File this one next to the earlier posts re: the "Medea Hypothesis," aka. "if the Earth really is a single organism, it's awfully mean." New Scientist's "Dawn of the animals: Solving Darwin's dilemma" discusses how life on earth as we understand it really got started by the mother of all mass extinctions:

"Put together, all the recent findings and ideas paint a picture of early evolution dramatically different from what we long imagined. The oceans did not suddenly become hospitable places for large animals 2.5 billion years ago when the atmosphere began to fill with oxygen, nor did animals suddenly appear during the early Cambrian. Instead, the first animals appeared much earlier but were limited to a thin layer of surface water in hostile oceans still dominated by bacteria. They were restricted in size by the lack of oxygen, starved of vital nutrients and regularly killed en masse by toxic upwellings.

Their deaths were not in vain, though. As their bodies sunk to the bottom of the toxic seas and were buried, carbon dioxide was sucked out of the atmosphere, triggering a series of deep ice ages [JK: so deep, some believe that even the oceans froze] that reset the chemistry of the oceans. The surviving animals seized the opportunity to wrest control of the oceans from the bacteria, producing clear waters rich in oxygen in which larger, more complex animals could evolve. Thus the stage was set for the Cambrian explosion."