| Imagine the entire planet covered with ice, from the poles to the Equator. It happened once long ago, according to some climatologists.
Doug Macdougall is an earth scientist at the University of California-San Diego and author of a new book called "Frozen Earth." He says the planet-wide freeze is known as "Snowball Earth."
"It's pretty well universally agreed by the scientists who work on ice ages that this was a period between about 600 million and 800 million years ago when the Earth overall was very cold. There's a little bit of debate about whether or not the entire Earth was frozen solid, including the oceans, but everyone agrees it was very, very cold."
Macdougall says the evidence includes deposits of sediments characteristic of glaciation. They're found on all continents dating back to the Snowball Earth period.
"This was a period when all the continents were actually clustered at low latitudes, and we have good evidence that there were glaciers at sea level in the tropics at that time. And that means that temperatures were below freezing in the tropics at sea level, and quite possibly the entire ocean was frozen as well."
The global freeze might have had an impact on evolution. Land plants and animals had not yet appeared 600 million years ago, and marine life was made up only of small, primitive creatures. But Macdougall says larger, more complex forms of soft-bodied marine animals appeared shortly after the global freeze ended. Within a few million years, undersea life exploded, with a wide variety of shell-covered animals and even some with primitive skeletons.
Macdougall says it's not known whether Snowball Earth affected evolution. But he says similar outbursts of life have followed other ice ages. |