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Extinction of megafauana was caused by overhunting by Paleoindians.

Sorry to be so long in responding. Overhunting by paleoindians is the conventional explanation and it may be correct but I have serious doubts. First, there's Africa where some experts say humans have lived longer than anywhere else and there are still plenty of megafauna around. There are still lots of large mammals in parts of Asia, too, where humans have lived a very long time. Why did such alleged overhunting occur only in places like the Western hemisphere, Australia and partially in Europe (fewer extinct species)?

There is certainly evidence that paleos hunted big mammals like mastadons. Yet when I put myself in their place -- armed with a stone-tipped spear and maybe an atlatl and lots of false bravado -- I ask why in the world I'd preferentially go after something utterly ferocious like a mastadon, mammoth, sabertooth tiger, etc. instead of a critter under 250 pounds, most of which didn't go extinct anywhere.

The disappearance of big mammals in Australia occurred between 40,000-20,000 years ago (dating is imprecise), and there’s evidence humans have been there for as much as 50,000 years. There's absolutely no physical proof the extinct large Aussie animals were hunted by people. Maybe they were but why aren't any spearheads or other human tools associated with animal remains as in Siberia and the U.S.?

Maybe the paleos used wasteful hunting methods like driving megafauna off cliffs and cutbanks. But how do you herd giant ground sloths, monster cats, grouchy giant bears and mega-kangaroos? Even if they did, why did the extinctions occur across the board and involve mammals of only a certain size? Even the Amerinds, who hunted buffalo by driving them off cliffs, didn’t succeed in diminishing their numbers by much. That was left to European colonists armed with rifles who were determined to eliminate the buffalo as a source of Indian food, fur and sinew. They nearly succeeded where untold generations of more primitive hunting methods failed.

I don’t have an answer or even a pet theory yet, but I don’t think hunting by paleoindians is the only answer.

The large mammals in North and South America either evolved in this hemisphere in the abscence of humans, or migrated over from Eurasia well before humans. They weren't accustomed to humans, and lacked defensive strategies to cope with them. It wouldn't have been to difficult to bring down mastadons with stone-tipped spears; African bushmen do it occasionally with tiny arrows, that have been dipped in poison; they soot up into the animal's soft underbelly, and then track it for several days until it dies. Large megafauna could have been hunted in a similar fashion; spear the animal in the intestines, then track it for several days until it dies of peritonitis. Large bears would probably have been located in their hibernation dens and speared while groggy. The young of dire wolves and large cats would have been located in dens and destroyed (that was what drove the wolf to near-extinction in North America).

The Clovis culture was an advanced paleolithic culture that revolved entirely around hunting. Those people lived to hunt, and by hunting. I have seen no convincing evidence of any other mechanism for the extinctions.

For some good background information, you might try reading a book entitled "The Eternal Frontier: an Ecological History of North America and its Peoples". This book traces the natural history of North America from Cretaceous times to present.