BOSTON — Wielding a unique high-tech probe to explore slices of ancient rock, scientists have found the oldest evidence yet of primitive life on Earth.
The discovery pushes back by at least 300 million years the earliest known traces of living organisms. The results also suggest that the tiny life forms were hardy enough to withstand a punishing meteor bombardment Earth endured more than 3.8 billion years ago, the researchers say. The meteor onslaught is thought to have sterilized the young planet.
As scientists gain more experience with the new probe, it could help provide details on living conditions for microbes on early Earth and offer a more precise means of unearthing geochemical signs of past life in meteorites.
"Our evidence establishes beyond a reasonable doubt that life [on Earth] emerged at least 3.85 billion years ago," says Stephen Mojzsis, a graduate student at the Scripps Institution of Oceanography in La Jolla, Calif., and lead author of a research paper in the latest issue of Nature. "We may well find that life exists even earlier."
Until now, 3.5 billion-year-old bacteria fossils found in western Australia represented some of the earliest evidence for life on Earth. William Schopf, a paleobiologist with the Center for the Study of Evolution and the Origin of Life at the University of California at Los Angeles (UCLA), discovered and analyzed these fossils over a seven-year period and reported his results in 1993. He uncovered eight new species and noted that the microbes' diversity and physical complexity suggested that their ancestors must have been much older than 3.5 billion years.
The evidence Mr. Mojzsis's team uncovered comes not from fossils, but from the chemical makeup of samples taken from a 3.85 billion-year-old rock formation on Akilia Island, off of southern West Greenland. The samples, once ancient ocean sediment, changed dramatically under immense pressure and searing temperatures born of geological change.
BUT it did encase carbon in tiny calcium-phosphate crystals, known as apatite, which formed as the rocks underwent change. The apatite, which can come from biological or nonbiological processes, was a significant clue, but the carbon held the key.
"Life processes produce a distinct signature" by splitting carbon into two different forms, or isotopes, explains Mark Harrison, a UCLA geochemist and member of Mojzsis's group, which included Britons and Australians. The signature lies in the ratio between the two isotopes, which differs for biological and nonbiological sources.
To determine the ratio, Dr. Harrison and his team used a new device called an ion microprobe to measure the chemical content of tiny portions of a sample without destroying it. Until now, he says, researchers have had to dissolve rock samples in acid and treat them with chemicals to get rid of contaminants, before using other equipment to decipher a sample's composition.
The microprobe shoots cesium ions into the sample. Aimed at a tiny carbon-hugging apatite crystal, the cesium ions are used to knock carbon ions free. The carbon ions are then analyzed.
"This is as good a job as any mortal could do" in building a case for 3.8 billion-year-old life forms, says John Hayes, of the Woods Hole Oceanographic Institution in Woods Hole, Mass. "The evidence will always be thin - it's in the nature of the rocks. They're so old and so chewed up, it's like trying to extract information from a shredded newspaper buried in a landfill for years."
Nor does he hold out much prospect for geologists finding a fossil-friendly formation that old. But he notes that if the new microprobe lives up to its potential, researchers could turn its ions on the Australian or other microfossils.
"With some clever deductions, it might be possible to reconstruct the microbial ecosystem and figure out the details of the ancient microbial community," he says.