Image: Evolution and Oxygen

Evolution of life on Earth as related to the appearance of oxygen in the atmosphere.  The plot of oxygen versus time is modeled after the data of Rye and Holland (1999), who have proposed this pattern as the most likely based on studies of ancient soils (paleosols).  The pictures of organisms are meant to emphasize that the early Earth was colonized by simple organisms, probably prokaryotic in nature, and that complex organisms (multicellular large creatures) did not appear until the oxygen levels were near to what they are now, approximately 500 million years ago.