The Early Earth: Creating the Conditions for the Emergence of Life

Summary Life emerged on a planet Earth that had little resemblance to the planet we know today. Trying to prise information about the environmental conditions reigning at the time life appeared during the Hadean Eon (4.56–4.0 Ga) is an arduous task because of the lack of preservation of a rock record from that eon. It is necessary to tease data from geochemical signals and long-lasting minerals, such as zircons, that occur in younger rocks but are inherited from deeply buried and recycled Hadean rocks. Comparison with data from younger, well-preserved rocks from the Eo-Paleoarchean (4.0–3.2 Ga), modeling, and comparative planetology provide the rest of the data. These data document a planet subjected to what would today be called extreme conditions: anoxic atmosphere, high flux of UV radiation, slightly acidic oceans, high degree of global volcanism and hydrothermal activity, high flux of extraterrestrial materials and impactors, different crustal behavior and basin effects, near-absence of clastic influx, higher tidal effects, small extents of generally low-lying landmasses, and high seawater temperatures at the interface with underlying rocks and sediments. However, a huge amount of variability existed at the local scale (i.e., the scale of prebiotic chemistry and abiogenesis [the origin of life]). Conversely, the numerous scenarios and pathways suggested by prebiotic chemists for the origin of life are very precise and often call for specific environmental conditions or the following of an exact sequence of chemical steps, one after the other, to reach the goal of recreating a living cell. Nature, however, does not follow the rules of recipe books. The proposed scenarios for the emergence of life—for example, those concerning hydrothermal environments, subsea or subaerial (including subaerial impact craters), the pumice raft theory, and radioactive placer sands or deep-seated faults—vary in their compatibility with current understanding of the early Earth’s environment. The evidence suggests that most of the hydrothermal scenarios would have been common, subaqueous or subaerial. The main difference between the scenarios is whether UV radiation was an essential component of prebiotic chemistry and abiogenesis, as some, but not all, hypothesize.