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By Tim Radford
Movements of the earth's crust may mean that global warming driven by greenhouse gases from power stations and vehicle exhausts isn't the only threat to life the world faces.
About 700 million years ago, global temperatures fell so low that glaciers may have reached the equator. Snowball Earth may have all but extinguished life on the planet. But the only life at the time was microbial and dispersed in the oceans.