Around 3.5 BYA the only signs of life were anaerobic (don't need oxygen to life) microorganisms called cyanobacteria and other prokaryotes ( single celled organisms. Because the atmosphere was very poisonous, all life was living in the ocean to be protected. The cyanobacteria were producing oxygen through photosynthesis.
As the cyanobacteria were producing more and more oxygen, the atmosphere and the oceans started to become oxidized.
I'm Dr. Lynn Margulis and I believe that the microorganisms that were being killed from the new oxygen in the ocean were very adaptive. Some died, however most used the oxygen in either low or high concentrations. Eventually they learned to use oxygen as an energy source and increased their activity. Overtime these bacterium became aerobic (using air to survive.)
Eventually the soft membrane microorganisms started to group together to survive as they were being attacked by hard membrane microoranisms. At this time these prokaryotes started to evolve into eukaryotes (multi celled organisms).
Hello i'm Professor Kwan Xian and after studying amoeba in a lab I learned that the two bacterium joined together to survive and have energy. Once they were separated, both bacteria died which tells us they needed each other. As they combined this allowed the period of evolution to come.
After million of years of evolving, around 1 BYA, 10,000 new species of life started to show up in all shapes in sizes and in different designs. All life still remained in the safe and vast oceans. Ever since then more special would evolve and evolve to have a greater chance of survival.
Due to the ozone layer and the atmosphere being oxidized, the aerobic life living in the sea would eventually venture out on to land and evolve from there. Plants and animals would start to evolve into the life see today. In fact, us humans, like all life, are ancestors to the once cyanobacteria and other microorgaisms of the ancient sea.