This matters because if extinction were truly random, we'd have a much richer evolutionary history, because at least some representatives of all living things would make it to the present. But because extinction tends to be clumped around certain lineages, when extinction occurs we lose whole groups of species. "The long-term consequences are therefore much worse for biodiversity," says Roy.

The Science study will provide needed ammunition for modern-day conservationists as well. We're in the middle of what some scientists have begun to call the sixth great extinction event, this one caused almost entirely by human beings. Human expansion, hunting, deforestation and ultimately climate change are eliminating species at a rate up to 1,000 times higher than the evolutionary norm. Species like the Yangtze River dolphin and the golden toad have disappeared, while a range of animals — from the Sumatran tiger to the silky Sifaka lemur of Madagascar are on the brink.

Since extinction tends to target groups of vulnerable species, conservationists would be smart to identify and focus their efforts on the most susceptible families. That means species that have a narrow geographic spread — always a risk factor for extinction, in case something happens to their habitat — and, interestingly, large body size, which also tends to be associated with extinction. "It's a quick and dirty way to get a better picture of which species are likely to be most impacted," says Roy. "Then you can go in and mark your priorities." Extinction may be a part of life, but as the dominant species, we have the ability to influence it — for better or for worse.