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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
years gone by just as new forms of life arose at different periods of history
of earth. There has been gradual evolution of life forms. Any population
Figure 7.1 Diagrammatic representation of Miller’s
experiment
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has built in variation in characteristics. Those characteristics which enable
some to survive better in natural conditions (climate, food, physical factors,
etc.) would outbreed others that are less-endowed to survive under such
natural conditions. Another word used is fitness of the individual or
population. The fitness, according to Darwin, refers ultimately and only
to reproductive fitness. Hence, those who are better fit in an environment,
leave more progeny than others. These, therefore, will survive more and
hence are selected by nature. He called it natural selection and implied it
as a mechanism of evolution. Let us also remember that Alfred Wallace, a
naturalist who worked in Malay Archipelago had also come to similar
conclusions around the same time. In due course of time, apparently new
types of organisms are recognisable. All the existing life forms share
similarities and share common ancestors. However, these ancestors were
present at different periods in the history of earth (epochs, periods and
eras). The geological history of earth closely correlates with the biological
history of earth. A common permissible conclusion is that earth is very
old, not thousand of years as was thought earlier but billions of years old.
7.3 WHAT ARE THE EVIDENCES FOR EVOLUTION?
Evidence that evolution of life forms has indeed taken place on earth has
come from many quarters. Fossils are remains of hard parts of
life-forms found in rocks. Rocks form sediments and a cross-section of
earth's crust indicates the arrangement of sediments one over the other
during the long history of earth. Different-aged rock sediments contain
fossils of different life-forms who probably died during the formation of
the particular sediment. Some of them appear similar to modern
organisms (Figure 7.2). They represent extinct organisms (e.g., Dinosaurs).
A study of fossils in different sedimentary layers indicates the geological
period in which they existed. The study showed that life-forms varied
over time and certain life forms are restricted to certain geological time-
spans. Hence, new forms of life have arisen at different times in the history
of earth. All this is called paleontological evidence. Do you remember
how the ages of the fossils are calculated? Do you recollect the method
of radioactive-dating and the principles behind the procedure?
Embryological support for evolution
was also proposed by Ernst
Heckel based upon the observation of certain features during embryonic
stage common to all vertebrates that are absent in adult. For example,
the embryos of all vertebrates including human develop a row of vestigial
gill slit just behind the head but it is a functional organ only in fish and
not found in any other adult vertebrates. However, this proposal was
disapproved on careful study performed by Karl Ernst von Baer. He noted
that embryos never pass through the adult stages of other animals.
Comparative anatomy and morphology shows similarities and
differences among organisms of today and those that existed years ago.
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