The oldest fossils known to date - 3,700-million-year-old microbial formations from Isua, Greenland - are described in a study published in Nature this week. These fossils predate the current earliest fossil evidence for life on Earth by 220 million years.

Allen Nutman and colleagues discovered 1-4-cm-high fossil stromatolites - sedimentary formations created by the layered growth of microorganisms - preserved in metamorphic rocks in Isua, southwest Greenland, that have previously been dated to about 3,700 million years old. The fossils, which were exposed by the recent melting of a perennial snow patch, are thought to have been deposited in a shallow marine setting. Several lines of evidence, such as details of the chemistry, sedimentary structures and minerals in the rocks, together indicate that the stromatolites were formed by live organisms.

These findings are consistent with previous genetic molecular clock studies that place the origins of life over 4,000 million years ago.