Next up is Proterozoic eon which started extending from 2500 to 541 million years ago its name comes from Greek means “earlier life”. The earth that time was completely froze into a snowball earth because the atmosphere that time was rich in oxygen where the ozone layer thickened and cyanobacteria were threatened. Cyanobacteria (blue-green algae) were microscopic bacteria that generates oxygen. What’s funny about these bacteria is that the oxygen they release were toxic substances that’s harmful and a threat to them. By the end of the Proterozoic Eon eukaryotes, cells that have a nucleus, appeared and were the ancestors of all modern living things. It was in the Phanerozoic Eon when hard-shelled species arrived after the Cambrian Explosion.