Tiny diamonds may set earlier date for first life: carbon isotope ratios suggest biological activity.

Microdiamonds embedded in ancient zircons found in Western Australia suggest that life may have existed on Earth as early as 4.25 billion years ago.

Chemical analyses indicate that the mini-gems contain higher-than-average concentrations of the carbon-12 isotope, researchers report in the July 3 Nature.

Experts strongly debate whether that anomaly is evidence that life existed on Earth so soon after the planet formed, 4.6 billion years ago. If true, however, the findings would put life's earliest appearance at least 400 million years earlier than previously thought.

"If the light carbon signature is from life, then this is very big indeed," says Craig O'Neill, a geoscientist at Macquarie University in Sydney. "The trouble is, there are quite a few other mechanisms that can form light carbon signatures."

Zircons, tiny crystals of zirconium silicate, are hard, durable and chemically inert. The rocks that contain these crystals eventually erode, but often the crystals are incorporated into younger rocks, says Martin Whitehouse, an isotope geochemist at the Swedish Museum of Natural History in Stockholm. Small bits trapped when zircons cool, also known as inclusions, stay protected as the host rocks degrade over time. The zircons become tiny time capsules, holding clues about the environment.

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Trace elements in zircons enable scientists to determine their age. Whitehouse and his colleagues recently measured the ratios of carbon isotopes that make up micrometer-sized diamonds embedded within some of the oldest zircons on Earth. These zircons are found in the Jack Hills of Western Australia, the remnants of rocks that originally solidified about 4.4 billion years ago.

The researchers analyzed 22 micro-diamond inclusions and found that the tiny gems formed and were then incorporated into the zircons. The concentration of the heavier isotope, carbon-13, in some of the microdiamonds was about the same as that typically found deep in Earth's mantle, where natural diamonds form. However, concentrations in others were as low as 58 parts per thousand below normal.

"That's a very unusual carbon signature," Whitehouse says.

Since metabolic processes that take place in cells produce isotopically light carbon, a high concentration of carbon-12 is often a sign that the carbon was generated by biologic activity, Whitehouse adds. But he agrees that other mechanisms, such as inorganic chemical reactions--including those between carbon oxides, methane, hydrogen and water, which were all constituents in Earth's early atmosphere--can yield isotopically light carbon.

The carbon-isotope ratios found in the zircons "are among the lightest ever measured," says Steven Shirey of the Carnegie Institution for Science in Washington, D.C. Although the isotopically light carbon isn't a sure sign that life existed on Earth 4.25 billion years ago, it's impossible to discount the notion, he adds.