Looking for life in the layers.

Some of the earliest evidence for life on Earth comes from peculiar dome-shaped rocks called stromatolites, which go back 3.5 billion years.

Geologists have long surmised that ancient microbial mats played a role in constructing stromatolites, but a new analysis shows that some of these deposits may be nonbiological structures unrelated to life.

The standard explanation for stromatolites holds that sticky microbial mats on the ocean floor captured particles of sediment and then covered them with a new layer of microbes, gradually building up large sedimentary domes. Modern stromatolites off the west coast of Australia grow this way.

John P. Grotzinger and Daniel H. Rothman of the Massachusetts Institute of Technology challenged this theory after studying 1.9-billion-year-old stromatolites from northwestern Canada. The scientists sliced through the formations and analyzed the thickness of calcite crystal layers within the rock.

Grotzinger and Rothman developed a mathematical model that includes only non- biological processes, such as chemicals precipitating out of solution and sediments falling to the ocean floor. This model could account for the types of layers within the Canadian stromatolite, they report in the Oct. 3 Nature.

"Our result demonstrates that the morphology of at least some, and perhaps many, types of stromatolites may be accounted for exclusively by abiotic mechanisms," they suggest.

Even without stromatolites, though, the evidence for life 3.5 billion years ago remains secure. Paleontologists have recovered fossils of cyanobacteria from that age, indicating that life evolved even further back in time.