Mankind may have unleashed the sixth known mass extinction in Earth's history, according to a paper released by the science journal Nature.
Over the past 540 million years, five mega-wipeouts of species have occurred through naturally-induced events.
But the new threat is man-made, inflicted by habitation loss, over-hunting, over-fishing, the spread of germs and viruses and introduced species, and by climate change caused by fossil-fuel greenhouse gases, says the study.
Evidence from fossils suggests that in the "Big Five" extinctions, at least 75 percent of all animal species were destroyed.
Palaeobiologists at the University of California at Berkeley looked at the state of biodiversity today, using the world's mammal species as a barometer.
Until mankind's big expansion some 500 years ago, mammal extinctions were very rare: on average, just two species died out every million years.