Approximately 65 million years ago, when the dinosaurs went extinct, 75% of the planets many species went with them.  It was the world's fifth, and most recent, mass extinction event. . 

Now, according to the journal Science, the Earth seems to be at the cusp of a sixth mass extinction. Only this time, an asteroid is not to blame. We are.

"Human impacts on animal biodiversity are an under-recognized form of global environmental change," the team of ecologists and biologists warn in Science. "Among terrestrial vertebrates, 322 species have become extinct since 1500, and populations of the remaining species show 25% average decline in abundance." 

A third of all vertebrates, the scientists write, "are threatened or endangered."ne driver of defaunation. Diseases that come from pathogens introduced by humans are another growing threat.