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Evolutionary Biology is the study of history of life forms
on earth. What exactly is evolution? To understand the
changes in flora and fauna that have occurred over millions
of years on earth, we must have an understanding of the
context of origin of life,
i.e., evolution of earth, of stars and
indeed of the universe itself. What follows is the longest of
all the construed and conjectured stories. This is the story
of origin of life and evolution of life forms or biodiversity on
planet earth in the context of evolution of earth and against
the background of evolution of universe itself.
7.1 ORIGIN OF LIFE
When we look at stars on a clear night sky we are, in a
way, looking back in time. Stellar distances are measured
in light years. What we see today is an object whose emitted
light started its journey millions of year back and from
trillions of kilometres away and reaching our eyes now.
However, when we see objects in our immediate
surroundings we see them instantly and hence in the
present time. Therefore, when we see stars we apparently
are peeping into the past.
The origin of life is considered a unique event in the
history of universe. The universe is vast. Relatively speaking
CHAPTER 7
EVOLUTION
7.1 Origin of Life
7.2 Evolution of Life Forms - A
Theory
7.3 What are the Evidences
for Evolution?
7.4 What is Adaptive
Radiation?
7.5 Biological Evolution
7.6 Mechanism of Evolution
7.7 Hardy - Weinberg
Principle
7.8 A Brief Account of
Evolution
7.9 Origin and Evolution of
Man
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the earth itself is almost only a speck. The universe is very
old
– almost 20 billion years old. Huge clusters of galaxies comprise the
universe. Galaxies contain stars and clouds of gas and dust. Considering
the size of universe, earth is indeed a speck. The Big Bang theory attempts
to explain to us the origin of universe. It talks of a singular huge explosion
unimaginable in physical terms. The universe expanded and hence, the
temperature came down. Hydrogen and Helium formed sometime later.
The gases condensed under gravitation and formed the galaxies of the
present day universe. In the solar system of the milky way galaxy, earth
was supposed to have been formed about 4.5 billion years back. There
was no atmosphere on early earth. Water vapour, methane, carbondioxide
and ammonia released from molten mass covered the surface. The UV rays
from the sun brokeup water into Hydrogen and Oxygen and the lighter H
2
escaped. Oxygen combined with ammonia and methane to form water,
CO
2
and others. The ozone layer was formed. As it cooled, the water vapor
fell as rain, to fill all the depressions and form oceans. Life appeared 500
million years after the formation of earth, i.e., almost four billion years back.
Did life come from outerspace? Some scientists believe that it came
from outside. Early Greek thinkers thought units of life called spores
were transferred to different planets including earth. ‘Panspermia’ is still
a favourite idea for some astronomers. For a long time it was also believed
that life came out of decaying and rotting matter like straw, mud, etc.
This was the theory of spontaneous generation. Louis Pasteur by careful
experimentation demonstrated that life comes only from pre-existing life.
He showed that in pre-sterilised flasks, life did not come from killed yeast
while in another flask open to air, new living organisms arose from ‘killed
yeast’. Spontaneous generation theory was dismissed once and for all.
However, this did not answer how the first life form came on earth.
Oparin of Russia and Haldane of England proposed that the first form
of life could have come from pre-existing non-living organic molecules
(e.g. RNA, protein, etc.) and that formation of life was preceded by chemical
evolution, i.e., formation of diverse organic molecules from inorganic
constituents. The conditions on earth were – high temperature, volcanic
storms, reducing atmosphere containing CH
4
, NH
3
, etc. In 1953, S.L. Miller,
an American scientist created similar conditions in a laboratory scale
(Figure 7.1). He created electric discharge in a closed flask containing
CH
4
, H
2
, NH
3
and water vapour at 800
0
C. He observed formation of amino
acids. In similar experiments others observed, formation of sugars,
nitrogen bases, pigment and fats. Analysis of meteorite content also
revealed similar compounds indicating that similar processes are
occurring elsewhere in space. With this limited evidence, the first part of
the conjectured story, i.e., chemical evolution was more or less accepted.
We have no idea about how the first self replicating metabolic capsule
of life arose. The first non-cellular forms of life could have originated
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BIOLOGY
3 billion years back. They would have been giant molecules (RNA, Protein,
Polysaccharides, etc.). These capsules reproduced their molecules perhaps.
The first cellular form of life did not possibly originate till about 2000
million years ago. These were probably single-cells. All life forms were in
water environment only. This version of a biogenesis, i.e., the first form of
life arose slowly through evolutionary forces from non-living molecules is
accepted by majority. However, once formed, how the first cellular forms
of life could have evolved into the complex biodiversity of today is the
fascinating story that will be discussed below.
7.2 EVOLUTION
OF LIFE FORMS – A THEORY
Conventional religious literature tells us about the theory of special
creation. This theory has three connotations. One, that all living organisms
(species or types) that we see today were created as such. Two, that the
diversity was always the same since creation and will be the same in future
also. Three, that earth is about 4000 years old. All these ideas were
strongly challenged during the nineteenth century. Based on observations
made during a sea voyage in a sail ship called H.M.S. Beagle round the
world, Charles Darwin concluded that existing living forms share
similarities to varying degrees not only among themselves but also with
life forms that existed millions of years ago. Many such life forms do not
exist any more. There had been extinctions of different life forms in the
Figure 7.1 Diagrammatic representation of Miller’s
experiment
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