Here's an interesting theory from the New Scientist:
No Moon, no life on Earth, suggests theory
12:17 18 March 04
Without the Moon, there would have been no life on Earth.
Four billion years ago, when life began, the Moon orbited much closer to us than it does now, causing massive tides to ebb and flow every few hours. These tides caused dramatic fluctuations in salinity around coastlines which could have driven the evolution of early DNA-like biomolecules.
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