Carbonaceous compounds found in sedimentary rocks were laid down by mats of photosynthetic organisms living in shallow seas 3.4 billion years ago, according to a paper in Nature this week. But the article does little to quell the debate over whether the origin of these compounds is biological, or simply the result of chemical reactions at the time.

Michael M. Tice and Donald R. Lowe, from Stanford University, California, believe their paper demonstrates unequivocally that organic matter found in the Buck Reef Chert in South Africa—one of the oldest sedimentary areas in the world—was the result of activity of living organisms, and not abiotic hydrothermal processes.

"Chert is a microcrystalline form of quartz," explained Tice, "and in the pictures, all the white stuff you see is chert. All the black stuff you see is carbonaceous mineral which is organic matter produced by organisms and which has since been heated...

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