The Origin of Animals

		This trilobite was one of the many multicellular animals to evolve during the Cambrian Explosion

Beneath the weight of all this evidence, the Snowball Earth hypothesis is gaining credibility. In fact, the geologic record indicates the global climate flip-flopped not just once, but two to five times. But what's most remarkable about the idea is not that the Earth ever froze over, but when. The last thaw occurred about 585 million years ago, just about the same time multicellular animal life proliferated on this planet. Hoffman and Schrag are convinced this is much more than coincidence.

"It would be remarkable," says Schrag, "if this catastrophic environmental event were followed by this incredible radiation of life and the two were not related."

How could the Earth's alternate freezing and thawing contribute to animal evolution? The freezing temperatures would have killed off most living things on Earth, leaving only the hardiest life forms behind. Small populations of these durable little creatures likely huddled in volcanic hot springs or deep-sea vents. Separated by miles of ice, these populations would have diverged from each other over many generations. As with Darwin's finches or African cichlids, "small populations and physical separation," says Hoffman, "are the key ingredients for creating new species."

Scientists propose that as the ice caps advanced, more sunlight was reflected, and the temperature plummeted.

When the ice receded, Hoffman and Schrag propose, the diverse species were poised to exploit the novel ecological opportunities the newly warm Earth presented - new habitats to colonize, new resources to make use of. In the end, according to Schrag, oxygen might have been the crucial resource that literally fueled animal evolution.

As the Earth's temperature rose, photosynthesis resumed full throttle, as Hoffman demonstrated with the carbon-13 levels in Namibia. Oxygen, a waste product of photosynthesis, must have risen as well, providing an abundant energy source for complex, multi-cellular life.

While it's impossible to prove which variable truly triggered animal evolution, for Hoffman and Schrag, the timing of the last worldwide glacial retreat and the Cambrian explosion is simply too much to over look.

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