Early birds

Rocks -- 3.85 billion year old --show that battered by meteors and supposedly sterile, ancient Earth still harboured life-forms

Published: Friday 31 January 1997

life existed on Earth 350 million years earlier than everyone predicted, says a researcher in California, us. It was a time when the planet was still being bombarded by meteors. Gustaf Arrhenius of the Scripps Institute of Oceanography in San Diego, us, claims that 3.85 billion-year-old rocks from Greenland contained a mixture of carbon isotopes that only living organisms produce. The ratio of carbon-12 and carbon-13 isotopes is known to be a sensitive indicator of life (New Scientist, Vol 152, No 2055).

Arrhenius used an ion microprobe to examine rock samples as small as 10 micrometres to prove existence of life 350 billion years ago. He focused on minute grains of a phosphate-rich mineral called apatite, formed from remains of living things. Crystallisation leaves small, carbon-rich inclusions embedded in the apatite. Each inclusion contained only about 20 trillionths of a gram (20 picogram) of carbon, but it was enough to measure isotope ratios.

The work suggests that life evolved on Earth as soon as environmental conditions allowed. Studies on isolated zircon crystals were done. It showed that a solid crust existed 4.2 billion years ago, but no intact rocks were discovered older than 3.96 billion years. The Moon was heavily bombarded by meteors until about 3.8 billion years ago, and the Earth probably too suffered the same fate, which some have argued could have sterilised the planet. But Arrhenius and his colleagues claim that their work shows that such a bombardment did not lead either to the extinction of life or the disruption of the finely laminated rock formations.

John Hayes of the Woods Hole Oceanographic Institution, Massachusetts, us, says that Arrhenius's measurement of isotope levels in such minute samples of carbon will open the door to further studies of early life, particularly in rocks which contain microfossils.

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