What the earliest evidence for life tells us about the early evolution of the biosphere.

Life emerged on Earth in an ultramafic world under anaerobic conditions and conditioned by particular environmental characteristics for which no record remains. Molecular clocks suggest that the Last Universal Common Ancestor, LUCA, lived in a well-established biome towards the end of the Hadean, between 4.33 and 4.09 Ga. They also suggest that the last bacterial common ancestor (LBCA) and the last archaeal common ancestor (LACA) may have diverged by the Palaeoarchaean, i.e. earlier than suggested by phylogenetic dating. Moreover, various geochemical and isotopic proxies for specific microbial metabolisms have been used to date the appearance of certain metabolic pathways in geological time. According to some molecular clocks, oxygenic photosynthesis arose in the Palaeoarchaean (3.5-3.2 Ga) and some geochemical studies point to oxygenic photosynthesis already in Eoarchaean times. The bulk of the geological evidence, however, indicates its appearance in the Mesoarchaean (3.2-2.8 Ga). This contribution explores the geological and palaeontological evidence for these interpretations and cautions the need to take into account other, abiotic influences on the proxy signatures, as well as the importance of basing interpretations of biogenicity on a complementary suite of proxies to ensure correct elucidations.This article is part of the discussion meeting issue 'Chance and purpose in the evolution of biospheres'.