Assessing the Earliest Evidence for Life in the Geologic and Genomic Records

Both the geologic and genomic records provide information on the earliest history of terrestrial life. Despite the challenges inherent to the fragmentary and highly metamorphosed earliest rock record, microfossils as old as ∼3.5 billion years (Ga) and potential morphological and isotopic traces in earlier rocks suggest the origin and diversification of life within the planet's first billion years. This is consistent with a growing body of evidence for liquid water in Earth's surface or near-surface environment by around 4.3 Ga, which provides at least one requirement of habitability. Genomic evidence further informs our understanding of early life, including some aspects of its environment, physiology, and metabolism. In addition, ancestral proteome reconstruction and molecular clocks provide clues to the nature and timing of the last universal common ancestor (LUCA), although the latter methodology remains challenging and highly uncertain. Improved molecular clock methods may be able to better constrain the timing of LUCA and the divergence of the major domains of extant life, while further interrogation of the earliest geologic and geochemical records may be able to reveal even earlier evidence for an inhabited planet.