WHATEVER THE OUTCOME of deliberations over snowball Earth, the theory has already served as a great stimulus and focus of the debate over the rise of animals, fostering many new ideas.
Patricia Vickers-Rich, a palaeontologist at Monash University in Melbourne, has switched from research on the origin of birds in past 65 million years to studying where animals came from, in part because of the excitement of working in a developing field.
"I'm just really pleased to be working in this area. It's fun to work in immature science, because you really feel that you might make a discovery that could make a fundamental difference in the way people look at things."
She believes the sudden appearance of mobile, complicated, multicellular animals at the end of the Neoproterozoic-era glaciations can be related to three events, or maybe a mix of them. One is an increase in oxygen in the atmosphere. The second is an injection of nutrients such as calcium and phosphates into the ocean, possibly due to the erosion of a huge Transgondwanan mountain range.
But the idea in which she is most interested at present is the freshening of the oceans as the ice melted. During significant glaciations the great bulk of freshwater is frozen as ice.
The majority of liquid water, underneath the ice cap in the oceans, would be highly saline, too salty for most marine life and certainly for multicellular animals. So Rich believes they would have had to retreat to less saline refugia, evolving and waiting for the great melt to make the oceans more habitable for them to rush out and colonise.
Another idea to emerge in the past couple of years – independent of snowball Earth but clearly a significant factor at the time of the rise of animals – is the formation and erosion of a huge Transgondwanan mountain range, between about 635 and 515 million years ago. A spin-off of the coming together of Gondwana, it would have been more than 8,000 km long and 1,500 km wide.
Evidence for its existence springs from the work of geologist Rick Squire, also at Monash University. He has studied the age of the mineral zircon within quartz grains in sands and sandstones all over the world. Wherever he went across what would have been Gondwana, he found a signal of rocks laid down within a narrow band between 650 and 500 million years ago.
And there is a pattern consistent with massive erosion during that time which moved about 100 million km3 of material into the oceans – enough to cover the entire contiguous part of Australia to a depth of about 10 km.
Squire believes this must have greatly increased levels of nutrients in the ocean, and that it also buried huge amounts of organic matter, allowing a build-up of oxygen in the atmosphere, necessary for complex animal life.
Wherever you stand on the snowball Earth idea, the glaciations of the late Neoproterozoic era provide a huge lesson in the power of the greenhouse effect, and the risks of modern climate change. It was real, it was fast, and it appears to have had a dramatic effect on evolution.
But snowball Earth itself is unlikely to happen again, says Hoffman, at least not in the near future.
"It gets more difficult because the Sun is getting closer and brighter all the time. Joe Kirschvink attributed the cooling to the fact that there were a lot of continents at low latitude, increasing albedo and weathering. So that's a very different geography from what we have today. We can't rule it out in the distant future, but that would be tens or hundreds of million of years away."
Tim Thwaites is a science journalist in Melbourne and the president of the Australian Science Communicators.
