The Mystery of the Megafauna Over 30 species of North American mammals – including the mastodon, giant beaver and saber-toothed tiger – became extinct within a few thousand years. Why? “The most widely held hypotheses offered for the Ice Age extinction fall into two general categories, one of which has to do with environmental change, and the other of which has to do with the appearance of a new predator—i.e., humans,” says Dr. Richard Tedford, curator of Vertebrate Paleontology at the American Museum of Natural History. Dramatic changes in climate and vegetation following the retreat of the continental glaciers 10,000 to 12,000 years ago are the primary components of the environmental change argument. “Over a period of just a few thousand years, there was very rapid warming, rising sea levels, and retreat of the glacial front,” points out Dr. Tedford. Many habitats changed, and some simply disappeared. “For animals that were large-bodied and slow-reproducing, this was quite something to contend with.” Meanwhile, archaeological evidence shows that humans were spreading throughout North America at about this same time. “Thus you have superimposed on the climatic changes the arrival of a new predator—a very adaptable predator that lived through all these changes and could shift from prey to prey when conditions were bad,” Dr. Tedford continues. Proponents of the so-called overkill hypothesis feel people were the determining factor. They use statistical models to show that human populations could indeed have grown swiftly enough to decimate the animals and doom the megafauna to extinction. What is Dr. Tedford’s position on the extinction debate? “Because there is such a coincidence between the extinctions and the appearance of human hunters, I think humans were an element—but not the element.  The other element was climatic change.”  He advocates a more balanced view that incorporates changes in vegetation, rearrangement of habitats, low reproductive rates, the spread of humans, and even factors that have yet to be studied more fully, such as the effects of disease. “All of these things were part of the dynamic of extinction.”