The Cambrian period, roughly 500 million years ago, was marked by a seeming explosion of diverse multicellular organisms. For the past 10–15 years, the carbon-isotope signature in Precambrian limestone has been studied for clues to what prompted this burst of life.

Wild fluctuations in carbon-isotope values during the late Precambrian were interpreted as sudden, drastic changes in the global carbon cycle, such as methane releases or glaciations that could have altered ocean conditions to such extremes that an evolutionary burst was plausible. But new analysis suggests that photosynthetic organisms spread over land earlier than previously thought, and the subsequent rise in atmospheric oxygen was what triggered the Cambrian explosion (see page 728).

Paul Knauth, a geochemist at Arizona State University in Tempe, and Martin Kennedy, a geologist at the University of California, Riverside, came this conclusion while trying to understand an isotopic puzzle. Studies published in the past decade that plotted values of carbon isotopes in the Precambrian have ignored the oxygen isotope — a key omission.

Some 25 years ago Knauth and others documented how carbon-isotope signatures get recorded in limestone. As mineral deposits on the ocean floor form into limestone, and sea level rises and drops, coastal fresh water often enters the rock pores. This fresh water carries with it carbon from terrestrial vegetation, dissolved as bicarbonate. The carbon shows up in marine limestone together with the oxygen isotope specific to the water the carbon was dissolved in. The oxygen can be used to track the timing of any freshwater entry into rock.

Researchers used the fluctuations in the limestone carbon isotopes to suggest that dramatic changes had occurred in the global carbon cycle, but, because of his past research, Knauth knew that plotting carbon- and oxygen-isotope levels in marine limestone might give a more accurate picture of what the carbon-isotope variations meant. “Everybody who works on how limestones form agrees you can't look at carbon alone,” says Knauth.

Knauth joined forces with Kennedy, who has extensive experience examining rocks from all over the world. Together they studied thousands of published records of carbon-isotope studies and plotted carbon alongside oxygen values. They found that the oxygen and carbon isotopes of the Precambrian limestones were identical to those of more recently formed Phanerozoic rocks, which were known to have received a groundwater influx of photosynthetic carbon from plants.

“This means that an extensive greening happened in the late Precambrian times,” says Knauth. He is not sure exactly when the greening began, but no rock older than 850 million years shows this carbon signature.

Knauth is concerned that some respected colleagues may take issue with his challenge to the accepted paradigm, but he's comfortable with his interpretation. “Isotopes speak with a robust voice — that's why I like them,” he says. “To me, this is one of the most beautiful examples of isotope geochemistry.”

Knauth is currently in Australia looking for fossil evidence of photosynthetic organisms from about 500–800 million years ago. “The search is on for what organisms caused the greening of Earth,” he says.