Over the last several billion years, the sediments of the earth have been raised and lowered, cooled and heated, twisted and straightened. Any possible fossil traces of life from such dim reaches of time have become extremely difficult to distinguish from the surrounding rock. Indeed, all of the oldest putative signatures of life are smudged and disputed. But a new analysis claims to significantly bolster the theory that some peculiar rocks from Australia contain some of the first signs of life.
Abigail Allwood of Macquarie University and her colleagues studied a 10 kilometer stretch of rock in Australia's Pilbara region identified nearly 30 years ago as the remnants of stromatolites. These distinctive, layered mounds are the result of colonies of cyanobacteria or other microorganisms capturing and cementing sediment and sand. But some researchers have challenged that theory, noting that chemical activity around hydrothermal vents could produce similar structures.
Allwood's team undertook a comprehensive survey of the disputed stromatolites, detailing the variety of shapes contained therein, the ancient setting revealed by the rocks as well as any similarities to present day microbial mats. They found more than seven types of rocks with different features corresponding to how the rock particles were deposited. Some have a surface resembling egg-carton ripples; others take the shape of large cones.
The researchers argue that it is unlikely that purely inanimate processes produced this wide variety of markings in one, relatively small, area. Additionally, the cone-shaped formations show signs of grains being cemented to their sides, suggesting that microbes fixed them there. Areas between these cones, on the other hand, appear to contain only sediment deposited by waves in a shallow sea.
Because this fossil layer rests between two volcanic layers, it can be precisely dated to 3.43 billion years old. Thus if Allwood and her collaborators are correct, the stromatolites rank among the oldest signs of life on the earth. "What is so striking about their scenario is that it is so 'normal'--in the sense that transgressive, tidally influenced carbonate platforms with stromatolites are common in the geological record. Stromatolites are also known to form in similar environments today," argues Stanley Awramik in a commentary accompanying the paper in today's Nature. "This is not the first report on stromatolites from this formation and it probably won't be the last."