Segre, D., and D. Lancet. “Theoretical and computational approaches to the study of the origin of life.” In Origins , pp. 91-120. Springer, Dordrecht, 2004.
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Investigating the evolutionary path that led to present-day cells is a little like tracing a maze, where the entrance and exit are known, but much work remains to find the paths that connect them. For the origin of life, the " entrance " is the multifaceted environment of a water-and carbon-rich planet and the " exit " is a protocellular structure subject to Darwinian evolution. Whether terrestrial life started on our planet or elsewhere in the Universe is a recurrent subject of debate, often nourished by proposals of Panspermia scenarios (Crick, 1981; McKay, 1997), and recently inspired by the finding of candidate relics of martian life forms (Kerr, 1996; McKay et al., 1996). An extraterrestrial origin cannot be absolutely discarded using the present knowledge, but there are two good reasons for setting aside this debate: (i) there is currently no solid evidence of extraterrestrial life, and (ii) moving the problem elsewhere still requires solving it. While exciting recent analyses have certified the presence of planetary systems other than the solar one (Lissauer et al., 2000), and future explorations might one day give us the opportunity to analyze frozen relics of ex-traterrestrial life, most origin of life scenarios assume that the cradle of life-as-we-know-Although the details of prebiotic Earth conditions and of the nature of the earliest forms of life are probably lost forever, a lot is known about the general features primordial Earth. For example, a time window for the transition from inanimate to animate matter has been estimated using geophysical and astronomical methods. The lower bound of this window, 3.9 billion years ago, corresponds to the end of a period of intense meteorite bombardment (Chyba and Sagan, 1992), as testified, for example, by the analysis of the Moon’s craters (Maher and Stevenson, 1988). For the upper bound we can rely on geological records, such as stromatolites and microscopic fossils, which provide evidence that 3.8 billion years ago life was already present on Earth h (Mojzsis et al., 1996; Mojzsis et al., 1998; Schopf et al., 1965).