Question

The first living organisms evolved in a reducing environment which had little free oxygen. Then, photosyntheitic prokaryotes arose and began "poisoning" Earth with free oxygen. This event is written in the geology of this planet, and had a profound effect on the evolution of living things.

1. How did this chemical revolution affect early life?
2. What innovations were made possible?
3. What happened to those first, anaerobic organisms? Where are they found today? What does that say about the primordial earth?

Answer 1

Earth originated around 4.5 billion years ago. The first living organisms to inhabit the earth were anoxygenic prokaryotes. These were simple cells without nucleus and membrane bound organelles. They contain ribosomes. Their DNA was enclosed in a membrane less area at the middle of the cell called nucleoid. They were chemoautotrophs, that is, they prepared their own food by oxidizing simple inorganic molecules like CO2, H2S, N2 etc. They were anaerobes because the respire in the absence of oxygen. There was no oxygen in the atmosphere on the early earth. This type of atmosphere was called reducing atmosphere.

After chemoautotrophs, photoautotrophs came onto this planet. They were capable of using light to obtain energy. This was called as photosynthesis, during which oxygen was released as well. The release of oxygen made the environment oxygenated and the atmosphere shifted from the reducing one to oxygenated one.

After photoautotrophs, organisms capable of using atmospheric oxygen arose. This included the algae early land plants pteridophytes gymnosperms and angiosperms. This was led by the origin of animal kingdom, ultimately resulting in the origin of invertebrates and vertebrates and mammals and hence, humans.

This is how use of different chemicals, first inorganic by earliest prokaryotes and then oxygen by subsequent organisms lead to chemical evolution on earth.

​​​The earliest anaerobic organisms are still present on this planet. These are the organisms which we call as nitrogen fixing bacteria, phosphate solubilizing bacteria, sulphur using bacteria and iron using bacteria.

Some of these anaerobes evolved to live in the most harsh conditions on this planet. These belong to the domain Archaea. These are methanogens halophiles and thermophiles which are still present on this planet.

​​​The presence of anaerobes indicated that primordial earth was devoid of oxygen and had very simple inorganic molecules to be used as energy sources by these organisms.

Please rate.