A close-up of one of the stromatolite layers
Image: Allen Nutman
Structures discovered in 3.7-billion-year-old rocks in Greenland, which appear to be evidence of microbes living in a shallow sea on early Earth, could be the earliest fossils ever found. The structures are no more than a few centimetres tall and look like stromatolites, which are layered mounds that were – and still are – formed by photosynthetic microbes living in water. The fossils come from a region of south-west Greenland called the Isua supracrustal belt and their age makes them about 220 million years older than any previous fossil found.
“This is one of the extremely few places where this kind of feature could still be preserved in the rock record,” says Allen Nutman at the University of Wollongong in Australia, who led the team. Most rocks as old as this have been heavily altered by heat and the effects of plate tectonics, geological processes that tend to obliterate fossils or leave them warped beyond recognition. However a small section of the Isua region has escaped complete transformation. Its rocks have been only partially altered, and this has allowed the area to retain some of its original characteristics, possibly including fossils, Nutman says.
Nutman and his colleagues found their possible stromatolites at two locations where the bedrock is now exposed, although in past years it was covered by snow all year round. Laboratory analysis of samples showed that the structures have the same chemical signature as sea water, pointing to a marine origin,and their shape – bulging up from the sediment layer below, with hints of layers visible in the rock – mirrors that of modern stromatolites.
However, some doubts remain. The main problem, says William Schopf at University of California Los Angeles, is that the Isua rocks contain no fossil remains of microbes. “To seal the case, you’d really like to have the microorganisms preserved, but that’s not possible in this type of rock in that sort of setting.”
If the rocky formations are stromatolites, the microbes that created them were likely part of a larger community that had already evolved on Earth, Nutman says. What we have in Isua is just a tiny sample of any life that may have been around at that time,” he says.