Earth has undergone a continual process of geological and physical change from the time of origin about 4.6 billions years ago.
2
Microbes have always been able to adapt and cope with these changes. The dating of rocks based on the decay of radioactive isotopes and the fossilized remains of cells provides evidence for early microbial life. Stromatolites, which are laminated microbial mats built from layers of filamentous bacteria that became fossilized are abundant in rocks up to 3.5 billion years old or younger.
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An interesting fact about stromatolites is that filamentous phototrophic bacteria, which might be related to the green non-sulfur bacterium Chloroflexus, may have formed ancient stromatolites.
Ancient stromatolites were more likely composed of anoxygenic phototrophs rather than oxygen-producing cyanobacteria, since molecular oxygen did not appear until the evolution of oxygenic photosynthesis by cyanobacteria. Therefore, It has been concluded that the first cells may have been anaerobic autotrophs due to the lack of oxygen and abundance of hydrogen and carbon dioxide. These anaerobic autotrophs obtained their carbon from CO
2
and used electrons derived from sulfides, hydrogen or ferrous iron to reduce CO
2
to cellular material from H
2.
-
2H
2
+ CO
2
→ H
2
O + [CH
2
O]
After the development of early forms of carbon and energy metabolism, microbial life underwent a long process of metabolic diversification; and as the different elements and nutrients were consumed and became limited; the competition for resources and natural selection led to the evolution of new forms of metabolism. Molecular evidence suggests that the ancestors of the modern day archaea and bacteria had diverged by 4.1 –3.9 billion years ago and that the development of distinct metabolisms where early bacteria may have used hydrogen and carbon dioxide to produce acetate or ferrous iron compounds as a source of energy
2
(i.e., electrons) had also occurred.
At the same time, archaea- developed the ability to use hydrogen and carbon dioxide as substrates for the production of methane.
-
4H
2
+ CO
2
→ CH
4
+ 2H
2
O
The modern day Bacteria and Archaea had existed as the only forms on our planet for about 2 billion years before eukaryotes evolved (Madison, Martinko, Dunlap, and Clark 2009). Eukaryotes originated after the rise in atmospheric oxygen. Oxygen was also a factor in endosymbiosis. The endosymbiotic theory suggests that eukaryotic cells were created from a symbiosis between different prokaryote cells that had the ability to use oxygen and generate ATP. These prokaryote cells have evolved into the mitochondria that are now part of multicellualr organisms. The other significant part of the theory states that other prokaryotic cells that were able to carry out photosynthesis have evolved into the chloroplast, which is the organelle necessary to carry out photosynthesis in plants and algae.
