If conditions were initially favourable for the origin of life, why did they cease to be so and why did only one lineage ultimately survive? First, conditions must have changed fairly rapidly as the planet and the rest of the solar system settled down. Comet and meteorite impacts would have become less frequent; the "late heavy bombardment" has been dated to 4,000-3,800 million years ago. Second, the Gaia principle tells us that when early life became established it altered the environment. Perhaps, therefore, life itself made the planet unsuitable for the origin of life. Photosynthesis might have been a key factor; even a trace of oxygen in the atmosphere could have permanently sterilised the inanimate world. Alternatively, perhaps one type of cell - our ultimate ancestor - ate the others.
It is easy to dismiss this aspect of the origin-of-life problem as relatively trivial. But as we argued earlier in the chapter, it is trivial only if the origin of life was a highly improbable process. If the origin of life was likely, in other words recurrent, then the question of why spontaneous generation ceased demands a convincing answer.
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