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.

Purported stromatolites from 3.7 to 3.8 Ga Isua Supracrustal Belt. (a) Field view of the interpreted conical stromatolitic structures from Isua. (b–f) Cut faces of rock sections illustrating strain features. (a–f from [82]). (g,h) Cut rock faces across the stromatolitic structures with detail in (h) illustrating interpreted biofilm onlap onto vertical structures (g, h from [81]). Philosophical Transactions B via PubMed

Time tree constructed by Moody et al. [13] which places LUCA (and its ecosystem) at about 4.2 Ga, showing also the estimated timings of appearance of the last universal, archaeal, bacterial and eukaryotic common ancestors (LUCA, LACA, LBCA and LECA, respectively); the last common ancestor of the mitochondrial lineage (Mito-LECA); and the last plastid-bearing common ancestor (LPCA). Arc, Archaea; Bac, Bacteria; Euk, Eukarya. The earliest parts of this figure are bracketed by the date of the Moon-forming impact (a date of 4.52 Ga is used by Moody et al. [13], but see discussion below), and the oldest known fossil evidence for cellular life. Likewise, the latter is grounded in the Betts et al. [14] paper, which refers to enigmatic structures in the 3.35 Ga Strelley Pool Chert, Panorama Formation, Pilbara [15]. We note that there are, in fact, older cellular fossils in the 3.45 Ga Kitty’s Gap Chert, Pilbara [9,16].Philosophical Transactions B via PubMed

• What the earliest evidence for life tells us about the early evolution of the biosphere, Philosophical Transactions B via PubMed (open access)
• What the earliest evidence for life tells us about the early evolution of the biosphere Philosophical Transactions B (open access)

Astrobiology,