Abiogenesis proposes that the first life-forms generated were very simple and through a gradual process became increasingly complex. Biogenesis, in which life is derived from the reproduction of other life, was presumably preceded by abiogenesis, which became impossible once Earth’s atmosphere assumed its present composition. Modern abiogenesis hypotheses are based largely on the same principles as the Oparin-Haldane theory and the Miller-Urey experiment. There are, however, subtle differences between the several models that have been set forth to explain the progression from abiogenic molecule to living organism, and explanations differ as to whether complex organic molecules first became self-replicating entities lacking metabolic functions or first became metabolizing protocells that then developed the ability to self-replicate.
The habitat for abiogenesis has also been debated. While some evidence suggests that life may have originated from nonlife in hydrothermal vents on the ocean floor, it is possible that abiogenesis occurred elsewhere, such as deep below Earth’s surface, where newly arisen protocells could have subsisted on methane or hydrogen, or even on ocean shores, where proteinoids may have emerged from the reaction of amino acids with heat and then entered the water as cell-like protein droplets.