But there is no question that a cosmic interloper will hit Earth, and we won't have to wait millions of years for it to happen. Once a disaster scenario gets the cheesy Hollywood treatment, it's hard to take it seriously. But the next statistic on the list could be us. At present, we may worry about snail darters and red squirrels in abstract terms. The current rate of extinctions is, by some estimates, 10,000 times the average in the fossil record. Human activity is severely disrupting almost all life on the planet, which surely doesn't help matters. The odds of being one of the people to witness doomsday are highest when there is the largest number of witnesses around - so now is not such an improbable time. Or turn the argument around: How likely is it that this generation will be the one unlucky one? Something like one fifth of all the people who have ever lived are alive today. By pure odds, it's unlikely that we would be among the very first hundredth of a percent of all those people. If humans were to survive a long time and spread through the galaxy, then the total number of people who will ever live might number in the trillions. In 1983, British cosmologist Brandon Carter framed the " Doomsday argument ," a statistical way to judge when we might join them. Yet 99 percent of all species that ever lived have gone extinct, including every one of our hominid ancestors. It's difficult to imagine it all coming to an end. In the 500,000 years Homo sapiens has roamed the land we've built cities, created complex languages, and sent robotic scouts to other planets.
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