The oldest convincing evidence of life is in 3.5 Ga rocks in the Pilbara region of WA and the Barberton Mountainland of South Africa. Diverse microbial communities lived in environments ranging from volcanic calderas to open marine settings. There is tenuous evidence for life in 3.8-3.9 Ga rocks of Greenland, but the record is obscured by the pervasive alteration of all known rock successions of this age and older. Thus there are no known well preserved rocks older than 3.5 Ga - so, no convincing fossil record. The best we can do at present is to infer the earliest stages of life from studies of extant organisms and from theoretical and experimental approaches to the origin of life.
The oldest known macroscopic organisms preserved in the rock record are dated at 2.1 Ga. Their affinities are obscure, but they may have been algae. There is chemical evidence for eukaryotes (nucleated sexually reproducing organisms) back to 2.7-2.8 Ga, and tentative morphological evidence at 3.0 Ga or even earlier.
Oxygen isotope studies hint at the possibility that the oceans of these times were hot, perhaps 60-800C or so, despite the low luminosity of the Sun. An enhanced greenhouse resulting from high CO2 concentrations in the atmosphere is the likely cause. The bulk of the evidence indicates an atmosphere and hydrosphere with little or no free oxygen and oceans with little sulfate. Oxygenation of the surface of the Earth resulted from oxygenic photosynthesis by cyanobacteria.
Malcolm Walter is Professor of Astrobiology at the University of NSW and Director of the Australian Centre for Astrobiology based there. He has worked for 45 years on the geological evidence of early life on Earth, and more recently on the search for life on Mars. He has also worked as an oil exploration consultant and a consultant to museums. In 2004 he was elected Fellow of the Australian Academy of Science. He has been awarded a Eureka Prize for his interdisciplinary research.
This talk was given at ISS2009: Genes to Galaxies, the 35th Professor Harry Messel International Science School, held at the University of Sydney in July 2009.
