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RISING levels of carbon dioxide could speed up evolution. It seems that the higher the levels of CO₂ in the atmosphere, the faster new species appear.

University of Kansas researchers modelled how the amount of CO₂ in the atmosphere has changed over the past 545 million years. Levels of this greenhouse gas used to be up to 20 times as high as today.

Then they turned to the fossil record to work out the rates at which species of marine animals appeared and disappeared over the same period. When they compared the variation in CO₂ with the fossil data, they saw a close match between high CO₂ levels and more new species.

Bruce Lieberman, a member of the Kansas team, says the correlation is striking. 鈥淭he two curves match each other extremely well,鈥� he says.

Daniel Rothman of the Massachusetts Institute of Technology had already sugg-ested that levels of atmospheric CO₂ over the past 400 million years were linked to the amount of biological diversity. But instead the new work has found a better match with the rate at which new animal species were appearing and disappearing. It also extends the study back in time to encompass almost the entire history of animal life, including the initial burst of animal evolution known as the Cambrian explosion.

The researchers aren鈥檛 sure how CO₂ levels could affect evolution. But if it is directly responsible for encouraging the formation of new and distinct species, or speciation, they say it鈥檚 conceivable that the rise in atmospheric CO₂ predicted for this century might encourage new species to emerge, increasing biodiversity. But not for a while: any increase in the rate of evolution will not be enjoyed by our children or grandchildren but by whatever is inhabiting the planet 5 to 10 million years down the road, says Lieberman.

It鈥檚 also possible that other factors, such as plate tectonics, could be influencing both evolution and levels of CO₂. When continents break apart, the geological activity not only releases massive amounts of CO₂, but could also drive speciation as populations become isolated. Either way, the researchers interpret the results as evidence that the planet is acting as a pacemaker for evolution.

But not everyone is convinced. While welcoming this new approach, Arnold Miller of the University of Cincinnati says the fall in average rates of speciation and extinction in marine animals since the Cambrian explosion could just as easily be explained by the disappearance of animals that evolve fast, such as trilobites.