. According to Arrhenius life was transferred from “cosmozoa” (life of outer space) to different
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planets small units called ‘spores’. The spores were covered by a thick protective covering. When the spores got
favourable conditions and temperature, the spore coat was dissolved and gave birth to initial living organisms. This
theory does not explain as to how the life originated in space and how the life originated in spores remain
impenetrable by ultraviolet and gama rays.
(e) Theory of sudden creation from inorganic material : Cuvier (1769-1832) believed in
catastrophism. According to him, the catastrophy destroys the whole life on earth, and after that, new life originates
called it as Mechanistic theory.
(f) Naturalistic Theory : Haldane, a British scientist, stated that in the early atmosphere of gas mixture
probably carbon dioxide, ammonia and water vapours were predominantly present. When ultraviolet rays reacted
on them, organic molecules were formed. Gradually, quantity of these oceans which later gave rise to amino acids,
proteins, carbohydrates, nucleic acids, etc.
(ii) Oparin's Modern Theory : Oparin (1924) proposed that “life could have originated from non-living
organic molecules.” He believed in Biochemical origin of life. Haldane (1929) also stated similar views. Oparin
greatly expended his ideas and presented them as a book “The origin of life” in 1936.
According to this theory, the Earth originated about 4,500
million years ago. When the earth was cooling down, it had a reduced
atmosphere. In this primitive atmosphere nitrogen, hydrogen,
ammonia, methane, carbon mono-oxide and water were present.
Energy was available in the form of electric discharges by lightening
and ultraviolet rays. As soon as the earth crust was formed, it was very
much folded. Torrential rains poured over the earth for centuries and
were deposited in deep places.
The atmospheric compounds, inorganic salts and minerals
also came in deep places oceans, these molecules gave rise to a
variety of compounds and finally to the self-duplicating molecules.
Ultimately these molecules were enclosed in membranes derived
cfrom lipids and proteins, along with water and chemical compounds,
giving rise to cell like units. Again random combinations may have
led to the formation of chlorophyll– containing organisms which
could produce their own food (autotrophs) by a process called
photosynthesis. These organisms had a better chance to live because
they synthesise starch from carbon dioxide and water in presence of According to Oparin’s hypothesis–Origin of cells from
sunlight. Starch could be used as further source of energy. During coacervates
photosynthesis, oxygen was produced. The oxygen was used by
other organisms for respiration. Also oxygen, when acted upon by ultraviolet rays, formed ozone layer through
which ultraviolet rays cannot pass. This layer is formed about 25 km. from earth's surface. After the formation of
ozone layer, organisms could come to the surface of the ocean and could survive even on land, if thrown out of
oceans. The Oparins's and Haldane's theory of origin of life is most accepted these days as it is supported by Miller's
experiment duly supported by David Buhal, Melvin Kelvin's experiment etc.
O2 is absent in the primordial atmospheres at the time of origin of life.
Miller's Experiment : An American scientist (Biologist) Stanley Miller (1953) performed an experiment
under support Oparin's theory of origin of life. He believed that basic compounds which are essential for life can be
Chapter 19 484
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synthesised in the laboratory by creation in the laboratory, on a small scale, the conditions which must have existed at
the time of origin of life on earth.
Miller took a flask and filled it with methane, ammonia and hydrogen in proportion of 2:1:2 respectively at
0°C. This proportion of gases probably existed in the environment at time of origin of life. This flask was connected
with a smaller flask, that was filled with water, with the help of glass tubes. In the bigger flask, two electrodes of
tungsten were fitted. Then a current of 60,000 volts was passes, through gases containing bigger flask for seven
days. At the end of seven days, when the vapours condensed, a red substance was found in the U-tube. When this
red substance was analyzed, it was found to contain amino acids, Glycine and nitrogenous bases which are found
in the nucleus of a cell.
Aerobic Photoautotrophs
Anaerobic Photoautotrophs
An experiment to prove that organic compounds were the basis of life, was performed by miller.
From the above theory we conclude that life first originated in water. Therefore, water still continues to be anChemoautotrophs
essential constituent of life.
Chemoheterotrophs
The entire process of the origin of life, as proposed by Oparin, can be summarised as under –
(a) The Chemical Evolution :Anaerobes
c(1) Step 1 : Formation of simple molecules : The reactions between simple atoms like carbon, hydrogenExperiment conducted by Stanley Miller
oxygen and nitrogen in the primitive atmosphere led to the formation of simple compounds like water, ammonia
and methane. But since the earth was very hot, all these substances remained in the form of vapours in the
atmosphere. Gradually, as the earth started cooling down, the dense clouds began raining on the earth. But the
liquid was still very hot. Therefore, as the liquid water touched the earth's surface, it again got vaporized to be
returned to the atmosphere. This process continued for millions of years. As a result of these heavy downpours, the
earth's surface got filled with water to form rivers and oceans. Ammonia and methane got dissolved in the oceanic
water. The mineral elements, which were dissolved in rivers, were also carried into the oceans when rivers joined it.
The scientists have found that simple unicellular organisms (resembling modern cyanobacteria) were present
on this earth about 3600 million years ago. It is believed, therefore, that life must have originated on this earth
about 4600 to 3600 million years ago.
It must be clear that the earth's atmosphere at that time was quite different from as it exists today. The earth's
atmosphere at that time was reducing, not oxidizing (as it is today). The primitive atmosphere of earth had
hydrogen, nitrogen, water vapours, carbon dioxide, methane and ammonia abundance. Oxygen was not available
in free state in sufficient quantities.
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(2) Step 2 : Formation of Simple organic compounds : Continuous rains provided opportunities for
different types of molecules to collide with each other and react. Methane is an active compound, and it reacted
with other compounds to form organic compounds like ethane, butane, propane, ethyl alcohol. From such organic
compounds which were formed in the ocean and which played a role in the origin of life include –
(A) Sugars, glycerol and fatty acids : These were formed by the combination of carbon, hydrogen and
oxygen.
(B) Amino acids : These were formed by the combination of carbon, hydrogen, oxygen and nitrogen.
(C) Pyrimidines and Purines : These were formed by the combination of carbon, hydrogen and nitrogen.
These compounds were formed at the time when sunlight could not reach earth because of dense clouds in
the sky. Under such circumstances, the energy required for the synthesis of above–mentioned chemical substances
must have been obtained from the cosmic rays and lightening in the sky. Even today, NH3 and other compounds
are formed in the atmosphere during electric discharges. Thus, it is clear that all the amino acids required by living
organisms could be formed in the condition of primitive atmosphere. However, this type of synthesis requires
reducing atmosphere because the substances produced through these processes get destroyed by oxidation on
coming in contact with oxygen. Since free oxygen is available in the atmosphere today, present day atmosphere is
no longer reducing–it has turned oxidizing. Therefore, in the present circumstances, such a synthesis of chemical
substances is not possible.
(3) Step 3 : Formation of complex organic compounds : The simple organic compounds combined in
different ratios to form complex organic compounds like polysaccharides, fats and proteins.
(A) Simple sugars combined in different ratios to form polysaccharides like starch, cellulose, glycogen etc. The
formation of such compounds had been very important for the origin of life because cell walls are made up of
cellulose and energy–giving molecules are stored in the form of starch and glycogen.
(B) The reaction between glycerin and fatty acids yielded fats.
(C) Different types of combinations between a variety of amino acids yielded different types of proteins. The
formation of protein was a very important step in the origin of life because proteins are not only structural
components of cell organelles, but many proteins, called enzymes, work as catalysts for biochemical reactions.
In fact, the present day organisms synthesise their complex molecules from simple organic substances with the
chelp of enzymes only. However, enzymes themselves are proteins. Therefore, first of all protein must have been
synthesized without the help of any enzyme.
(4) Step 4 : Formation of nucleic acids and nucleoproteins : You have already learnt that the reaction
between methane, ammonia and water resulted in the formation of purines and pyrimidines. Some of the purines
and pyrimidines combined with sugar and phosphorus to form nucleotides. Many molecules of nucleotides
combined to form nucleic acids–DNA and RNA. The formation of nucleic acid in the oceanic water was a big steps
in the direction of origin of life. Nucleic acids combined with the proteins to form nucleoproteins. Some of the
nucleoproteins developed the capability to synthesise molecules similar to themselves, from organic and inorganic
substance present in the ocean. In other words, the capability to reproduce had evolved. As a result of continuos
reproduction, the number of nucleoproteins went on increasing. Since, organic substance were required for this, the
organic substances started being depleted resulting in competition between the nucleoproteins. Physical and
chemical changes sometimes led to the changes in the competition of nucleoproteins, and new types of
nucleoproteins came into existence by mutations. Those new nucleoproteins which were successful in the
competition, increased in number.
Chapter 19 486
Evolution
N Atmosphere
H CH
N CN
H C HN C H
C N CH
HC N
Lava Fe Carbides Nitrides CH4 NH3 HCN
Ni Methane, ammonia
Other
Metals
(b) Organic Evolution : UNSATURATED O2
HYDROCARBONS
(1) Step 5 : Formation of CoacervatesO2
Oparin believed that the formation of protein was a very important step in towards the origin of life. Due toe. g., Ethylene; Acetylene
their Zwitterionic nature, the protein molecules. This enabled these colloidal structures to maintain their identify
inspite of being surrounded by water molecules–forming a type of emulsion. The coalescence of these colloidalAlcohols; Aldehydes,Energy of ultraviolet raysAmino Acids
structures led to the formation of structures called coacervates. These coacervates had the ability to exchangeKetones; Organic acidsand electric dischargee. g., Alanine; Leucine; Glycine
substances with the surrounding water and accumulating required substances within them.
e. g., Glycerol; Formaldehyde
c Under certain conditions scientists have obtained cell like structures. These are known as coacervates.
Sydney F. Fox's experiment : Sydney F. Fox of Florida University, obtained some complex molecules by
heating upto 90°C a dry mixture of many amino acids found in living organisms. The molecules so obtained very
much resembled the proteins. He heated these molecules in water and allowed the mixture to cool down. In the
fluid so obtained, he could observe minute structures resembling the cells. He called them microspheres. The
microspheres are surrounded by membranes, and these also reproduce vegetatively just like years. Surprisingly the
biochemical processes like breakdown of glucose also occur their. However, electron microscopic examination of
these does not reveal any cellular structure. On keeping in distilled water, these become turgid, but these get shrunk
if immersed in salt solution. Oparin's coacervates and Fox's microspheres are infact, similar structures.
(2) Step 6 : Formation of Primitive cell : A primitive cell membrane was formed by the arrangement of
lipid molecules between the surface of coacervates and external watery medium. This provided stability to the
coacerrates. It is believed that a primitive cell was formed when–
(A) Nucleic acids having the property of self-duplication entered the coacervates.
(B) Rearrangement of molecules occurred inside the coacervate surrounded by lipid molecules.
Earliest organisms : Evidences available so far indicate that the cells of the earliest organisms did not
contain either nucleus or cell organelles. The molecules of nucleic acid were surrounded by a colloidal mixture (may
be called protoplasm) of proteins and organic compounds. This, in turn, was surrounded by a thin membrane.
Chapter 19 487
Evolution
Water and soluble substances would pass through this membrane. Such cells which lacked nuclei were called
prokaryotic cells. In 1966, some fossils have been discovered from 300 million years old rocks. These fossils are of
prokaryotic organisms. Earliest organism is chemoheterotrops.
(3) Step 7 : Origin of autotrophism : In the primitive organisms, the process of metabolism began
because all the substances required for reduction were available in water. Of course, oxygen was not available
which was required for oxidation of substances to yield energy. Therefore, the primitive cells respired anaerobically
i.e., these used to obtain energy by fermenting the organic compounds obtained form the water, with the help of
enzymes due to fast nutrition, growth and multiplication, their number in the ocean increased greatly. As a result,
scarcity of organic substances developed in the ocean. A struggle started between the cells for obtaining nutrition.
cIn this way, the prokaryotic cells which were chemoautotrophs, became photo autotrophic. These cells
At such a time, some of these organisms developed the capability of synthesizing organic substances. Such
organisms began synthesizing energy– giving substances (carbohydrates) from simple inorganic substances
abundantly available in the environment. In this way, evolution of autotrophs from heterotrophs took place.
This was the beginning of autotrophic nutrition. However, it was quite different from the photosynthesis which
is carried out by green plants, because it utilized energy obtained by anaerobic respiration (not solar energy).
Therefore, such type of nutrition is also called chemoautotrophic nutrition. Such type of nutrition is observed even
today in the sulphur bacteria. 6 CO2 + 12 H2S Fermentation→ C6 H12O 11 + 6 H2 O + 12 S
Chemical energy
At the same time, from different chemicals present in the oceanic water evolved porphyrins which where like
modern chlorophyll led to the evolution of present chlorophyll, so that these cells started utilizing H2 O instead of
H2S for photosynthesis. 6 CO2 + 12 H2S Solar → C6 H12O6 + 6 H2 O + 12 S
Energy
Till then, oxygen was not freely available in the atmosphere. However, gradually molecular changes in the
bacteriocholorophyll led to the evolution of present chlorophyll, so that these cells started utilizing H2O instead of
H2S for photosynthesis. 6 CO2 + 12 H 2 O Solar → C6 H12O6 + 6 H2 O + 6O2
Energy
resembled modern cyanobacteria. In 1968, the forms of such types of cells have been recovered from 320 million
years old rocks. These have been given the name Archaeospheroides barbertonensis. Due to the absence of well-
defined nuclei in them, these have been included under the kingdom ‘Monera’. Thus, release of O2 in the
atmosphere and its free avilability was the result of photosynthesis. This was a revolutionary change which greatly
affected the course of organic evolution.
(4) Step 8 : Origin of Eukaryotic cells : As a result of photosynthesis, oxygen was released in the
atmosphere which started reacting with methane and ammonia in the atmosphere. Its reaction with methane
yielded CO2 and H2O. On the other hand, reaction between oxygen and ammonia resulted in the formation of CO2
and nitrogen. In the course of these changes, Ozone (O3) gas was formed from oxygen; the ozone spread in the
form of an envelope surrounding the earth, the distance between the ozone layer and the earth's surface being
approximately 15 miles. As free oxygen became available on the earth, gradual changes took place in cell structure
also. Membrane – bound organelles i.e., mitochondria, chloroplasts, golgi bodies, lysosomes evolved. Thus,
prokaryotic cells. Most of the organisms on the earth today are eukaryotic.Gradual changes in the earth's
atmosphere led to gradual changes in the eukaryotic cells also. Instead of living separately, the cells started living
together in the form of colonies. Simultaneously, multinucleation of multicellular structures forming tissues. Different
Chapter 19 488
Evolution
types of tissue combined to form special organs. From the organs, organ systems and ultimately complex bodies of
organisms were formed.
Organic evolution would have not been taken place if individuals in a population did not show genetic
variation.
Synthetic theory is the most accepted theory of organic evolution.
The greatest evolutionary change enabling the land vertebrates to be completely free from water, was the
development of shelled eggs and internal fertilization.
The material for organic evolution is mutation.
Evidences of Organic Evolution.
(i) Evidences from Classification : All the known living animals and plants have been classified into
various species, genera, families, order, classes, phyla and kingdoms. The classification of a particular animal isThe following are the evidences in favour of Organic Evolution :
attempted only after its extensive study. It is seen that every living being is related with other living being. The
relations may be very close or may be quite apart. On their relationship, they are put under various orders, classes,(i) Evidences from Classification(ii) Evidences from Comparative Anatomy
phyla etc. On the superficial examination one can hardly believe that they are interrelated. But after their careful
study they can be arranged in definite order, Protozoa (acellular) being at the base while Chordata at the top. No(a) Analogy and Homology
doubt, there is some sort of gap between chordates and non-chordates today but who known that this gap may be
filled some day by some further discoveries. Moreover, the present-day types represent only the terminal twigs of a(b) Vestigeal organs
vast phylogenetic tree and for establishing relationship we should focus our attention on the main trunk. The
(iii) Evidences from Physiology (iv) Evidences from Serology
canimals can be classified as described above in a systematic order.
(v) Evidences from Embryology (vi) Evidences from Palaeontology
(vii) Evidences from geographic distribution (viii) Evidences from Genetics
(ii) Evidences from Comparative Anatomy : In all the living animals, the basic substance of life is
Protoplasm. If the species had been created separately, then there should be no relationship in the various organs
and systems of animals. But on the contrary, we see that large number of animals although unlike in appearance
show most of the systems and organs made on the same plan. The resemblance are very close in the members of
the same group. For example –
(a) Analogy and Homology : While examining the various structures in the bodies of different animals, one
may come across certain organs of same origin but of different functions. For example, the forelimbs of salamander,
crocodile, bird, bat, whale, and man, all have the same origin and essential structures but different functions to
perform. Such structures are called homologous. On the other hand, the wings of insect, pterodactyle, bird and bat
perform the same function, though they have different origin and entirely different structures. Such structures are
termed analogous.
The homologous structures give us the answer that how the two different types of animals or organs have the
same origin and functions to perform; this shows that they have changed themselves according to their different
needs.
Analogous organs are those, which are functionally similar.
Chapter 19 489
Evolution
Homologous organs explains divergent evolution.
Similarity developed in distantly related groups as an adaptation to the same function is called
convergent evolution.
The wings of an insect and a bat exhibit analogy.
(b) Vestigeal organs : There are present in the body of animals certain structures which have no function(a) Homology in the forelimbs of vertebrates(a) Homology in the forelimbs of vertebrates
and are very much reduced. These structures are quite developed and
functional in allied animals. Such useless organs are termed as
vestigeal and are of frequent occurrence. In human body alone, there
are as many as ninety such organs. For example, vermiform appendix
is vestigeal in man but functional in rodents, horse and other
herbivorous animals.
Vermiform appendix is the Vestigeal organ in man.
Hind limbs is a vestigeal organ of python.
Muscles of ear pinna is vestigial organ of man.
Wisdom teeth is vestigeal organ of man.
(iii) Evidences from Physiology : Various types of chemical
ctests exhibit many basic similarities in physiological and chemical
properties that show a physiological relationship among animals.
Take the example of thyroxin hormones, which are similar in all Different vestigial organs in man
metabolism. The thyroid of human beings can be very easily replaced
by the thyroid of cattle without any ill-effects and so on.
(iv) Evidences from Serology : This is a method by which the reactions of blood serum is observed. From
the blood are also extracted the crystals of Oxyhaemoglobin. The structure differs in different vertebrates, but in a
definite order. The reaction is nearly identical in man and anthropoid monkeys, but slightly less identical with other
mammals.
(v) Evidences from Embryology : With the exception of a few, every multi-cellular animal originates from
a zygote. The development from zygote to adult shows many similarities in various organisms. The development is
termed as ontogeny.
Haeckel, a German biologist, gave a theory “Ontogeny repeats Phylogeny”. An individual organism during
its development (ontogeny) tries to repeat the history of its race by different stages (phylogeny). The vertebrate
development confirms the above statement. For example, the embryos of all vertebrates pass through a gilled stage.
In fishes, the gills are present in the adult condition. In amphibians, gills are present in some forms of frog and in
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tadpole. While in reptiles, birds and mammals these gills are never functional but nevertheless they are always
present in the embryo.
Haeckel, believed that all those stages, which have occurred during the course of evolution of a particular
animal, also pass through its development.
There are objections to this theory, but is throws enough light to show the interrelationship of animals.
(vi) Evidences from Palaeontology : The study of fossils and their interpretation forms one of the great
evidences of evolution. An Italian scientist, Leonardo da Vinci, was the first person to recognize their importance
and said they were either remains of organisms of their impressions on some sort of clay or rock.
A number of fossils have been discovered from time to time. Some of these fossils are very prefect in their
state of preservation. For example, the fossils Archaeopteryx shows the characters of reptile on the one hand, and
the characters of birds on the other hand, meaning that the birds have evolved from reptiles, the Archaeopteryx
being a connecting link.
In the same way all the evolutionary stages of horse, elephant, camel and man etc., can be constructed. The
earliest horse was known as Eohippus. It was eleven inches in height and made its appearance in Eocene time from
some unknown five-toed ancestor. The Eohippus, after various evolutionary stages, transformed itself into the
present-day horse Equus, which shown many dissimilarities from its great grand, grand ancestor. The various stages
in its evolutionary life had been preserved as fossils. The same is the case with camel, elephant and man, etc. So
this science of paleontology helps in a great deal in understanding the process of evolution.
Wallace gave a theory very similar to that of Darwin.
In Cenozoic era mammals and birds were evolved.
There was no life in Azoic era.
Fossils are remains of organisms present in the rocks.
Ruling reptiles were dominant during Mesozoic era.
c0.1
Era Age million years Period Epoch Dominated Fauna
Recent Modern man, mammals, birds, fishes,
Quaternary (Holocene) insects.
Pleistocene Extinction of grate mammals. Primitive
2.5 man common. Evolution of human
society & culture.
Cenozoic Tertiary Pliocene Evolution of primitive man–like forms
(Age of Modern life)
7 from man like apes. Formation of modern
mammals.
2.5 Miocene Mammals at peak. Evolution of man–like
apes.
Oligocene Decline of egg laying and marsupial mammals.
3.8 Rise of first monkeys and apes.
5.4 Eocene Diversification of placental mammals.
6.5 Paleocene Rise of first primates, placental mammals
and modern birds.
Mes 135 Cretaceous Extinction of dinosaurs and toothed
ozoi birds. Rise of first modern birds.
c
(Era
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Paleozoic 195 Jurassic (Age of Giant Reptiles) Origin of advanced lizards, crocodiles,
(Era of Ancient life) alligators, marsupials toothed birds.
240 Triassic Dinosaurs became large. Reptiles
285 dominant.
Permian Origin of dinosaurs & primitive mammals
375 (egg–laying mammals). Extinction of
420 Carboniferous primitive amphibians.
450 (Age of Amphibians) Extinction of many marine invertebrates
520 Devonian like trilobites. Rise of modern insects.
570 (Age of fishes) Evolution of mammals like reptiles.
2300 Silurian
3600 Ordovician Origin of reptiles and winged insects.
3800 (Age of invertebrates)
4600 Cambrian cOrigin of first landvertebrates
Pre–cambrian (amphibians) Fishes abundant.
Proterozoic Origin of jawed fishes and wingless
(Era of early life) insects.
Origin of chordates with first jaw– less
Archeaozoic fishes (origin of vertebrates). Invertebrates
(Era of invisible abundant.
All invertebrate phyla established.
life) Trilobites (swimming crustaceans which
do not exist today) dominant
Origin of marine metazoans including
sponges, cnidarians, annelids, molluscs
and arthropods.
Scanty fossils.
Origin of prokaryotes (Monera) and
Eukaryotes (Protista). Origin of life.
No life?
(a) Fossils :
(1) Direct evidences of organic evolution are provided by fossils (L. fossil, dug up).
(2) The science of discovering and studying fossil record is called Palaeontology (Gr. palaeo, ancient; logy, study of).
(3) Fossils are remnants, models and impressions of extinct organisms.
(4) Fossils are found preserved in earth's sedimentary rocks.
(5) Fossil may be an entire organism buried in sediment or snow, small part of ancient organism or impression of
ancient leaf or stem.
(6) Fossilization occurs where organisms are buried and preserved by natural processes.
(7) Petrifaction is the most usual type of fossil in which hard parts like bones, shells, teeth or trunk of trees are
preserved.
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(8) Other types of fossils are moulds, casts and compressions. A compression is a common type of plant fossilThorium 232 → Lead 206
formation.
(9) A compression is formed when the internal structure of plant is lost leaving a thin carbon film which gives the
outline of the original plant.
(10) When the contents of intestine of ancient animals are preserved, it is known as coprolite.
(11) Palynofossils are tiny microscopic spores, pollen and other vegetal remains of the past.
(12) Pseudofossils are formed by minerals which crystalize and develop into patterns resembling outlines of plants.
(13) Twenty million years old fossil forests have been discovered and studied by the Birbal Sahni Institute of
Palaeobotany, lucknow.
(14) About 25,000 years old frozen elephant-like mammoths were found buried in ice in Siberia in the early part of
this century.
(15) Leonardo da Vinci (1452-1519) is called the father of Palaeontology.
(16) The modern palaeontology was established by Georges Cuvier (1769-1832) and referred to as 'Founder of
modern Palaeontology'.
(17) Palaeontological study helps in understanding and locating coal and hydrocarbon sources.
(b) Age of fossils :
(1) The age of the fossils or rocks can be determined by ‘Clock of the rock method' or 'Radioactive clock’.
(2) 'Clock of the rock' method is based on conversion of unstable radioactive nuclei into stable nuclei over a fixed
period.
(3) Radioactive clock method was introduced by Boltwood in 1907.
(4) The common radioactive elements which lose their radioactivity and change into their non-radioactive isotopes at
a fixed rate are :
Potassium 40 → Argon 40
Carbon 14 → Nitrogen 14
Uranium 238 → Lead 207
cRubidium 87 → Strontium 87
Important living fossils
1. Peripatus (Arthropoda) 2. Limulus (Arthropoda)
2. Nautilus (Mollusca) 4. Neopilina (Mollusca)
5. Lingula (Brachiopoda) 6. Latimeria (Coelacanth fish)
7. Sphenodon (Reptilia) 8. Didelphis (Opossum)
(vii) Evidences from geographic distribution : If the study of horizontal distribution of animals on the face of
this earth is made, it would be seen that animals are not evenly distributed. Two identical places with the same climate and
vegetation may not have same sort of animal fauna Alfred Russel Wallace (1823-1913) divided the whole world into six
major biogeographical regions or realms. For example, Elephants and Rhinoceros occur in India and Africa only and not in
Brazil, although the climatic conditions are alike. Again, members widely separated areas. For example, lung fishes are
found in South America, Australia, Africa and nowhere else.
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The explanation of this uneven distribution is quite obvious, and can be explained on the basis of organic
evolution. A particular species, after arising
from one place, migrates to other far off
places. While doing so, it has to come across
various climatic changes for which it tries to
modify it self. In this way, the new species
are added. After its dispersal, if some
barriers arise, the species becomes confined
to that particular region and is isolated from
the parent species. This provides a very cThe biogeographical realms of the world
interesting theory on the mechanism of
organic evolution.
Charles Darwin during his voyage
around the world studied the fauna and
flora of Galapagos islands (off the west coast
of south America). Galapagos islands are
called 'a living laboratory of evolution'. Darwin studies
20 related varieties of the bird belonging to family
Geospizidae different in shape and size of beak. These
birds are now called 'Darwin's finches'.
(viii) Evidences from Genetics : Johan Gregor
Mendel in 1866 published his work on experimental
breeding. He bred two individuals differing in certain well-
defined characters, and observed the ratio in which various
contrasting parental characters appeared in successive
generations. Since then, selective breeding has started in the
case of domestic animals and plants to obtain the evidences of
organic evolution. These furnish the direct evidences of
evolution.
Conclusion : According to the above description,
we have seen certain evidences in favour of organic
evolution. Although the evidences are indirect, merely
interpretations based on certain phenomenon, certain
organs, systems and other facts but they throw enough
light to show us that the present day forms of life have
originated from simpler forms in millions and millions of
years. The process was continuous gradual, and
accompanied by trial and error.
Recapitulation theory is given by E. Haeckel.
(a) Atavism : Atavism or reversion is the sudden reappearance of some ancestral features. Such as Large
canines, thick body hairs, short temporary tails.
(b) Protective resemblance :
(1) A close resemblance with environment for defence and offence is found in many insects.
(2) Some organisms resemble other organisms and thus escape from enemies. This phenomenon is known as
'mimicry'.
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(3) Phyllium is the leaf-insect because it resembles leaves. It is an example of mimicry or protective
colouration.
(4) Peppered moth, Biston betularia, is found abundantly in industrial cities of Britain.
(5) The change of the lighter coloured variety of peppered moth, Biston betularia, to its darker variety
(carbonaria) is due to mutation of a single Mendelian gene for survival in smoke-laden industrial environment.
(6) Phenomenon of "Industrial melanism" demonstrates natural selection.
(7) Biston betularia shows industrial melanism and it is the example of 'evolution taking place before the eyes'.
(c) Connecting links : Intermediate or intergrading forms between two groups of organisms :
Organism Connecting link between
1. VirusesTheories of organic evolution. Living and nonliving
2. Euglena (Protozoa) Plants and animals
3. Proterospongia (Protozoa)(i) Lamarckism : Lamarck (1744 –1829) was one of the most brilliant stars on the horizon of the history ofProtozoa and Porifera
4. Peripatus (Arthropoda)evolution. Though he was a man of great intellect yet he had to cut a sorry figure because of poverty, and secondly,Annelida and Arthropoda
5. Neopilina (Mollusca)as he could not get the approval of the famous and influential Cuvier. His work was recognized only few years afterAnnelida and Mollusca
6. Balanoglossus (Chordata)his death. Nonchordata and Chordata
7. Dipnoi (Lungfish) Pisces and Amphibia
8. Archaeopteryx (Aves)He was the first naturalist to put forward a general theory of evolution in his famous book. PhilosophicReptiles and Birds
9. Prototheria (Mammalia)Zoologique published in 1809. His evolutionary theory may be summarised in the form of following laws:Reptiles and Mammals
(a) The internal forces of life tend to increase the size of an organism.
c(b) The necessity in animals to produce new structures.
(c) The effect of use and disuse.
(d) Inheritance of acquired characters.
(a) The internal forces of life tend to increase the size of an organism : Lamarck believed that there
is some kind of internal force which is constantly working in living beings. Its tries to make the animal bigger in size.
(b) The necessity in animals to produce new structures : According to this law, each organ and
structure is the product of some continuous necessity in the animals, to develop it. He believed that it was not due
to the direct influence of the environment, but acted through the nervous system, the process being very slow. The
plants, he said, were directly influenced by their environmental conditions.
(c) The effect of use and disuse : Lamarck thought that the continuous use of a particular organ or
structure tends to increase its size and its development. On the other hand, disuse results in disappearance of that
structure, the process being slow and gradual. He supported this by a number of well-known examples. The long
neck of giraffe was the result of continuous stretching to obtain food from the trees (long neck was acquired due to
excessive use). The wings of certain birds Ratitae were not used for long, as there were no formidable enemies and
food was available in abundance. So they did not fly and consequently lost the power of flight. Their wings became
rudimentary (flightless condition acquired due to the disuse of wings).
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(d) Inheritance of acquired characters : Lamarcks stated that all the characters, which are acquired in
one's own lifetime are inherited by offsprings.
Cirticism : The greatest draw-back in the Lamarck's work was that it was too theoretical and there were no
proofs to support it. His ideas were theoretically sound but practically they had no standing. He met a severe
criticism from various workers –
(1) The most serious blow came from Weisman who put his theory of continuity of germplasm which states
that the inheritance is the sole concern of germ cells. Characters introduced in the germ cells will only be inherited
and not those which are present in somatic cells.
(2) If the acquired characters were to be inherited, as Lamarck said, the world would have been full of
cripples, blinds and deformed persons, as most of these characters are acquired.
(3) Some workers have practically proved that mutilations are not inherited by offspring’s even if practiced for
generations.
Lamarck's second and third laws show much truth in them; but they are not the sole cause of evolution. The
laws of heredity of Weisman are practically opposed to the fourth law of Lamarck. Nevertheless, there came a
number of workers who supported Lamarck and modified his laws. They were known as Neo-Lamarckian and the
names of a few of them are : Cope, Haeckel, German savant, Gadow and Spencer.
(ii) Germplasm theory was given by weisman : Later on, Weisman also admitted that the germplasm
may become modified to a certain extent by some environmental factors.
There is always going on a struggle for existence among various agencies. The struggle for existence may be–
(a) Intra-specific : When two different groups of animal species are opposed to each other, i.e., lion and
deer or birds and insects.
(b) Inter- specific : When there is struggle between the members of the same species, as all of them have
same wants and requirements.
(c) Environmental : It is the struggle between the animals and their environment (i.e., climate, vegetation,
open spaces, jungles and water, etc.,) A change in climate may affect adversely, resulting in the extinction of some
cand the survival of others.
Key point to lamarck's view about organic evolution is that every offspring inherits characters acquired by
the parental generation.
Law of “Inheritance of acquired characters” is presented by Lamarck.
Lamarck’s theory was most severely criticized by curvier who greeted it by calling it “Nouvelle Folie.”
Difference between Lamarckism and Neo–Lamarckism
Lamarckism Neo–Lamarckism
It is original theory by Lamarck. It is a modification of the original theory of Lamarck in order to
make it more suitable to modern knowledge.
The theory lays stress on internal force, appetency and use and
disuse of organs. Neo–Lamarckism does not give any importance to these factors.
It believes that changes in environment brings about a conscious
reaction in animals. The theory stresses on the direct effect of changed environment
According to Lamarckism the acquired characters passes on to on the organisms.
the next generation.
Normally only those modification are transferred to next
generation which influence germ cells or where somatic cells give
rise to germ cells.
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(iii) Darwinism : Charles Robert Darwin was undoubtedly the first naturalist who put the idea of organic
evolution on sound footing. His statements and theories were based upon practical experiences and large number
of proofs which he collected directly from the nature. He devoted his whole life for the purpose of finding out proofs
in support of the theory of organic evolution.
Charles Darwin wrote Orgin of species.
Darwin was appointed up on a world survey ship of British government H.M.S. Beagle.
His main ideas about the evolution are given below –
(a) Over – production of offsprings (b) Limited supply of food and shelter (c) Struggle for existence :
(1) Intra –specific (2) Inter –specific (3) Environment
(d) Survival of the fittest : During the process of struggle for existence, some individuals perish; because
cthey could not adapt themselves accordingly, and others survive. Those which survive always show some changes
(d) Survival of the fittest (e) Universal occurrence of variations (f) Inheritance (g) Natural selection
The theory natural selection is given by Darwin.
(a) Over-production of offsprings : The power of reproduction is enormous in the living beings. The single
Paramecium (Protozoa) divided about 600 times in a years. If all the progeny survive their total amount would
exceed that of earth in a few months. Again if all the eggs of a lobster were to produce young ones, in about ten
years time the sea would be full of lobsters and there will be hardly any space for other animals. Darwin gave one
more interesting example of elephants. An elephant lives for about a hundred years and starts reproducing at the
age of thirty. If each female produce six young ones, in 750 years, about 1,90,00,000 would be alive.
(b) Limited supply of food and shelter : The amount of food and shelter is limited in a particular area. It
is sufficient only for a definite number of individuals.
(c) Struggle for existence : It is a common experience, that even with the enormous rate of reproduction,
the number of species mostly remains stationary. The nature has provided a number of checks over their
population. There is limited food, breeding places, shelter, presence of predators and parasites, etc.
and departures from their original pattern. Darwin attributed this cause of the origin of species and said that in this
way new species were formed which were most fit to live.
(e) Universal occurrence of variations : The “departures form the original pattern” and changes in
animals were termed as variations. Darwin said that these variations occur universally in all the living beings, no
matter how closely they are related with each other. Variation is perhaps the most fundamental evolutionary factor.
(f) Inheritance : The useful variations are inherited by the progeny. For this, Darwin used the term
‘pangenesis’
“Theory of pangenesis” is given by Darwin.
(g) Natural selection : Darwin used the term ‘natural selection’ for all the forces (namely, over-production
of offsprings, struggle for existence, survival of the fittest, variation and inheritance) when working together.
Most of the biologists agree with the Darwin's theory as the best explanation of organic evolution. But there are a
number of objections to this theory.
By performing the replica plating experiment, lederberg supported the “Natural Selection Theory”.
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Evolution
Objections
(i) Darwin's theory does not explain that the effects of ‘use and disuse’ of organs are inherited.
(ii) He considered the minute fluctuating variations as the cause of natural selection, but most of the variations
are non-heritable.
(iii) He did not distinguish between germinal and somatic variations. His theory of Pangenesis has no basic at
all.
(iv) He believed that variations occur in all directions haphazardly. But now it is established that the variations
occur only on definite lines of change.
(v) Darwin considered mutation too infrequent for the necessary basis of selection.
(vi) Darwin's theory explains survival of the fittest and arrival of the fittest.
(vii) It does not explain how natural selection could make use of certain adaptive characters in their initial
stages, i.e., what would be the use of electric organs, electric fishes, until they have enough of power to produce a
shock.
(viii) It does not explain the over-specialised and vestigeal organs.
(ix) According to him, only useful characters are inheritable but on the contrary certain useless and non-
adaptive characters are also passed on.
(x) Geologists and astronomers think that the time required producing organic world is much more than the
actual age of the earth today.
In the light of these criticism and objections various workers after him modified his theory. Most of the work
was done after the rediscovery of Mendel's work in 1900. This modified theory of Darwin is known as Neo-
Darwinism.
(1) Examples of Natural selection :
c(a) Industrial melanism : Industrial melanism is a phenomenon where the moths living in the industrial
areas, develop black colour (melanin pigments) to match the body to soot-covered background, on the bark of
trees.
The industrial melanism is observed and worked out by a number of evolutionists like Fisher, Ford and
Kettlewell.
Industrial melanism was observed in a peppered moth Biston betularia living, Manchester, in an industrial city
of Great Britain.
The peppered moths exist in two forms, namely melanic forms and non-melanic forms. The melanic forms are
black in colour because they contain melanin pigments. The melanic forms are also called carbonaria. The non-
melanic forms are light coloured. The light colour is due to the absence of melanin pigments.
(b) Resistance to DDT : In agriculture, insect pests are controlled by the spraying of insecticides like DDT.
However, DDT cannot bring about 100% deaths in any species of insects. A certain number of individuals survive.
These surviving individuals are resistant to DDT. The resistance is a character controlled by genes. The resistant flies
arise as a result of the application of DDT. They reproduce more and more resistant flies. Thus the resistant
populations are evolved as a selective advantage against insecticides.
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Evolution
(c) Resistance of bacterium to drugs : L.L. Cavalli and G.A. Maccacro (1952) experimentally proved
that the colon bacteria Escherichia coli develop resistance to the antibiotic chloramphenicol 250 times as great as
that tolerated by normal bacteria by exposing the bacteria to increased concentration of the drug.
Difference between Darwinism and Neo–Darwinism
Darwinism (Natural Selection) Neo–Darwinism
1. It is the original theory given by Charles Darwin (1859) to 1. Neo–Darwin is a modification of the original theory of
explain the origin of new species. Darwin to remove its short–comings.
2. According to this theory accumulation of continuous 2. Instead of continuos variations, mutations are believed to
variations causes changes in individuals to form new help form new species.
species.
(iv) Mutation theory :
(a) Hugo de Vries (1901), a Dutch Botanist, pioneered the theory of mutations to explain the mechanism of3. It believes in the selection of individuals on the basis of 3.Variations accumulate in the gene pool and not in the
evolution.accumulation of variation. individuals.
(b) The plant on which de Vries had experimented was Oenothera lamarckiana (Evening primrose).4. Darwinism does not believe in isolation.4.Neo–Darwinism incorporates isolation as an essential
component of evolution.
(c) The mutation observed by de Vries in Oenothera essentially was chromosomal number variant.5. It can explain the origin of new characters.5.
The theory can explain the occurrence of unchanged
c(d) Mutations are discontinuous variations, called 'sports' by Darwin and 'saltatory variations' by Bateson.6. Darwinism cannot explain the persistence of certain forms 6.forms over millions of years.
in the unchanged condition.
Normally only those modification are transferred to next
generation which influence germ cells or where somatic
cells give rise to germ cells.
(e) Mutations are generally harmful and recessive.
(f) Role of mutations in evolution is genetic variations.
(g) Mutations are due to changes in chromosomes, genes or DNA.
(h) To be a successful event for evolution, a mutation must occur in germplasm DNA.
(i) Mutations are the changes which may or may not be inherited.
(j) Germinal mutation is a change that is inherited.
(k) Hereditary variations in plants have been produced by use of X-rays.
(l) Frequency of a mutated gene in a population is expected to increase if that gene is selected by nature.
(m) The possibilities of hereditary and evolutionary changes are greater in species that reproduce by sexual
means.
(n) Organic evolution would not have taken place if individuals in a population did not show gene variations.
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Difference Amongst Lamarckism, Darwinism and Mutation Theory
Lamarckism Darwinism Mutation Theory
1. Vital force The theory believes that every Darwinism does not believe in No internal vital force is
organism has an internal vital internal vital force. involved.
force that tends to increase its size
upto a certain limit.
2. Conscious Animals with well developed nervous Darwinism does not involve any No conscious reaction is
Reaction system react consciously to any conscious reaction. believed to take part in the
change in environments process of evolution.
3. Appentency The theory considers appentency It is not a constituent of the Appentency in not involved.
cdesire r use and disuse of organs.or desires on the part of animalstheory.
an important force in the
development of modifications.
4. Use and Disuse The organs put to more use are The theory is silent about use The theory is silent about it.
believed to develop more while and disuse of organs.
organs not used begin to
degenerate.
5. Inheritance of The characters acquired by an According to Darwin, all the Only those variations are
Acquired
Characters organism during its life are living cells produce minute transferred to the offspring
believed to get transferred the particles or pangenesis, which which originate in germ cells
next generation. pass into germ cells for or in the cells which form germ
transmission to the offspring. cells.
6. Struggle for The theory does not clearly spell Organisms produce more The theory believes in the
Existence out struggle for existence in offspring than the available food struggle for existence.
relation to high biotic potential. and space so that a struggle for
existence ensues amongst them.
7. Origin of Variations appear in organisms in Variations appear automatically. Variations appear due to
Variations response to change in change in genetic make up.
environment, conscious reaction,
8. Continuous The theory is silent about them It is based on the origin and The theory is based on
Variations though it believes in a continuous selection of continuous discontinuous variations or
modification of organs in a variations. mutations.
particular direction.
9. Natural The theory does not take into Darwinism is based on natural Mutations theory believes in
Selection account natural selection or selection or survival of the fittest. natural selection or survival of
survival of the fittest. the fittest.
10. Progress of Evolution is a continuous process Evolution is a continuous Evolution is a jerky process,
Evolution
which moves in a direction process, the direction of which is the direction of which is
governed by environment and governed by nature. unpredictable though
appentency. ultimately it is governed by
nature.
(v) Variations : Dissimilarities between members of the same species are called variations. Tendency to differ
helps organisms in their adaptations for different environmental conditions and these variations are inherited from
one generation to another. Heritable variations are responsible for changes in a species to form new species.
Variations are thus important in evolution.
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Evolution
Variations are progressive factors in evolution. Members of the same species exhibit variations by structural,
physiological or psychological dissimilarities.
(a) Types of variations : Variations are classified in three sets.
(1) Germinal and somatic variations : Germinal variations arise in germplasm of the organism. They
occur in the gene pattern and are inheritable. These variation reach the zygote through gametes and hence are
inherited from generation to generation. For example colour of eyes or hair occur since birth whereas characters of
height and body built develop later in life. Germinal variations are also called blastogenic.
Somatic variations are produced due to environmental factors. They develop in somatic cells or
somatoplasm and are also called acquired variations. These are non-heritable. Darkness of skin due to working in
the sun, development of intelligence by better education, achievements of a musician, muscular body of an athlete,
bored nose and pinna in ladies, are some examples of acquired variations. Somatic or acquired variations are not
important in evolution.
(2) Continuous and discontinuous variations : Continuous variations are small and graded variations
which are found in the members of same species. Darwin called them as fluctuating variations and realised their
importance in evolution. Darwin considered all fluctuating variations inheritable from one generation to other to
form new species. They were considered important factor for natural selection. For example children of the same
class and age show continuous variations in their height and intelligence but they will be much different from
children of other classes.
Discontinuous variations arise suddenly and they are distinctly visible in the group. These ungraded
variations deviate so much from the average character that they are seen in new form among members of the same
species. There are no grades or intermediate stages in such variations. Darwin called these variations as sports while
Hugo de Vries called these as mutations. These discontinuous variations are stable and inheritable. Polydactyly or
more than five digits in hands or feet in man, occurrence of four horns instead of two in goat etc. are mutations.
(3) Determinate and Indeterminate variations : Determinate variations arise in a definite direction and
time and are due to adaptations. These are found continuously and progressively in organic evolution and are
affected by strong gene combination from generation to generation. Leaf eating moth Diabrotica soror is a good
example of this variation. In this moth many different colours are found other than main colour.
cIndeterminate variations do not arise under any special condition and can arise suddenly in any direction.
These can develop to any extent.
(b) Causes of variations :
(1) Environmental conditions (2) Inherent tendency to vary
(3) Dual parentage (4) Nuclear reorganization
(5) Change in the gene pattern
(vi) Adaptations : All living species in the world have special character to adapt themselves according to their
environment. If species are unable to adapt, it will be difficult for them to live in new environment and gradually
they will perish.
(a) Definitions of adaptations : Somatic or structural and physiological changes in an organism,
developed according to environmental conditions so that the organism is better fitted in its new environment, are
called adaptations.
(1) Post-adaptation : Adaptation in a species to the environmental conditions in which it is already living is
called post adaptation. Such adaptations cause perfection because a particular species already living in the
environment gradually becomes more and more adapted.
Chapter 19 501
Evolution
(2) Pre-adaptation : Some species living in a particular environment develop adaptations for new
environmental conditions and later migrate to the new environment. Such adaptation is called pre-adaptation.
Simpson (1953) called it prospective adaptation.
(b) Types of adaptations : Adaptation can be of following types according to the habit and habitat of the
organisms :
(1) Cursorial adaptation : Animals which run fast on ground develop such adaptations like in horse and
antelope.
(2) Fossorial adaptation : These animals live under ground and they dig burrows like rats and insectivores
etc.
(3) Aquatic adaptation : Animals that live in the water as fishes, whale, siren etc.
(4) Scansorial adaptation : These animals are adapted for climbing on walls, trees, hills etc. like reptiles
and some birds.
(5) Volant or aerial adaptations : Animals that fly are mainly birds. Some animals like bats are also
adapted for flying in air.
(6) Cave adaptations : Some animals live in caves like salamander and some species of fishes.
(7) Deep sea adaptation : Some species of fishes live in deep sea and are adapted accordingly.
(8) Desert adaptation : Some animals are adapted to living in dry climate of deserts like camel and
kangaroo etc.
(c) Adaptive radiation : When different species evolve from one common ancestor and develop different
modifications in their body according to their environmental conditions, it is called adaptive radiation. They exhibit
cadaptive divergence or radiant evolution. The concept of adaptive divergence was given by Osborn.
Chapter 19 502
Evolution
(vii) Synthetic theory :
(a) Dobzhansky (1937) in his book ‘Genetics and Origin of Species' provided the initial basis of synthetic
theory.
(b) ‘Modern synthetic theory of evolution' was designated by Huxley in 1942.
(c) Some of the important workers who have contributed to the modern synthetic theory are : Th.
Dobzhansky, R.A. Fisher, J.B.S. Haldane, Sewall Wright, Ernst Mayr and G.L. Stebbins.
(d) According to synthetic theory there are five basic factors involved in the process of organic evolution.
These are :
(1) Gene mutations,
(2) Changes in chromosome structure and number,
(3) Genetic recombinations,
(4) Natural selection and
(5) Reproductive isolation.
(e) While the first three factors are responsible for providing genetic variability, the last two are responsible for
giving direction to the evolutionary processes.
(f) Besides the five factors outlined above, there are two accessory processes, namely migration of individuals
from one population to another and hybridization between races, species and even related genera, which contribute
to the evolution.
(g) The most accepted and recent theory of organic evolution is the synthetic theory.
(viii) Hardy-weinberg equilibrium :
(a) Mutations introduce new genes into a species resulting a change in gene frequencies.
(b) G.H. Hardy, an English mathematician, and Wilhelm Weinberg, a German physician, in 1908
established a simple mathematical relationship to the study of gene frequencies.
c(c) If certain conditions existed, gene frequencies would remain constant.
(d) The conditions necessary for gene frequencies to remain constant are :
(1) Mating must be completely random.
(2) Mutations must not occur.
(3) Migrations of individual organisms into and out of the population must not occur.
(4) The population must be very large.
(5) All genes must have an equal chance of being passed to the next generation.
(e) According to Hardy-Weinberg concept, the gene frequencies will remain constant if all above five
conditions are met.
(f) The distribution of genotypes could be described by the relationship A2 + 2Aa + a2 = 1, where A2
represents the frequency of the homozygous dominant genotype, 2Aa represents the frequency of the heterozygous
recessive genotype and a2 represents the frequency of the homozygous recessive genotype.
(g) Constant gene frequencies over several generations indicate that natural selection and evolution are not
taking place.
(h) Changing gene frequencies would indicate that evolution is in progress.
Chapter 19 503
Evolution
Human Evolution.
During the course of evolution different animal species evolved special organs for a successful life like wings in
insects, birds and bats for flying, claws for holding, burrowing and climbing in rats and squirrels and fins or paddles
in aquatic life for swimming. Similarly, most significant event in human evolution has been evolution of brain which
enabled him to become most superior member of animal kingdom. The large and more complex brain evolved
greater ability of thinking, logical power and capacity of taking decisions depending on the situation.
Present human species is named as Homo sapiens sapiens (Sapient = wise). T. H. Huxley (1863) in his
book ‘Man's Place in Nature’ made first attempt to explain scientific grounds of upbringing of man. Charles
Darwin (1871) in his book ‘The Descent of Man’ gave his ideas about ancestry of man.
(i) Systematic position of man in animal kingdom :
Phylum Chordata
Subphylum Vertebrata
Class Mammalia
Order Primates
Suborder Anthropoidea
Superfamily Hominoidea
Family Hominidae
Genus Homo
Species sapiens
(ii) Place of descent of man : Available fossils give the evidence that most of the fossils of prehuman
ancestors including monkeys and apes have been recovered from Africa, Asia and Europe. However, descent of
man is supposed to have occurred in Asia due to following reasons – (i) Civilization of Asia is oldest. (ii) Asia is the
land of origin of all domesticated animals and crop plants. (iii) Migration of many animal species has taken place in
Asia. (iv) Fossils of many ancestors of man have been found in Java and China which are part of Asia. (v) Climate
of Asia was favourable at that time for the evolution of man.
(iii) Time of descent of man : Although time of descent of man is a controversial subject but fossil evidence
indicates that time of evolution of man is from Miocene epoch to the beginning of Pliocene epoch of Tertiary period
cin Coenozoic era. It can be estimated that ancestral man evolved from man like apes about 1 crore 32 lakh years
ago.
(iv) Evolutionary characteristics of man : The modern man possesses following special features, which
have been acquired during the course of evolution–
I. Bipedal locomotion. II. Large brain and cranial cavity.
III. Grasping hands and feet. IV. Erect posture.
V. Stereoscopic (binocular) vision. VI. Sensitivity
VII. Social organisation. VIII. Expression by speech.
Morphological changes that have occurred in man during the process of evolution are :
I. Increase in brain size and intelligence. II. Attainment of erect posture.
III. Flattening of face. IV. Shortening of body hair and reduction in their number.
V. Elevation and narrowing of nose. VI. Increase in height.
VII. Reduction of brow ridges. VIII. Rounding and enlargement of cranium.
IX. The bowl like form of pelvic girdle and broad ilia to support the viscera.
X. Formation of chin.
Chapter 19 504
Evolution
The vertebrates of class mammalia as well as other primates have common ancestory with monkeys and apes.
Mammals evolved from primitive reptiles in early Jurassic period (about 210 million years ago).
Humans belong to family hominidae in which Homo sapiens is the only living species. The evolutionary history of
man has been built up on the basis of study of fossils and molecular homology.
The earliest hominid stock included fossils of Ramapithecus and Sivapithecus unearthed from Africa and Asia.
Several species belonging to genus Homo can be recognised from fossil record. Human evolution took place in
Africa and Asia.
A common ancestory for great apes and man has been deduced on the basis of similarities in DNA content,
chromosome number and banding pattern of chromosomes.
(i) Early human ancestors : The fossils Ramapithecus and Sivapithecus which lived in Africa and Asia
(about 1015 million years ago) are believed to be the forerunners of Hominids. These were first man–like primates.
The first fossil of Ramapithecus was a fragment of upper jaw recovered from the Shivalik Hills of India.
Ramapithecus and Sivapithecus must have a sort face, small brain case, thickly enameled large teeth and they must
have been used to walk on their knuckles.
(ii) Australopithecus (the first man–ape)– Its fossils were described by Raymond Adart in 1925 from
South Africa. These were intermediate between Ramapithecines and genus Homo.
Australopithecines are considered to be ancestral to all hominids of genus Homo.
Australopithecines must have been small statured averaging about four feet. They walked nearly or completely
straight. The vertebral column had a distinct lumbar curve with pelvis broad and basin–like. The teeth were larger
than those of modern man. though jaws and teeth were larger than those of modern man. Their face was
prognathous and a chin was absent. The bulge of occipital region was small. Eyebrow ridges projected over the
eyes.
Their brain capacity ranged from 450–600 ml i.e., slightly larger than that of modern adult Chimpanzee.
Thus, Australopithecine’s represented man with an ape–brain.
• Ramapithacus has been known from shivalik hills in India.
Australopithecus stood erect.
G. E. Lewis discovered fossils of Ramapithacus.
c Australopithacus have 500 c.c. cranial capacity.
(iii) Homo erectus (The forerunner of Modern Humans) : In the Middle Pleistocene period,
Australopithecines were succeeded by large brained form which were described under the name Pithecanthropus or
Java man. Its first fossils were obtained by Dubois (1891). These were named Pithecanthropus erectus (erect ape–
man). Similar fossils were found in a cave near Peking, China, and were named Sinanthropus pekinenis.
Mayer (1950) has replaced these names by Homo erectus–
(a) Java Man (Homo erectus = Pithecanthropus erectus) : Its fossils occurred in the Pleistocene
deposits about 500,000 years ago. Its cranial cavity was about 940 c.c; (intermediate between that of
Australopithecus (600–700 c.c.) and modern man (1400–1600 c.c.) It was more than five feet tall with skeleton
much like ours. Its forehead was low and slanting. The face was prognathous, and jaws were massive with huge
teeth. The chin was absent and bony eye. He might have learnt the use and construction of tools and knew how to
lit fire.
Fire was first used for protection and cooking by Java man.
(b) Peking man (Homo erectus pekinensis = Pithecanthropus pekinensis–Sinanthropus
pekinensis) : These perhaps lived 500,000–2,00,000 years ago. It was very similar to Java man with heavy bony
eyebrow ridges, low slanting forehead and chinless face. However, their cranial cavity was much larger as compared
to Java man ranging from 850–1200 ml. and averaging 1075 c.c.
Chapter 19 505
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