The Second Global Glaciation

As we saw in Chapter 5, by the next round of Snowball Earth events, those spanning the time interval from 800 to 600 million years ago, animal life was present on Earth, but it was newly formed. Either simultaneously with or soon after the appearance of the new animal phyla, Earth was once more locked into a global icehouse. once again, there must have been a period of mass extinction, as the warm planet froze and the heat-loving organisms of

Earth had to retreat to oases of heat, such as around volcanoes and hydrothermal vents, or die. Yet the very severity of these events may have benefited the newly arisen animals. The great stress inflicted by environmental conditions imposed by the Snowball events would have stimulated inordinately rapid evolution among the newly evolved animals. It would also have caused the isolation of various populations, because the small populations of life huddled around the undersea volcanoes would have been cut off from any exchange of genes with other animal groups. This very isolation may have been largely responsible for the diversity of phyla that emerged at the other end of these crises, for when the final Snowball Earth event ended, about 600 million years (or less) ago, an entirely new group of creatures was ready to take over the planet. This is the interval when animal life began to diversify dramatically, in an event known as the Cambrian Explosion, the subject of the next chapter.

Would this have happened if the glaciations had not occurred? Kirschvink and Hoffman suggest that there is a causal link between the cessation of these major glaciations and the emergence of animals. Hoffman has noted, "Without these ice events, it is possible there wouldn't be any animals or higher plants." He believes that the melting of the ice at the end of these ice ages boosted biological productivity—and in the process stimulated evolutionary activity. This idea has yet to be confirmed, but it remains a tantalizing possibility.

Both of the two great episodes of Snowball Earth nearly ended life on Earth, as we know it. But each, ultimately, may have been crucial in stimulating the great biological breakthroughs necessary for animal life: the evolution of the eukaryotic cell and then the diversification of animal phyla. This leads us to ask whether Snowball Earth events are necessary to produce animal life as diverse as that seen on Earth today.

The end of the last Snowball Earth event brought the time interval known as the Precambrian to a close. Soon thereafter, abundant skeletons of larger animals began to fill the sea, in the Cambrian Explosion. If the two groups of scientists led by Joseph Kirschvink and Paul Hoffman are correct about Snowball Earth, a good case can be made that life on Earth is to some extent due to these events.

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