The prevailing view in 1976 blamed man. The book quotes Charles Darwin, "What havoc the introduction of any new beast of prey must cause in a country, before the instincts of the indigenous inhabitants have become adapted to the stranger's craft or power." The theory is that man traveled over the Bering Straight and hunted the North American megafauna to death. Like all theories, this is based on observable data, namely the increasing sophistication of the Clovis spearpoints found in archaeological digs. But this brings up so many questions. Why wasn't there a die-out of megafauna in Africa as humans learned to make tools? Does anyone have an example of an animal that did adapt to human hunting techniques? And were there enough Clovis people spread throughout the entire continent to kill off 33 distinct types of large mammals? If so, how did the bison survive?
Today, some scientists believe that the large animal populations were already weakened by the time the Clovis hunters came on the scene. Disease and climate change could be accomplices. A really interesting idea is the second-order predation theory: as humans killed off carnivores, herbivores became overpopulated and overgrazed the land, creating boom-or-bust cycles for the vegetation, prey, hunters, and everyone involved. Unfortunately, this doesn't explain why bears and jaguars didn't regulate herbivore populations.
Finally, some scientists propose a comet theory, because that's what scientists do. A bunch of species die? Let's talk giant explosion! In 2009, evidence was found of large comets falling in North America, but nobody's found the gigantic crater you might expect from such a disaster.
The Illinois State Museum now states on their fun and excellent website "scientists do not know for sure" why these animals went extinct. We thought we knew in 1976, but now we don't. We have better, more sophisticated tools at our disposal to find new observable evidence, like the comet evidence. It's like questioning cold cases with new DNA technology! Which is cool, I think. And I can have my opinion, but in the end, my opinion doesn't matter. Science is fact. Just because we're no longer sure what happened doesn't mean there isn't one answer for us to discover.