Earliest Earthlings

Chemical signatures in rocks unearthed in Greenland suggest that life began nearly 4 billion years ago--about 400 million years earlier than the oldest known fossils, says a report in the 7 November issue of Nature.

A team led by Gustaf Arrhenius and Stephen Mojzsis of the Scripps Institution of Oceanography in La Jolla, California, collected samples from a 3.85-billion-year-old layer in the Isua rock belt in western Greenland and from nearby Akilia Island--two of the world's oldest known sediment beds. Using a tool called an ion microprobe, they measured the ratio of isotopes of carbon in apatite, "a mineral that acts as a protective capsule, preserving such chemical traces of life," says Arrhenius. The researchers found an isotopic ratio skewed toward carbon-12 and carbon-13, which is "characteristic of biological debris," says John M. Hayes, senior scientist at Woods Hole Oceanographic Institution.

Popular theories suggest that a constant meteorite bombardment kept Earth sterile from its formation--some 4.5 billion years ago--until about 3.9 billion years ago, roughly the time the rock sediments in Greenland were formed. If so, the carbon ratios indicate that life originated at about the earliest possible time on Earth.

Scientists caution, however, that the findings aren't proof positive. The sediment samples are "badly preserved," because the Greenland rocks were exposed over the eons to heat and high pressure, says Hayes, who nevertheless believes that some unidentified life-form is the most likely explanation for the isotopic ratios. "It's difficult to tell what kind of life it was," says Mojzsis. "We can speculate that it was bacteria, but we don't know what kind," he says.

The next step will be to test the ion microprobe on a variety of other ancient samples. After that, Hayes says, researchers studying martian meteorites and ancient microfossils "should start getting in line."