The earliest life on Earth consisted of prokaryotes — small single-celled organisms without nuclei. These earliest organisms were anaerobic — they did not require oxygen to live.(Image from Lunar And Planetary Institute)
From Astrobiology:
Oldest Evidence for Life in Doubt
Chemical biomarkers in ancient Australian rocks, once thought to be the oldest known evidence of complex life on Earth, may have infiltrated long after the sediments were laid down, new analyses suggest.
The evidence was based on biomarkers - distinctive chemical compounds produced today by modern-day relatives of cyanobacteria and other complex life forms. In 1999, a team of researchers contended that the biomarkers in the 2.7-billion–year-old rocks pushed back the origins of cyanobacteria by at least 550 million years and of eukaryotes by about a billion years.
Although some scientists interpret the new findings, published in the Oct. 23 Nature, as disproving the older dates, others contend that the results still allow for the presence of the organisms or their kin at that time.
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