WAS complex life triggered by an asteroid impact? That is the claim of Australian researchers, who are challenging the 鈥淪nowball Earth鈥� explanation, which says a series of mega ice ages kick-started the evolution of complex life forms.
For most of Earth鈥檚 history, simple single-celled organisms predominated. So the sudden appearance of complex multicellular creatures has long perplexed evolutionary biologists. The most popular explanation is that a series of extreme ice ages that struck roughly between 750 million and 590 million years ago 鈥� the Snowball Earth era 鈥� destroyed most of the simple organisms around and created new ecological niches that encouraged the proliferation of more sophisticated creatures.
There isn鈥檛 any direct evidence for this, but a few fossils of early Ediacarans 鈥� the first known multicellular animals 鈥� started to appear just 5 million years or so after the end of Snowball Earth. And it was the only plausible explanation around, until now.
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A team led by Malcolm Walter of the Australian Centre for Astrobiology at Macquarie University in Sydney has been studying kilometre-long core samples of local rocks, containing hundreds of delicate fossils dating back 850 million years. The team found that single-celled green algae, or phytoplankton, survived Snowball Earth (Geology, vol 31, p 459). More complex algae did not appear until at least 15 million years after the glaciers melted 鈥� too late to fill new niches created by the freeze, says team member Kath Grey of the Geological Survey of Western Australia.
But the timing coincides perfectly with a well-documented hit by a 5-kilometre-wide asteroid that blasted out Lake Acraman in South Australia 580 million years ago, leaving clear traces in the core samples. 鈥淎fter the impact, the phytoplankton suddenly increase in size,鈥� says Grey. 鈥淭here are more of them, and they are more complex.鈥� Some 60 new species appear in the core at that time, resembling certain modern algae with spines and a hard outer coating. The Acraman asteroid would have spawned huge dust clouds that blotted out the Sun for months, creating environmental mayhem 鈥� ideal conditions for the diversification of life, says Grey.
Other recent findings also weaken the idea that the Snowball triggered the evolution of complex creatures. Fossils in California鈥檚 Death Valley dating from one of the Snowball鈥檚 early ice ages show that single-celled organisms expected to be wiped out actually survived unchanged (Proceedings of the National Academy of Sciences, vol 100, p 4399).
But not everyone is convinced. Diversification happened too soon after the asteroid impact for evolution to have wreaked significant changes, says Jim Gehling of the South Australian Museum in Adelaide. 鈥淭here must be some lead-up time. The timing still supports Snowball.鈥�
It should be relatively easy to settle whether the asteroid was responsible. 鈥淚t鈥檚 a great idea, and very testable,鈥� says Harvard geologist Paul Hoffman, who proposed the idea of a Snowball Earth with his colleague Dan Schrag in 1998. 鈥淚f you find one place where there are big spinies before the impact layer, the hypothesis is wrong.鈥�