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Rock of agesAs part of an international team, Monash scientists have found evidence that conditions for life to form on earth existed 400 million years earlier than previously thought, writes JUNE YU Life needs an atmosphere and an ocean system – they absorb and reflect some of the sun’s UV radiation that would otherwise destroy living cells. But when did our earth develop an atmosphere and ocean system sophisticated enough for life to form and survive? Monash Professor Ray Cas , from the Department of Earth Sciences, says one way of determining when life began is by studying fossils. The planet itself is believed to be just over 4.5 billion years old, and the oldest known fossil remains of any type – dated at 3.5 billion years old – can be found in the Pilbara region of Western Australia.
According to Professor Cas, the Monash researchers’ goal was to evaluate the nature of the earth’s earliest known surface environments. "These rocks gave us the best opportunity to see if the earliest known conditions on the earth’s surface were conducive to supporting life," he says. Professor Cas and Dr Beresford battled hostile conditions and flew by helicopter to reach the Isua Greenstone Belt (IGB). The area is accessible only for a six-week period during July and August when the ice recedes far enough to allow exploration. "We spent two weeks documenting the characteristics of the volcanic and sedi-mentary rocks and determined that both rock types formed in deep water conditions," Professor Cas says. This means that conditions would have been favourable for life to form on earth at that time, he says. The volcanic rocks in the IGB consisted of basalt pillow lava, which only forms underwater – strong evidence of an ocean system. Furthermore, there was no evidence to indicate that any volcanic eruptions occurred, which is consistent with the existence of water deep enough to suppress eruptions and further evidence of a developed ocean system. As well, the sedimentary rocks in the IGB consisted of two principal types – deep water sand layers called turbidites and banded iron formations. Turbidites are produced by very short-lived, avalanche-like currents in deep water – further evidence of a deepwater ocean system. Banded iron formations consist of alternating ferric oxide rich and poor layers. The ferric oxide rich layers need high levels of free oxygen, which is produced by photosynthesis. The presence of this type of rock suggests that photosynthetic organisms had evolved by 3.8 to 3.9 billion years ago, evidence of an atmosphere capable of sustaining life, Professor Cas says. He says his work in Greenland advanced earlier results of the Danish team, and his findings will be presented at an international symposium in Perth in early September. His research is also consistent with a recent finding in north-west Australia of a zircon mineral grain that has been dated at 4.4 billion years old, indicating that the earth already had a solid crust at that time. This combined data now indicate that the solid earth and its oceans and atmosphere developed much earlier than previously thought, Professor Cas says. ACTION: For further information about research in the Department of Earth Sciences, visit www.earth.monash.edu.au/Department/research.html
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But another way of establishing the origins of life is through
investigating the composition of the earth’s oldest rocks. Last year,
Professor Cas and postdoctoral fellow Dr Steve Beresford joined an
international team of scientists, led by Danish researchers, to analyse
rocks in the Isua Greenstone Belt of Greenland. Using lead isotope
dating methods, scientists have estimated these rocks are up to 3.9
billion years old.