About 3.5 billions years ago the earth surface was very different from now, particularly it was less hospitable. Oceans were larger, very often there were volcanic eruptions and meteor showers. Temperatures were probably similar to those registered now but the sunlight was less vivid. The most important feature of the ancient environment was the absence of free oxygen. Indeed it was almost lacking in the atmosphere whose main components were carbon dioxide, methane and ammonia.
But life was already present!
Which kind of organisms could survive in a such hard environment?
They were only unicellular prokaryotes (like the bacteria and microalgae living today) able to gain energy without oxygen. They were very numerous and different from each other for their metabolism based on different energetic sources (methane, ammonia, sulphur and so on) predators were lacking: nobody was eaten. So they grew and spread everywhere.
Then, about 2 billion years ago, a new evolutionary metabolism appeared.
Aquatic organisms called blue-green algae began using energy from the Sun to split molecules of H2O and CO2 and recombine them into organic compounds and molecular oxygen (O2). This solar energy conversion process is known as photosynthesis.
Yes, the same photosynthesis that practically support life on the earth even now.
According to modern hypothesis photosynthesis originated through the fusion of various evolutionary lines which pulled part of their genetic material. This fusion of genes, at present referred to as horizontal gene transfer, created a metabolism much more productive than the others already existing, also considering that the sunlight was in the meantime increased. Some of the photosynthetically created oxygen combined with organic carbon to recreate CO2 molecules. The remaining oxygen accumulated in the atmosphere, touching off a massive ecological disaster with respect to early existing anaerobic organisms.
A true atmospheric pollution!
Most species died and disappeared for ever.
Other species could survive in particular environments with a low oxygen concentration.
Others “learned” to breathe oxygen while others were able to survive by the fusion with those breathing oxygen organisms.
In this way the eukaryotic cell originated.
Thus, starting from a great atmospheric transformation, lethal for the majority of the forms of life at that time existing, new forms arose, from which during the time pluricellular organisms (man included) derived.
To cut a long story short: perhaps if the atmospheric pollution in progress now will increase beyond measure we, existing living beings, could succumb while new life forms could be originated on the earth.
Impressive!
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