ONE OF THE MOST TELLING images of our age was taken from Mars in May 2003. It shows the lonely, finite, comfortingly blue ball of Earth and its much smaller moon against a dark sky. Nothing else.
But if the picture had been taken between 600 and 800 million years ago, an influential group of Earth scientists argue, chances are it would have been even more stark – and in black and white. The familiar, friendly blue and green of ocean and continent would be replaced by alien white. Earth would look like a seemingly lifeless snowball hanging in space.
It was an era of global glaciation, these researchers say. Antarctica all over. There was ice at least a kilometre thick at the coastlines, much thicker in the continental interior, and extending right across the oceans.
At temperatures below -40°C, all moisture would have been frozen out of the air and the only clouds would be volcanic in origin. The only sounds would have been the eerie cracking of the creeping glacial ice and the occasional rumble of an earthquake or a volcano.
A little over 10 years after its first publication, in the U.S. journal Science, caused a flurry of debate, the 'snowball Earth' hypothesis still seems bizarre, and contentious. But, as I discovered at an international symposium on the topic, held at the University of Melbourne in September 2008, proponents of the theory believe it is on the way to evolving into an established scientific theory.
"The evidence is [now] a lot stronger," says geologist Paul Hoffman of Harvard University, in Cambridge, Massachusetts, the lead author of the original paper, and still an enthusiastic supporter of the hypothesis.
But doubters are concerned that the data they see does not fit.
"I can't make it work from my point of view," says Kath Grey, another symposium participant and chief palaeontologist at the Geological Survey of Western Australia, in Perth. She has been trying to match up the details of the story she reads in rock cores from Australia with what is being found overseas.
Snowball Earth is a creative idea that, like evolution by natural selection, the Big Bang and continental drift before it, provides an explanation for a set of disparate and unusual observations.
How is it that sediments of glacial origin directly abut rocks typically laid down in the tropics? Why is there evidence pointing to a stagnation of the oceans and the disappearance of almost all life between about 800 and 600 million years ago?
But the real excitement of snowball Earth, and the real source of controversy, is its power to explain one of the great mysteries of the fossil record – the appearance, almost overnight, of large, fully-fledged, multicellular animals, in great diversity.
First, around 575 million years ago, come the weird, mostly soft-bodied marine forms known as Ediacarans. Found in South Australia, Newfoundland, South China and northern Russia, most were attached to the ocean floor, and some reached up to four metres in length.
About 33 million years later, within a short (in evolutionary terms) period of only 20 to 30 million years, the ancestors of almost all modern animals appeared. This sudden burst of diversification is known as the Cambrian explosion.
