Resumo

BERALDI-CAMPESI, Hugo. Early life on Earth and the first terrestrial ecosystems. Bol. Soc. Geol. Mex [online]. 2014, vol.66, n.1, pp.65-83. ISSN 1405-3322.

Terrestrial ecosystems have been largely regarded as plant-dominated land surfaces, with the earliest records appearing in the early Phanerozoic (< 550 Ma). However, the presence of biological components much older than plants in habitats as different as soils, peats, ponds, lakes, streams, and dune fields, suggests that much earlier types of terrestrial ecosystems appeared in Earth at least 2700 Ma ago. Microbes were abundant ~3500 Ma ago, and they surely adapted to live in subaerial conditions in peritidal and inarid and semiarid environments, as presently done by terrestrial microbes, which have great and rapid capacity of adapting themselves to changing conditions as suggested by fossil records. Yet, this evidence is rare and indirect in comparison with fossils from shallow or deeper marine environments, and its record has been largely overlooked. Consequently, the notion that microbial communities may have formed the earliest land ecosystems has not been widely accepted nor integrated into our general knowledge. Nowadays, an ample record of shallow-marine and lacustrine biota in ~3500 Ma-old deposits is known, together with evidence of microbial colonization of coastal environments ~3450 Ma ago, and indirect evidence that suggest biological activity in > 3400 Ma-old paleosols. The type of ambiances from where this evidence derives, endorses the idea that life on land perhaps occurred in parallel with aquatic life back in the Archean. The rapid adaptations seen in modern microbes, their outstanding tolerance to extreme and fluctuating conditions, their early and rapid diversification, and their old fossil record, collectively suggest that they constituted the earliest terrestrial ecosystems. It is likely that microbes contributed in forming the biomass-rich cover where plants later evolved. Understanding how life diversified and adapted to terrestrial conditionsis critical to comprehend its impact on the Earth's systems over millions of years.

Palavras-chave : Primitive terrestrial ecosystems; cyanobacteria.