When the Earth Froze

The rocks tell us that at least twice, the earth has frozen over from the poles to the equator

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The answers, of course, depend on what we think caused snowballs in the past. The scientists I talked to didn't seem too concerned — after all, it's been about 600 million years since the earth has had this sort of problem. Everyone noted that the sun was 6 to 7 percent dimmer back in those days.

What interests me most is the question of how any living thing survived the event. When I ask scientists about this, they become a little evasive. Remember, though, that at this time life on earth was confined to the ocean, and consisted predominantly of single-celled organisms. If you think of a planet populated exclusively by green pond scum, you won't be far off. It's possible that photosynthetic algae could have survived in localized warm refuges (around undersea volcanoes, for example), in water trapped in spongy ice, or in seasonal meltwater lakes near the tropics. Life can survive in these sorts of environments in today's Antarctica. But which of these escape routes life actually took, or whether it used a different, as yet unimagined strategy, is a question for future research.

Daniel Schrag points out one interesting thing about the relation between snowball events and life. "Multicelled life on earth developed soon after the last snowball ended," he says, "and there is some (equivocal) evidence that it tried to start up between snowballs, only to be wiped out each time." It may be, in other words, that only single-celled life can survive such total freezes and that the development of complex life-forms had to wait until they no longer occurred.

Only when the freezes stopped did evolution produce beings so complex that they can read the story in the rocks and tell us that the earth was once much different from what it is today.

By James Trefil


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