Older, not Better

Tuesday, April 01, 1997
The oldest fossils, in 3.5- billion-year-old rocks from Australia, indicate that life was present on Earth about one billion years after the planet itself formed. Now geochemists Stephen Mojzsis and Gustaf Arrhenius, of the University of California at San Diego, and their colleagues have pushed back the date for life’s debut even further. They believe they have found unmistakable traces of life in the oldest known sedimentary rocks on Earth, from Akilia Island on the southwest coast of Greenland. The Akilia rock contains microscopic, crystal-encased grains of carbon of a type usually produced only by living things.

Geochemists can tell whether ancient rocks once hosted life, even in the absence of visible fossils, simply by looking at the carbon in them. That’s because carbon comes in two varieties, or isotopes: a heavy form called carbon 13, and the lighter carbon 12. Because living organisms concentrate the lighter isotope in their tissues, life’s distinctive signature is usually detectable in sedimentary rocks enriched with carbon 12.

The rock the researchers studied contained cell-size grains of a mineral called apatite, a component of all organisms. They suspected the apatite might be a marker of ancient life, so they sliced open about 50 grains and looked at them under an electron microscope. The grains, the researchers found, turned out to have carbon cores.

To determine the carbon’s source, the researchers made use of an ion microprobe. They aimed a beam of intensely energetic cesium ions at each apatite grain. The cesium beam vaporized the carbon into a plume of charged ions that they captured and separated by mass in a magnetic field. They found that each grain contained on average 3 percent more carbon 12 than an inorganic origin would allow. There is no known inorganic process on Earth that can mimic this isotopic signature, says Mojzsis.

If the researchers are right, their findings push back the origin of life on Earth by about 350 million years. It appears from our findings, says Mojzsis, that life was doing just fine at least 3.85 billion years ago.
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