Seeding life on Earth ow plausible is it that life on Earth got its start from organic materials brought here by comets or asteroids (page 366)? In a relatively thin atmosphere like that of today's Earth, even small comets would have hit Earth with such velocity that a great percentage of their organic constituents would have been lost en route or would have burned up on impact. But in a thick carbon dioxide atmosphere that may have existed some 4.5 to 3.8 billion years ago (when heavy

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... nt was occurring and when life may have just begun to evolve), the thick atmosphere would have slowed comets, and their organics (some 25% of their mass) would have arrived at Earth intact. Chyba et al. discuss the likelihood of a cometary-source scenario for the origin of life and calculate how much organic material-some 1020 kilograms--could have been delivered to Earth in this way. The best estimate is that during the time of heaviest bombardment, 106 to 107 kilograms of cometary organics would have accreted on Earth per year.