● `color{Violet}"Evolutionary Biology"` is the study of `color{Violet}"history of life forms"` on earth.

● To understand the changes in `color{Violet}"flora and fauna"` that have occurred over millions of years on earth, we must have an understanding of the context of `color{Violet}"origin of life"`, i.e., evolution of earth, of stars and indeed of the universe itself.

● When we look at `color{Violet}"stars"` on a clear night sky we are, in a way, `color{Violet}"looking back in time"`.

● `color{Violet}"Stellar distances"` are measured in `color{Violet}"light years"`.

● What we see today is an object whose emitted light started its `color{Violet}"journey millions"` of year back and from `color{Violet}"trillions of kilometres away"` and reaching our eyes now.

● However, when we see objects in our `color{Violet}"immediate surroundings"` we see them instantly and hence in the `color{Violet}"present time"`.

● Therefore, when we see `color{Violet}"stars"` we apparently are `color{Violet}"peeping into the past"`.

● The `color{Violet}"origin of life"` is considered a `color{Violet}"unique event"` in the history of universe.

● The `color{Violet}"universe is vast"`.

● Relatively speaking the `color{Violet}"earth"` itself is almost only a `color{Violet}"speck"`.

● The universe is very old – almost `color{Violet}"20 billion years old"`.

● Huge `color{Violet}"clusters of galaxies"` comprise the universe.

● Galaxies contain `color{Violet}"stars"` and `color{Violet}"clouds"` of gas and dust.

● The `color{Brown}"Big Bang theory"` attempts to explain to us the `color{Violet}"origin of universe"`.

● It talks of a `color{Violet}"singular huge explosion"` unimaginable in physical terms.

● The universe `color{Violet}"expanded"` and hence, the `color{Violet}"temperature came down"`.

● `color{Violet}"Hydrogen"` and `color{Violet}"Helium"` formed sometime later.

● The gases `color{Violet}"condensed under gravitation"` and formed the `color{Violet}"galaxies"` of the present day universe.

● In the solar system of the `color{Violet}"milky way galaxy"`, earth was supposed to have been formed about `color{Violet}"4.5 billion years"` back.

● There was `color{Violet}"no atmosphere"` on early earth.

● `color{Violet}"Water vapour"`, `color{Violet}"methane"`, `color{Violet}"carbondioxide"` and `color{Violet}"ammonia"` released from molten mass covered the surface.

● The `color{Violet}"UV rays"` from the sun broke up water into `color{Violet}"Hydrogen"` and `color{Violet}"Oxygen"` and the lighter `H_2` escaped.

● `color{Violet}"Oxygen"` combined with `color{Violet}"ammonia"` and `color{Violet}"methane"` to form water, `CO_2` and others.

● The `color{Violet}"ozone layer"` was formed.

● As it `color{Violet}"cooled"`, the water vapor fell as `color{Violet}"rain"`, to fill all the depressions and form `color{Violet}"oceans"`.

● `color{brown}"Life"` appeared `color{Violet}"500 million years"` after the formation of earth, i.e., almost `color{Violet}"four billion years"` back.

● Some scientists believe that life came from `color{Violet}"outer space"`.

● `color{Violet}"Early Greek thinkers"` thought units of life called `color{Violet}"spores"` were transferred to `color{Violet}"different planets"` including earth.

● `color{Brown}"Panspermia"` is still a favourite idea for some astronomers.



● For a long time it was also believed that life came out of `color{Violet}"decaying and rotting matter"` like straw, mud, etc.

● This was the `color{Brown}"theory of spontaneous generation"`.

● `color{Brown}"Louis Pasteur"` by careful experimentation demonstrated that life comes only from `color{Violet}"pre-existing life"`.

● He showed that in `color{Violet}"pre-sterilised flasks"`, life did not come from killed yeast while in another flask `color{Violet}"open to air"`, new living organisms arose from ‘killed yeast’.

● `color{Violet}"Spontaneous generation"` theory was dismissed once and for all. However, this did not answer how the first life form came on earth.



● `color{Brown}"Oparin of Russia"` and `color{Brown}"Haldane of England"` proposed that the first form of life could have come from pre-existing `color{Violet}"non-living organic molecules"` (e.g. RNA, protein, etc.).

● They also theorized that formation of life was preceded by `color{Violet}"chemical evolution"`, i.e., formation of diverse `color{Violet}"organic molecules"` from inorganic constituents.

● The conditions on earth were – `color{Violet}"high temperature"`, `color{Violet}"volcanic storms"`, `color{Violet}"reducing atmosphere"` containing C`H_4`, N`H_3`, etc.

● In 1953, `color{Brown}"S.L. Miller"`, an American scientist created similar conditions in a `color{Violet}"laboratory scale"`.

● He created `color{Violet}"electric discharge"` in a closed flask containing C`H_4`, `H_2`, N`H_3` and water vapour at `color{Violet}"8000 degree C"`.

● He observed formation of `color{Violet}"amino acids"`.



● In similar experiments others observed, formation of `color{Violet}"sugars"`, `color{Violet}"nitrogen bases"`, `color{Violet}"pigment"` and `color{Violet}"fats"`.

● Analysis of `color{Violet}"meteorite content"` also revealed similar compounds indicating that `color{Violet}"similar processes"` are occurring elsewhere in space.

● With this `color{Violet}"limited evidence"`, the first part of the conjectured story, i.e., `color{Violet}"chemical evolution"` was more or less accepted.

● We have no idea about how the first `color{Violet}"self replicating"` `color{Violet}"metabolic capsule"` of life arose.

● The first `color{Violet}"non-cellular forms"` of life could have originated `color{Violet}"3 billion years"` back.

● They would have been `color{Violet}"giant molecules"` (RNA, Protein, Polysaccharides, etc.).

● These `color{Violet}"capsules"` reproduced their `color{Violet}"molecules"` perhaps.

● The `color{Violet}"first cellular form"` of life did not possibly originate till about `color{Violet}"2000 million years"` ago.



● These were probably `color{Violet}"single-cells"`.

● All life forms were in `color{Violet}"water environment"` only.

● This version of a `color{Violet}"biogenesis"`, i.e., the first form of life arose slowly through `color{Violet}"evolutionary forces"` from `color{Violet}"non-living molecules"` is accepted by majority.

● However, once formed, how the `color{Violet}"first cellular forms"` of life could have evolved into the `color{Violet}"complex biodiversity"` of today is another fascinating story

● `color{Violet}"Conventional religious literature"` tells us about the theory of `color{Brown}"special creation"`.

● This theory has `color{Violet}"three connotations"`.

● One, that all `color{Violet}"living organisms"` (species or types) that we see, today were `color{Violet}"created as such"`.

● Two, that the `color{Violet}"diversity"` was always the `color{Violet}"same since creation"` and will be the same in future also.

● Three, that earth is about `color{Violet}"4000 years old"`.

● All these ideas were `color{Violet}"strongly challenged"` during the `color{Violet}"nineteenth century"`.

● Based on observations made during a sea voyage in a sail ship called `color{Violet}"H.M.S. Beagle"` round the world, `color{Brown}"Charles Darwin"` concluded that existing living forms `color{Violet}"share similarities"` to varying degrees not only among themselves but also with life forms that `color{Violet}"existed millions of years"` ago.

● Many such life forms `color{Violet}"do not exist"` any more.

● There had been `color{Violet}"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 `color{Violet}"gradual evolution"` of life forms.

● Any population has built in `color{Violet}"variation in characteristics"`.

● Those characteristics which enable some to `color{Violet}"survive better"` in natural conditions (climate, food, physical factors, etc.) would `color{Violet}"outbreed"` others that are `color{Violet}"less-endowed"` to survive under such natural conditions.

● Another word used is `color{Violet}"fitness of the individual"` or population.

● The fitness, according to `color{Violet}"Darwin"`, refers ultimately and only to `color{Violet}"reproductive fitness"`.

● Hence, those who are `color{Violet}"better fit"` in an environment, leave `color{Violet}"more progeny"` than others.

● These, therefore, will `color{Violet}"survive more"` and hence are `color{Violet}"selected by nature"`.

● He called it `color{Brown}"natural selection"` and implied it as a `color{Violet}"mechanism of evolution"`.

● `color{Brown}"Alfred Wallace"`, a naturalist who worked in `color{Violet}"Malay Archepelago"` had also come to similar conclusions around the same time.

● In due course of time, apparently `color{Violet}"new types of organisms"` are recognisable.

● All the existing life forms `color{Violet}"share similarities"` and share `color{Violet}"common ancestors"`.

● However, these ancestors were present at `color{Violet}"different periods"` in the history of earth ( `color{Violet}"epochs, periods and eras"`).

● The `color{Violet}"geological history"` of earth closely correlates with the `color{Violet}"biological history"` of earth.

● A common permissible conclusion is that `color{Violet}"earth is very old"`, not thousand of years as was thought earlier but `color{Violet}"billions of years old"`.