In biology, abiogenesis or the origin of life[3][4][5][a] is the natural process by which life has arisen from non-living matter, such as simple organic compounds.[6][4][7][8] While the details of this process are still unknown, the prevailing scientific hypothesis is that the transition from non-living to living entities was not a single event, but an evolutionary process of increasing complexity that involved molecular self-replication, self-assembly, autocatalysis, and the emergence of cell membranes.[9][10][11] Although the occurrence of abiogenesis is uncontroversial among scientists, its possible mechanisms are poorly understood. There are several principles and hypotheses for how abiogenesis could have occurred.[12]
The study of abiogenesis aims to determine how pre-life chemical reactions gave rise to life under conditions strikingly different from those on Earth today.[13] It primarily uses tools from biology, chemistry, and geophysics,[14] with more recent approaches attempting a synthesis of all three:[15] more specifically, astrobiology, biochemistry, biophysics, geochemistry, molecular biology, oceanography and paleontology. Life functions through the specialized chemistry of carbon and water and builds largely upon four key families of chemicals: lipids (cell membranes), carbohydrates (sugars, cellulose), amino acids (protein metabolism), and nucleic acids (DNA and RNA). Any successful theory of abiogenesis must explain the origins and interactions of these classes of molecules.[16] Many approaches to abiogenesis investigate how self-replicating molecules, or their components, came into existence. Researchers generally think that current life descends from an RNA world,[17] although other self-replicating molecules may have preceded RNA.[18][19]
The classic 1952 Miller–Urey experiment and similar research demonstrated that most amino acids, the chemical constituents of the proteins used in all living organisms, can be synthesized from inorganic compounds under conditions intended to replicate those of the early Earth. Various external sources of energy may have triggered these reactions, including lightning and radiation. Other approaches ("metabolism-first" hypotheses) focus on understanding how catalysis in chemical systems on the early Earth might have provided the precursor molecules necessary for self-replication.[20]
Earth remains the only place in the universe known to harbour life,[21][22] and fossil evidence from the Earth informs most studies of abiogenesis. The age of the Earth is 4.54 Gy (billion years);[23][24][25] the earliest undisputed evidence of life on Earth dates from at least 3.5 Gya (Gy ago),[26][27][28] and possibly as early as the Eoarchean Era (3.6–4.0 Gya). In 2017 possible evidence of early life on land was found in 3.48 Gyo (Gy old) geyserite and other related mineral deposits (often found around hot springs and geysers) uncovered in the Pilbara Craton of Western Australia.[29][30][31][32] Other discoveries suggest that life may have appeared on Earth even earlier. As of 2017[update], microfossils (fossilised microorganisms) within hydrothermal-vent precipitates dated 3.77 to 4.28 Gya in rocks in Quebec may harbour the oldest record of life on Earth, soon after ocean formation 4.4 Gya during the Hadean Eon.[1][2][33][34][35]
The NASA strategy on abiogenesis attempts to identify interactions, intermediary structures and functions, energy sources, and environmental factors that contributed to the diversity, selection, and replication of evolvable macromolecular systems,[36] and to map the chemical landscape of potential primordial informational polymers. The advent of polymers that could replicate, store genetic information, and exhibit properties subject to selection likely was a critical step in the emergence of prebiotic chemical evolution.[36]