Question

reviewing key concepts: major evolutionary milestones

Answer 1

Answer:There are various theories which explain the origin of life but the most accepted theory is Chemogenetic Theory or Modern Theory. Before understanding about this theory we have to clear what was the conditions on the earth on that time.

1. The temperature on that time was very high(around 800 °C)

2. The lighter elements were present in gaseous forms( CH₄,NH₃, He, H₂ and water vapours)

3. Heavy element form the core of earth(Fe, Ni)

4.U.V. rays and lightening were the source of energy that time.

5.No molecular oxygen were present as the atmosphere were reducing that time.

So various theories are there for the explanation of evolution of life as:

Theory of special creation

Theory of Abiogenesis

Theory of Biogenesis

Panspermiatic Theory

Chemogenetic / Modern theory

The most accepted scientifically is Modern theory

According to this theory the origin of life takes place in three steps

a) Chemogeny: Chemogeny means chemical evolution. This theory was explained by Oparin ans Haldane

According to this when the temperature was very high and there were two energy source as U.V. light and Lightening (Sun source) so due to this weather change takes place as in continuous raining form through out year. This result in the formation of huge water bodies where minerals and other substances present in the earth crust were found in dissolved state in the water bodies. Or in other ways we can say broth formation(liquid nutrient media formation takes place)

By this excessive raining the temperature also slows down (below 100 °C)

Now the gases were CH₄, NH₄,CO₂ but still no molecular oxygen. In the presence of U.V.light . other lightening source certain molecule and compounds react to form bigger compounds which can replicate as sugar, aldehyde, purines, pyrimidines, fatty acids. But due to reducing atmosphere the conditions were anaerobic.

b) Biogeny: Now some of simple compounds aggregates to form complex bio molecules as DNA,RNA and Protein.

c) Cogenogeny: Here the protein and broth (all substances dissolved in water) aggregates to form a structure called as Protobiont but they did not reproduce.

Now the first life form originated from Protobiont were Eiobiont/ Coacervates which can reproduce by budding.

As the coacervates were firstly chemo-heterotrops where they only use chemicals to obtain energy for food .

When they use the light as energy source they are called as Chemo-phototrops. As the atmosphere were reducing that time so the sulphur, iron and other kind of chemicals were used but the bacteria are not using CO₂ so they are anoxygenic.

But some bacteria develop chlorophyll (pigments) to trap sun energy so they now start sifting towards aerobic condition as they have now the pigment so they used CO₂ and H₂O for photosynthesis and evolved oxygen. So the first aerobic bacteria is Cynobacteria.