There's no doubt any longer. The Earth has entered the sixth great extinction in its 4.5-billion-year history. Species are disappearing 100 times faster than the background rate — normal rate between mass extinctions — a pace unparalleled since the dinosaurs died out 66 million years ago. And the reason is not asteroids, volcanic eruptions, or massive tectonic movements. It is us — human beings. 

Five mass extinctions have thus far been recorded in the Earth's geological history. Proof that a sixth is upon us lies in showing that current rates of extinction are above the 'background' rate prevailing in the previous five extinctions. New research by biologists at multiple North American universities, published last week in Science Advances, shows that even assuming a highly conservative background rate of 2 mammal extinctions per 10,000 species per 100 years (2E/MSY), the average rate of vertebrate species loss over the last century has been up to 114 times higher than the background rate. By the 2E/MSY rate, it should have taken, depending on the vertebrate taxon, 800-10,000 years for the same number of species to go extinct.