When Mr. Bak traced the lengths of time that ancient groups of animals had survived, he found a power law popping up in those data as well. Much of the time, life was relatively quiescent, and few species were going extinct. Occasionally, though, the bottom dropped out, and most species vanished. That's an indication that the system is at a critical state. When one species died off, it forced others to evolve or vanish, occasionally producing a cascade of extinctions.
Asteroids can provide the initial trigger for such cascades, but so can relatively minor events.
In 1997, another complexity theorist, Ricard V. Sole, extended that work by conducting a more rigorous statistical test of extinction data. Ms. Weil shook her head when she first read that work. Recuperating from ankle surgery, she was catching up on her pile of journals. As I was reading the paper, I thought instinctively that I disagreed with the conclusions, and I wasn't sure why I disagreed. Then I realized that they had done their study without a null hypothesis, which in science is not a safe thing to do.