In "Noah's Snowball", Paul Hoffman and Dan Schrag theorize that the rapid warming of a previously ice-bound Earth triggered the Cambrian Explosion- the spasm of evolution that produced the first multicellular animals. Fossil evidence tells us this animal explosion occurred some 600 million years ago. But what evidence is there of such a dramatic change in the Earth's climate?
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Global Glaciers
Hoffman and Schrag propose that roughly 700 million years ago, the Earth's climate cooled and the polar ice caps expanded. As the ice advanced, more and more sunlight was reflected back into space, causing the temperature to drop even further. By the time the ice caps covered roughly half of the globe, the cooling process was rapid and unstoppable. Soon the Earth was completely entombed in ice one kilometer thick.

The "Snowball Earth" hypothesis is not a new one. Clues to this episode in Earth's history have stumped geologists since the 1960's. Only recently have Hoffman and Schrag made sense of it all. And the evidence they have pooled, geological, biological and chemical, comes from all over the globe.

Back in 1964, Cambridge geologist Brain Harland discovered glacial deposits near the equator. The hottest place on Earth covered in glaciers? For years, scientists resisted the idea, assuming the deposits were not native to the tropics, but had migrated to the equator over time. Twenty-two years later, researcher Joe Kirschvink discovered the perfect sample in Adelaide, Australia- a glacial deposit with undeniable tropical origins. The deposits' telltale sign- the "magnetic signature" found only in rocks forged near the equator- is the central bit of evidence that the Earth was once entirely locked in ice.

Samples from cliffs like this one in Namibia first sparked Hoffman and Schrag's theory

Halfway around the globe, in Namibia on the southwest coast of Africa, Paul Hoffman fit together another piece of this geologic puzzle. Hoffman measured the carbon levels in sediments found both below and above known glacial deposits. In the sediments beneath the glacial deposits, he discovered an excess of carbon-13, a geologic indicator of photosynthesis. The finding showed that the region, submerged at the time, had been temperate and teeming with algae.

Immediately beneath the glacial deposits, however, the amount of carbon-13 drops off dramatically, indicating an abrupt halt in photosynthetic activity. In the layers above the glacial deposits, the carbon levels gradually rise. These findings suggest that a rapidly cooling Earth brought plant life to a halt. Life remained suspended during the long period of global glaciation and then resumed and blossomed in the spring-like thaw that followed.

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photos: Daniel Schrag