Over the past half-billion years, there have been five major mass extinctions, with the dinosaurs wiped out in the most recent of these. We now face the sixth mass extinction, which threatens to tear apart the fabric of the biosphere, with drastic consequences for most life on this planet, including us. In better times, species losses tick along at a barely discernable rate—perhaps one every five years. At present, somewhere between 50 and 150 species disappear every day, never to be seen again. (Once again, uncertainty in the actual value comes mostly from a lack of basic knowledge about how many species exist.)
 
This time around, a single species—Homo sapiens—has become the external force driving the decimation of millions of other species. Yes, we are the asteroid now colliding with the planet.

The list of anthropogenic factors is all too familiar, among them habitat destruction, overhunting, toxic pollution, and climate change. In particular, the duo of global warming and environmental destruction has eradicated habitats at a pace far exceeding the abilities of many species to track or evolve. At the current rate of extinction, about half of all species alive today may be extinguished by the close of the 21st century, an eco-evolutionary experiment not run since the end of the Cretaceous. Paleontology teaches us that the biosphere takes up to 10 million years to recover from a major mass extinction. So the decisions we make today will have cascading consequences well into the unimaginable future.