Snowball Earth

"Let it snow, let it snow, let it snow."

—Christmas song

It is hard to hide our genes completely.

—Philip Kitchner, The Lives to Come, 1996

Spring is universally associated with birth, growth, and fertility. It is a time of warmth and renewal after the frigid lifelessness of winter. And so it would seem that the emergence of animals long ago on Earth should have resulted from a protracted period of warm and fertile, spring-like conditions. But new information uncovered by several insightful scientists suggests that the birth of animal life on Earth was initiated not by a time of warmth but, rather, by the most fearful winter ever to grip the planet. If this phenomenon, known as Snowball Earth, turns out to be linked to the origins of animal life, what will it mean for the possibility of animal life on other planets?

As we noted earlier, a majority of astrobiologists believe that the temperature of early Earth from the time of the first life, about 3.8 billion years ago, until the origin of eukaryotic cells, about 2.5 billion years ago, was high—probably too hot for the existence of animal life. (Yet there are others who suggest that Earth may have undergone a "cool start," because the sun at that time was giving off much less energy than now). Both camps agree that the planet's atmosphere was almost devoid of oxygen. Those who believe in a "hot start" suggest that gradually, as greenhouse gases in the atmosphere were reduced in volume, the temperatures declined. But Earth may have cooled too much (or, if you are of the "cool start" persuasion, failed to warm up enough), at least in the short term. There is evidence of as many as four major episodes of glaciation on a scale far exceeding anything before or since—times of cold and ice that make the last ice age, the Pleistocene epoch of 2.5 million to 10,000 years ago, seem but a brief cold snap.

The first known Snowball Earth episode began about 2.45 billion years ago, and a second protracted siege of several such events occurred between 800 and 600 million years ago. These two dates are of great interest, because they are also the times of the two most signal events in biological history since life's first appearance here: Around 2.5 billion years ago the first eu-karyotic cells appeared, and the fossil record reveals that about 550 million years ago, diverse and abundant animal life blossomed, in the event known as the Cambrian Explosion, the subject of the next chapter. Perhaps it is just coincidental that these two spectacular and far-reaching biological events occurred immediately after the two most severe episodes of glaciation and ice cover in Earth history. But according to a controversial new theory, both may have been triggered by the Snowball Earth episodes.

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