|Themes > Science > Life Sciences > Physical Anthropology > Human Genetic Evolution > Early evolution of life on Earth|
Life of Earth appeared in a specific geological context and its early evolution took place alongside the geological evolution of the Earth. However, the greatest problem facing investigation of the first billion years in the history of the Earth is the lack of material "evidence": the dynamic activity of plate tectonics has all but obliterated the early rock record. Moreover, there is much dissention as to the interpretation of the few, highly metamorphosed remnants found in the ancient cratons. What information do we have about the earliest period in Earth's history and what are the changes that brought about the "modern" Earth? How do they affect the evolution of life?
Ancient detrital zircons dating to 4.4. b.y. ago provide evidence for a considerable amount of water at the surface of the Earth by that date and testify to the presence of protocontinents . Since life needs water, organics and energy, it could conceivable had originated already by 4.4 b.y. Extermination of life by massive impacts has been hypothesised  but there is no proof that this occurred. The oldest (equivocal) indications of life, based on carbon isotope studies of the oldest preserved supracrustal rocks at Isua, W. Greenland, date back to >3.75 b.y. ago . Although there are older metamorphosed terrains, the oldest well-preserved supracrustal rocks occur in the Pilbara, NW Australia, and in the Barberton greenstone belt, W. South Africa. Both terrains cover a critical transition period between an early Earth characterised by an apparently mixed, vertical plume and shallow plate tectonic regime and a basically modern plate tectonic regime (>3.5-ca. 2.9 b.y.). The Early Archaean portion of this rock record documents widespread life in the form of microbial mats in shallow water environments that were probably hot (pervasive hydrothermal activity) and salty . The mats were constructed and inhabited by a variety of microorganisms including small filamentous, rod-shaped and coccoidal forms (probably anoxygenic photosynthesisers, chemolithotrophs and heterotrophs).
Despite previous descriptions , there is no evidence for the presence of oxygenic cyanobacteria in the Early Archaean (indeed, genomic timescales suggest that the latter appeared about 2.6 b.y. ago ). The oldest bona fide observations of cyanobacteria date back to the 2.59 b.y.- old Cambellrand Subgroup in South Africa , although carbonaceous biomarkers from the 2.7 b.y. Hammersley Group shales in NW Australia suggest that they were present when those sediments were deposited . By this period, the Earth was tectonically similar to the modern Earth: cratonisation had led to the formation of true continental masses, there is clear evidence for lateral plate tectonic motion, and wide continental platforms were forming around the continents. In fact, the domal stromatolites, so characteristic of much of the Proterozoic, typically occurred on such shallow carbonate platforms .
In terms of eukaryote evolution, it appears that the lineage giving rise to the eukaryotes spilt off from the archaebacteria already by about 4.0 b.y. ago . Steranes, a group of macromolecular derivatives of eukaryotes, occur in the 2.7 b.y.-old Hammersley Group shales, hinting at organisms with some eukaryote characteristics by this time . However, genomic studies suggest that two specific occurrences of lateral gene transfer took place by symbiosis: a premitochondrial transfer at about 2.7 b.y. ago and a later mitochondrial transfer (involving cyanobacteria) at about 1.8 b.y. . It is probably impossible to directly identify the first eukaryote microfossils on the basis of morphology alone since they are likely to have had similar size and shape relationships to bacteria. In fact, the oldest interpreted eukaryotic microfossils consist of acritarchs dating back to 2.1 b.y. .
One phenomenon, which occurred at the same time as the evolution of cyanobacteria and eukaryotes, is the rise in atmospheric oxygen . For a long time it was believed that oxygenic photosynthesis by cyanobacteria was responsible but the removal of carbon from the atmosphere through the burial of organic matter and carbonates by plate tectonic activity may have been equally or even more significant in this process. Whatever the underlying reason, there seems to have been a clear relationship between the rise of oxygen in the atmosphere and the evolution of eukaryotes.
Thus we see that the early geological evolution of the Earth and the early evolution of life occurred in parallel. However, it is important to recall that microbial processes are surface-specific and that, although large-scale geological events form a global context for the evolution of life, there may be no direct cause and effect mechanism.
 Wilde, S.A. et al. (2001) Nature, 409 :
Date and place of birth: 20.6.1955,