Estimates of the maximum time required to originate life.
Fossils of the oldest microorganisms exist in 3.5 billion year old rocks and there is indirect evidence that life may have existed 3.8 billion years ago (3.8 Ga).  Impacts able to destroy life or interrupt prebiotic chemistry may have occurred after 3.5 Ga.  If large impactors vaporized the oceans, sterilized the planets, and interfered with the origination of life, life must have originated in the time interval between these impacts which increased with geologic time.  Therefore, the maximum time required for the origination of life is the time that occurred between sterilizing impacts just before 3.8 Ga or 3.5 Ga, depending upon when life first appeared on Earth.  If life first originated 3.5 Ga, and impacts with kinetic energies between 2 x 10(34) and 2 x 10(35) were able to vaporize the oceans, using the most probable impact flux, we find that the maximum time required to originate life would have been 67 to 133 million years (My).  If life first originated 3.8 Ga, the maximum time to originate life was 2.5 to 11 My.  Using a more conservative estimate for the flux of impacting objects before 3.8 Ga, we find a maximum time of 25 My for the same range of impactor kinetic energies.  The impact model suggest that it is possible that life may have originated more than once.
Oberbeck
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NASA Ames Research Center, Moffett Field, CA 94035, USA.
Fogleman
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D018534:Earth, Planet; D019418:Evolution, Chemical; D005580:Fossils; D019110:Meteoroids; D008962:Models, Theoretical; D000069497:Origin of Life; D013997:Time Factors