BUT WHAT TRIGGERED the runaway glaciation that led to snowball Earth? One answer is the same as what caused the more recent ice ages only more so: a wobble in the Earth's orbit took our planet further from the Sun resulting in less intense solar radiation.
But this cooling typically self-corrects, as it slows down the weathering of rocks that would normally absorb CO2. Retention of the gas in the atmosphere leads to global warming, which prevents glaciation from taking hold.
But in the late Neoproterozoic era, Kirschvink suggested, there was one major difference – albedo, again. It turns out that the massive supercontinent Rodinia was breaking up, leaving a great conglomeration of land masses (those now found in the Southern Hemisphere) strung along the equator, which eventually formed the more well-known Gondwana. This increased the albedo of the Earth just where it generally absorbed most solar radiation, and thus lowered the Earth's average temperature further.
The latest supporting evidence for snowball Earth, says Hoffman, comes from work published in Nature in June 2008 by a group led by Huiming Bao of Louisiana State University, in Baton Rouge, on rocks from South China, West Africa and Svalbard (the group of islands containing Spitsbergen, between Norway and the North Pole).
Bao and his colleagues have discovered unusual oxygen isotope signatures in these rocks related to photochemical reactions in the stratosphere. "These could only be preserved at the Earth's surface under conditions of very high CO2 concentrations and almost no photosynthesis and respiration," says Hoffman. "That's exactly what you expect in the snowball Earth state."
Even so, Hoffman thinks the simplest evidence remains the most compelling for the snowball Earth hypothesis. "The idea that there was ice at sea level in the warmest parts of the world, so therefore the colder parts must have been frozen as well, is not going to go away. There is no doubt about tropical glaciation."
Although no one believes the oceans froze solid, whether there was a thick, unbroken, crust of ice is still a bone of contention. This has significant evolutionary implications, because such an ice crust would prevent photosynthesis in the ocean.
Snowball critics argue that this would make life extinct, but supporters say organisms could persist as deep-frozen spores, in pools formed in cracks and under the ice, and in the small areas around hydrothermal vents, such as hot springs, fumaroles and black smokers under the sea.
Life would certainly be put through the wringer, however, having to adapt to tough conditions in small, far-flung patches – a recipe for rapid evolution and speciation. Perhaps enough to prompt the emergence of varied mobile, multicellular creatures; the rise of the animals.
Compared to such a snowball Earth, which he favours, Hoffman says an incompletely frozen slushball Earth would be relatively benign and not as evolutionarily selective. Unfortunately, it's not a question easily settled one way or the other. "We don't have any evidence of what was going on in the open ocean because the ocean floor has all been subducted."
But, he says, the time gap between the last snowball event and the first evidence of multicellular animals in the fossil record has now been reduced to less than two million years. These dates come from microfossils of animal eggs and embryos from the Doushantuo formation in South China, and traces of organic compounds typical of sponges left in rocks in Oman. "There now seems to me to be a much stronger case that these extreme climate events have something to do with the first appearance of multicellular animals."
