From: Australian Centre for Astrobiology
Posted: Tuesday, May 6, 2003
Scientists studying rocks near an ancient asteroid impact structure in South Australian have uncovered evidence that could change current theories explaining how life on Earth rapidly diversified about 580 million years ago.
Dr Kath Grey of the Western Australian Department of Industry and Resources' Geological survey and an ACA associate researcher, Prof Malcolm Walter, Director of the ACA and Dr Clive Calver of the Tasmanian Department of Mineral Resources challenge the idea that 'Snowball Earth' - an intense period of glaciation about 600 million years ago, triggered the evolution of simple life forms into more complex and familiar species.
In the May edition of the international journal Geology, Dr Grey and her team put forward an alternative radical idea that 580 million years ago an asteroid impact played a pivotal role in this evolutionary jump. The impact, known as the Acraman event, smashed a hole in South Australia about four times the size of Sydney.
Up until then, for the first three billion years of Earth's 4.5 billion year history, bacteria and simple algae had dominated life on Earth. "Then almost overnight geologically speaking, the ancestors of modern day animals and plants appeared in the fossil record about half a billion years ago," Dr Grey said. "The big question is what caused the rapid proliferation of life at that time?"
Research by other scientists suggests the evolutionary burst of life between 600 and 540 million years ago was the result of an intense period of global glaciation. However, if the findings of Dr Grey's research prove correct, the cause could lie beyond our planet.
Dr Grey, who has studied fossil plankton (single-celled green algae) from drill holes across Australia, has found that, as predicted by the Snowball Earth theory, bacterial mats and a few simple spherical species of plankton were the only organisms that managed to survive the intense ice age.
"As the sea level rose at the end of the ice age, these spherical forms increased in number," Dr Grey said. "But there is no sign of a new species emerging at the end of the intense ice age to support ideas of the rapid diversification of life at this time."
Dr Grey believes it wasn't until about 20 million years later more than 50 new and highly complex species suddenly replaced the small number of simple species in the fossil record.
"What is really interesting is that the more complex spiny fossils appear just above a layer of rock in South Australia associated with the Acraman impact," Dr Grey said.
In a related study, Dr Calver found significant carbon isotope changes mirrored Dr Grey's observations. Prof Walter has also noted that patterns associated with the Acraman impact were similar to those of mass-extinction and recovery events, and that a large asteroid impact could have produced conditions ideal for evolutionary change.
"Later impacts, like the 65 million year old Chixulub collision in Mexico wiped out a diverse range of species, including the dinosaurs," Dr Grey said. "But with the Acraman impact, there were only a small number of species around and the time to cause a mass extinction event.
"Most of the species that did survive were highly resilient, and had the ability to remain dormant through the cosmic winter that followed. When conditions improved, these species had an advantage over their competitors and were able to proliferate and diversify."
Dr Grey and her team have reasoned that the ensuing plankton diversification must have played a vital role in the subsequent development of the animals dependent on plankton as a food source.
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