<P> The classic 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 that may have triggered these reactions have been proposed, 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 . Complex organic molecules have been found in the Solar System and in interstellar space, and these molecules may have provided starting material for the development of life on Earth . </P> <P> The biochemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the age of the universe was only 10 to 17 million years . The panspermia hypothesis alternatively suggests that microscopic life was distributed to the early Earth by space dust, meteoroids, asteroids and other small Solar System bodies and that life may exist throughout the Universe . The panspermia hypothesis proposes that life originated outside the Earth, but does not explain its origin . </P> <P> Nonetheless, Earth remains the only place in the Universe known to harbour life, and fossil evidence from the Earth informs most studies of abiogenesis . More than 99% of all species of life forms, amounting to over five billion species, that ever lived on Earth are estimated to be extinct . The age of the Earth is about 4.54 billion years; the earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago, and possibly as early as the Eoarchean Era (between 3.6 and 4.0 billion years ago), after geological crust started to solidify following the molten Hadean Eon . In May 2017, evidence of the earliest known life on land may have been found in 3.48 - billion - year - old geyserite and other related mineral deposits (often found around hot springs and geysers) uncovered in the Pilbara Craton of Western Australia . However, there have been a number of discoveries that suggested the earliest appearance of life on Earth was even earlier . Currently, microfossils within hydrothermal vent precipitates dated from 3.77 to 4.28 billion years old found in Quebec, Canada may be the oldest record of life on Earth, suggesting life started soon after ocean formation 4.4 billion years ago . According to biologist Stephen Blair Hedges, "If life arose relatively quickly on Earth...then it could be common in the universe ." </P> <P> The Hadean Earth is thought to have had a secondary atmosphere, formed through degassing of the rocks that accumulated from planetesimal impactors . At first, it was thought that the Earth's atmosphere consisted of hydrogen compounds--methane, ammonia and water vapour--and that life began under such reducing conditions, which are conducive to the formation of organic molecules . According to later models, suggested by study of ancient minerals, the atmosphere in the late Hadean period consisted largely of water vapour, nitrogen and carbon dioxide, with smaller amounts of carbon monoxide, hydrogen, and sulfur compounds . During its formation, the Earth lost a significant part of its initial mass, with a nucleus of the heavier rocky elements of the protoplanetary disk remaining . As a consequence, Earth lacked the gravity to hold any molecular hydrogen in its atmosphere, and rapidly lost it during the Hadean period, along with the bulk of the original inert gases . The solution of carbon dioxide in water is thought to have made the seas slightly acidic, giving it a pH of about 5.5 . The atmosphere at the time has been characterized as a "gigantic, productive outdoor chemical laboratory ." It may have been similar to the mixture of gases released today by volcanoes, which still support some abiotic chemistry . </P>

Where are the two places that life is hypothesized to have started
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