You don't have to look all the way back to the dinosaurs to learn a valuable lesson about the fine line between global extinction and survival. A recent study of mitochondrial DNA has shown that the total human population on earth could once, fairly recently, have fit comfortably into your average-sized Broadway theater.

Because of a series of droughts in Africa a little over 70,000 years ago, the study shows, only about 2,000 or so human beings survived, clustered together in small groups. This may be the closest the human race has ever come to extinction; fortunately, the huddled survivors were able to repopulate the species to the extent that human beings soon began migrating out of Africa to other continents.