After more than a decade, these questions remained unanswered. The debate raged on - until August 1993, when, at a weekly seminar of the Santa Fe Institute, an iconoclastic mathematician named Ian Malcolm announced that none of these questions mattered, and that the debate over a meteoric impact was "a frivolous and irrelevant speculation."

"Consider the numbers," Malcolm said, leaning on the podium, staring forward at his audience. "On our planet there are currently fifty million species of plants and animals. We think that is a remarkable diversity, yet it is nothing compared to what has existed before. We estimate that there have been fifty billion species on this planet since life began. That means that for every thousand species that ever existed on the planet, only one remains today. Thus 99.9 percent of all species that ever lived are extinct. And mass killings account for only five percent of that total. The overwhelming majority of species died one at a time."

The truth, Malcolm said, was that life on earth was marked by a continuous, steady rate of extinction. By and large, the average lifespan of a species was four million years. For mammals, it was a million years. Then the species vanished. So the real pattern was one of species rising, flourishing, and dying out in a few million years. On average, one species a day had become extinct throughout the history of life on the earth.

"But why?" he asked. "What leads to the rise and decline of earth's species in a four-million-year life cycle?

"One answer is that we do not recognize how continuously active our planet is. just in the last fifty thousand years - a geological blink of an eye - the rain forests have severely contracted, then expanded again. Rain forests aren't an ageless feature of the planet; they're actually rather new. As recently as ten thousand years ago, when there were human hunters on the American continent, an ice pack extended as far down as New York City. Many animals became extinct during that time.