Early Earth and the Emergence of Life
Early Earth would have been very different and inhospitable compared to the Earth today. Throughout the Archean period there was little to no free oxygen in the atmosphere (<1% of presence levels). During the Proterozoic the amount of free O2 in the atmosphere rose from 1 - 10%. Most of this was released by the cyanobacteria in stromatolites. The nearest thing on Earth to living fossils, stromatolites are rocks made from millions of microscopic layers comprised of the remains of bacteria. They dominated the Earth for over 2 billion years and without the oxygen they produced, we may not have evolved. They can still be seen in Shark Bay, Australia.