ORIGIN OF LIFE
Big Bang Theory states that universe originated
about 20 billion years ago, by a thermonuclear explosion (big bang) of a dense
entity.
The
earth was formed about 4.5 billion years ago.
There was no atmosphere on early earth. Water vapour, CH4,
CO2 & NH3 released from molten mass covered the
surface.
H2O → H2
+ O2
NH3
+ O2 → water
CH4
+ O2 → CO2
Then
the ozone layer was formed. As it cooled, the water vapour fell as rain to form
oceans.
Theories of origin of life
1.
Theory of spontaneous generation (Abiogenesis): It
states that, life came out of decaying and rotting matter like straw, mud etc.
Louis Pasteur demonstrated that life comes only
from pre-existing life and disproved this theory.
He showed that in pre-sterilized flasks, life did not come from
killed yeast while in another flask open to air, new living organisms arose.
2.
Biogenesis: Proposed by Francisco Redi, Spallanzani & Louis Pasteur. It states that, life originates from pre-existing
life.
3.
Cosmic theory (Theory of Panspermia): It
states that, the units of life (spores) were transferred to different planets
including earth.
4.
Theory of special creation: It states that,
living & non-living was created by some supernatural power (God).
5. Theory of chemical evolution: Proposed by Oparin & Haldane. It states that, the
first form of life was originated from non-living inorganic & organic
molecules such as CH4, NH3, H2O, sugars,
proteins, nucleic acids etc. i.e. “Abiogenesis
first, but biogenesis ever since”.
Urey-Miller experiment
Harold
Urey & Stanley Miller conducted an experiment to prove
theory of chemical evolution. They created a condition similar to that of
primitive earth (i.e. high temperature, volcanic storms, reducing atmosphere
containing CH4, NH3, H2O, H2 etc).
They
made electric discharge in a closed flask containing CH4, NH3,
H2 and water vapour at 800o C. As a result, some amino
acids are formed.
In
similar experiments, others observed formation of sugars, nitrogen bases,
pigment and fats.
First non-cellular form of life originated 3 billion years ago.
They were RNA,
proteins, Polysaccharides etc.
EVIDENCES FOR
EVOLUTION
1.
Paleontological evidences
Paleontology: The study of fossils.
Fossils: These are remnants of life forms found in rocks (earth crust). Fossils are written documents of evolution.
Significance of fossils:
a.
To study phylogeny
(evolutionary history or race history). E.g. Horse evolution.
b.
To study the connecting
link between two groups of organisms. E.g. Archaeopteryx.
c.
To study about extinct
animals. E.g. Dinosaurs
d.
To study about geological
period by analyzing fossils in different sedimentary rock layers.
The study showed that life forms varied over time and certain life forms are
restricted to certain geological time spans.
2. Morphological and Anatomical evidences
Comparative
anatomy and morphology shows that different forms of animals have some common
structural features. This can be explained as follows:
a.
Homologous organs and Homology
-
Homologous organs are the organs
having fundamental similarity in structure and origin but different functions. This phenomenon is called Homology.
-
E.g. Human
hand, Whale’s flippers, Bat’s wing, and Cheetah’s foot. All these perform
different functions, but are constructed on the same plan.
-
Homology can be
seen in skeleton (e.g. humerus, radius, ulna, carpals, metacarpals &
phalanges), heart, blood vessels, excretory system, brain etc.
-
Homology in plants: E.g. Thorns of
Bougainvillea and tendrils of Cucurbita.
-
The origin of
homologous organs is due to Divergent evolution (It is the
process by which related species
become less similar in order to survive
and adapt in different environmental
condition).
-
Homology
indicates common ancestry.
b.
Analogous organs and Analogy
Analogous
organs are the organs having similar function but different structure and
origin. This phenomenon is called Analogy.
E.g.
§ Wings of
insects (formed of a thin flap of chitin) and wings of birds (modified forelimbs).
§ Eyes of Octopus
(retina from skin) and mammals (retina
from embryonic brain).
§ Flipper of
Penguins and Dolphins.
§ Sweet potato (modified
root) & Potato (modified stem).
§ Trachea of
insects (from ectoderm) and lungs
of vertebrates (from endoderm).
Origin
of analogous organs is due to Convergent evolution (It is the
process by which unrelated species
become more similar in order to
survive and adapt in similar
environmental condition).
3. Adaptive radiation (Biogeographical evidences)
Adaptive radiation (evolution by adaptation) is the
evolution of closely related species in a given geographical area starting from
a point. E.g.
o Darwin’s finches (seen in Galapagos Islands).
o Australian marsupials.
o Placental mammals in Australia.
Placental
mammals: Mole, Anteater, Mouse, Lemur, Flying squirrel, Bobcat, Wolf.
When more than one adaptive radiation is
appeared in an isolated geographical area, this leads to convergent evolution.
E.g. Australian Marsupials and Placental
mammals.
Placental mammals
|
Australian Marsupials
|
Mole
|
Marsupial
mole
|
Ant eater
|
Numbat (Ant
eater)
|
Mouse
|
Marsupial
mouse
|
Lemur
|
Spotted
cuscus
|
Flying
squirrel
|
Flying
phalanger
|
Bobcat
|
Tasmanian
tiger cat
|
Wolf
|
Tasmanian
wolf
|
4. Biochemical
evidences
-
Similarities in
proteins and genes
- Similarities in other biomolecules and metabolism.
5. Evidences
for evolution by natural selection
Natural
selection is the process by which the organisms that are best suited for their
environment survive and reproduce. Some evidences are given below.
Industrial
melanism (In England)
§
Before industrialization (1850s): There were
more white winged moths (Biston betularia)
on trees than dark winged or melanised moths (Biston carbonaria).
Reason: There was white coloured lichen covered the trees.
In that background the white winged moths survived but the dark coloured moths
were picked out by predators.
§
After industrialization (1920): More dark
winged moths and less white winged moths.
Reason: The
tree trunks became dark due to industrial smoke and soot. No growth of lichens.
Under this condition the white winged moth did not survive because the
predators identified them easily. Dark winged moth survived because of suitable
dark background.
o Excess use of herbicides, pesticides, antibiotics
or drugs
etc resulted in selection of resistant varieties (Natural selection by
anthropogenic action).
THEORIES OF
BIOLOGICAL EVOLUTION
1.
Lamarckism
(Theory of Inheritance of
Acquired
characters)
Proposed
by Lamarck. It states that evolution of life forms occurred by use and disuse
of organs. E.g.
o
Evolution by use of organs: Long neck of giraffe is due to continuous elongation to
forage leaves on tall trees. This acquired character was inherited to succeeding
generations.
o Evolution by disuse: Disappearance
of limbs in snakes.
This theory was eliminated out because it is proved that the
characters are inherited only through genes.
2.
Darwinism
(Theory of Natural selection)
Proposed
by Charles Darwin. It is based on 2
key concepts
·
Branching descent
|
·
Natural selection
|
Branching
descent: It explains that all organisms are
modified descendants of previous life forms.
Natural
selection: Consider a bacterial colony (say A) growing on a given medium. If the medium composition is changed,
only a part of the population (say B)
can survive under new condition. This variant population outgrows the others
and appears as new species, i.e. B is
better than A under new condition. Nature selects for fitness.
Work of Thomas Malthus on
populations influenced Darwin.
Natural
selection is based on the following facts:
· Heritable minor variations
· Overproduction by organisms
· Limited natural resources
· Struggle for existence
· Survival of the fittest
Population size grows exponentially if everybody
reproduces maximally (E.g. bacterial population). In fact, population size is
limited due to competition for resources (Struggle
for existence). Only some survives (Survival
of the fittest).
Darwin said that the organisms with heritable variations make
resource utilization better. They reproduce and leave more progeny. It leads to
a change in population characteristics and new forms appear.
MECHANISM OF
EVOLUTION
Darwin
ignored about origin of variation and mechanism of speciation.
Hugo de Vries
proposed Mutation Theory of
evolution. He conducted some experiments on Oenothera
lamarckiana (evening primrose) and believed that evolution takes place
through mutation and not by minor variation.
Differences between Darwinian variation & mutation
Darwinian variation
|
Mutation
|
Minor variation
|
Large
variation
|
Slow and directional
|
Random, sudden & directionless
|
Gradual evolution
|
Speciation by saltation (single step, large mutation)
|
Hardy- Weinberg Principle
-
It says that
allele frequencies in a population are stable and constant from generation to
generation.
-
The gene pool (total genes and their
alleles in a population) remains a constant. This is called genetic equilibrium (Hardy-Weinberg
equilibrium).
-
Sum total of
all the allelic frequencies = 1
-
E.g. In a
diploid, p and q are the frequencies of alleles A & a
respectively.
The
frequency of AA = p2 (i.e. the probability of an allele A with
frequency p is the product of the probabilities, i.e. p2)
The
frequency of aa = q2
The
frequency of Aa = 2pq
Hence p2 + 2pq + q2
= 1 [binomial expansion of (p+q)2]
Change of frequency of alleles in a
population causes disturbance in genetic equilibrium. This is due to evolution.
Factors affecting Hardy-Weinberg equilibrium
a. Gene migration:
Gene flow from one population to another. Here gene frequencies
change in both populations. There would be a gene flow if migration happens
multiple times.
b. Genetic drift: The accidental gene flow causing change in
frequency. Sometimes, the change in frequency is so different in the new sample
of population that they become a different species. The original drifted
population becomes founders and the effect is called founder effect.
c. Mutation: Mutations result in formation of new phenotypes.
Over few generations, this leads to speciation.
d. Genetic
recombination: It is the reshuffling of gene
combinations during crossing over resulting in genetic variation.
e. Natural
selection: 3 types.
i. Stabilizing selection: Here, more individuals acquire
mean character value and variation is reduced.
ii. Directional
selection: Here, individuals of one extreme are more favoured.
iii. Disruptive
selection:
A BRIEF ACCOUNT OF EVOLUTION
} Proterozoic era: 2000 million years
ago (mya):
◦ First cellular forms of life.
◦ Some of the cells had the ability to
release O2 as the light reaction in photosynthesis.
◦
Single celled organisms → Multicellular organisms
} Paleozoic era:
◦ 500 mya: Invertebrates.
◦ 400-600 mya: First land
organisms (plants).
◦ 400 mya: Arthropods invaded the land.
◦ 350 mya: Jawless fish. Fish with stout and strong
fins could move on land and go back to water.
◦ 320 mya: Sea weeds and few plants.
◦ Amphibians to reptiles. They lay thick-shelled eggs which do not dry up in sun unlike those of
amphibians.
◦ In the next 200 million years reptiles dominated on earth. Giant ferns (Pteridophytes) were
present but they all fell to form coal deposits slowly.
} Mesozoic era:
◦
200 mya: Some of the land
reptiles went back into water to evolve into fish-like reptiles (E.g. Ichthyosaurs).
◦ The land reptiles were dinosaurs. They
include
} Tyrannosaurus rex: Largest dinosaur (20 feet in height, huge fearsome
dagger-like teeth).
}Triceratops
|
}
Pteranodon
|
}Stegosaurus
|
}
Brachiosaurus
|
} Coenozoic era:
◦ 65 mya: Dinosaurs suddenly disappeared
◦ First mammals (shrew-like). Their fossils are small sized.
◦
In South America, there were
mammals resembling horse, hippopotamus, bear, rabbit etc. Due to continental
drift, when South America joined North America, these animals were overridden
by North American fauna.
◦
Due to continental drift, Australian
marsupials survived because of lack of competition from any other mammals.
ORIGIN AND
EVOLUTION OF MAN
·
Dryopithecus &
Ramapithecus (15 mya):
Hairy. Walked like gorillas & chimpanzee.
Dryopithecus: ape-like.
Ramapithecus: man-like.
Fossils of man-like bones found in Ethiopia & Tanzania.
·
Man-like primates (3-4 mya): Height up to 4 feet.
·
Australo-pithecus (2 mya):
In East African grass lands.
Hunted with stone weapons. Ate fruits.
Homo habilis:
First human-like being (hominid).
Brain capacity: 650-800 cc. Did not eat meat.
·
Homo erectus (1.5 mya):
Large brain (900 cc). Ate meat.
·
Neanderthal man (40,000- 1 lakh yrs ago):
Brain 1400 cc.
Lived in East & Central Asia.
Used hides to protect their body.
Buried their dead.
·
Homo sapiens (Modern man): 10,000 to 75,000 yrs ago.
Pre-historic cave art
developed about 18,000 years ago. Agriculture & settlements: 10,000 years
ago.