Extinctions are more frequent now than in the 60 million years before people came along.

"This reinforces the urgency to conserve what is left and to try to reduce our impact. It was very, very different before humans entered the scene," said Jurriaan de Vos, a post-doctoral researcher at Brown University.

The current extinction rate is more on the order of 100 extinctions per million species per year revising the earlier figure of one extinction per million species per year.

The new study examined evidence from the evolutionary family trees phylogenies of numerous plant and animal species. Phylogenies, constructed by studying DNA, trace how groups of species have changed over time, adding new genetic lineages and losing unsuccessful ones.

By comparing that rise of the number of species from the yet unchecked speciation rate with the historical trend evident in the phylogenies, researchers created a predictive model of what the counteracting historical extinction rate must have been.

"In most cases, the main cause of extinction is human population growth and per capita consumption, although the paper also notes how humans have been able to promote conservation," said senior author Stuart Pimm, a Duke University professor.

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