The original version of this story appeared in Quanta Magazine.
Once upon a time, long ago, the world was encased in ice. That’s the tale told by sedimentary rock in the tropics, many geologists believe. Hundreds of millions of years ago, glaciers and sea ice covered the globe. The most extreme scenarios suggest a layer of ice several meters thick even at the equator.
This event has been called Snowball Earth, and you’d think it would be a terrible time to be alive—and maybe, for some organisms, it was. However, in a warmer period between glaciations, the first evidence of multicellular animals appears, according to some interpretations of the geological record.