For decades the origin and evolution of life was restricted to the fossil record that recorded hard-shelled life. We now know, through determination of absolute ages by radioactive decay, that this record only record the last 500 m.y. or so of life. Prior to that, life existed as soft-bodied organisms, or even earlier, as single cell bacteria (prokaryotes) or single-celled organisms with nuclei (eukaryotes). The oldest microfossils, composed of single-celled organisms that probably were similar to cyanobacteria, are 3.5 b.y. old, and are found in Western Australia (not the same locality where the very old zircon mineral grains were found). More convincing evidence for life in the Archean comes from fossil layered microbial communities called stromatolites. Although the 3.5 b.y. old microfossils are still debated, people pretty much agree that the fossil record for life is undisputable by about 3.0 b.y., and stromatolites are part of this evidence. Fossil bacteria are universally accepted for the Proterozoic, where the images (and chemical compositions) are much more clear than the fuzzy images for the 3.5 b.y. old microfossils.
The Proterozoic microfossils are much more similar to the modern cyanobacteria. The occurrence of cyanobacteria early in earth's history is critical, since their metabolic "waste product" is oxygen, and it was essential to produce high levels of oxygen in the earth's atmosphere before more complex life (which requires different means of metabolism and energy storage) could evolve. In the latest part of the Proterozoic (~ 600 m.y. ago), multi-cellular, complex life is recorded in the fossil record.
The figure shown above casts the origin and evolution of life into a 24 hour clock.
Via Dr. Stefan Gruenwald