Abstract

The hypothesis that life’s rapid appearance on Earth justifies the belief that life is widespread in the universe has been investigated mathematically by Lineweaver and Davis (Astrobiol- ogy 2002;2:293–304). However, a rapid appearance could also be interpreted as evidence for a nonterrestrial origin. I attempt to quantify the relative probabilities for a non-indigenous ver- sus indigenous origin, on the assumption that biogenesis involves one or more highly im- probable steps, using a generalization of Carter’s well-known observer-selection argument. The analysis is specifically applied to a Martian origin of life, with subsequent transfer to Earth within impact ejecta. My main result is that the relatively greater probability of a Mar- tian origin rises sharply as a function of the number of difficult steps involved in biogene- sis. The actual numerical factor depends on what is assumed about conditions on early Mars, but for a wide range of assumptions a Martian origin of life is decisively favored. By con- trast, an extrasolar origin seems unlikely using the same analysis. These results complement those of Lineweaver and Davis. Key Words: Origin of life—Mars—Probability theory— Carter—Transpermia. Astrobiology 3, 673–679.