Based on evidence that an asteroid impact helped to reduce the dinosaurs to dust 65 million years ago, scientists have reasoned that other large impacts might produce similar extinctions — and that humans could be next on the hit list. But John Alroy of the University of California at Santa Barbara finds that life may be surprisingly resilient. He examined the size and ages of major craters in North America and compared them with the mammalian fossil record over the past 65 million years. Contrary to the predictions of one prominent extinction model, known as Raup's Kill Curve, Alroy could detect no correlation between impact size and the rate of extinction. He argues that life is far more tenacious than some scientists make it out to be. Furthermore, mass extinctions are very unusual, he says, and are rarely caused by a single catastrophic event. They are much more likely to result from slower, less dramatic processes such as species migration, climate change, competition, and disease.