That's why the Liberals are desperate to find a comet or asteroid to blame all those extinctions on, they need it to keep their Noble Savage myth going.
To believe it was not man, you would have to believe that whenever man first showed up at a new location, by some amazing coincidence an asteroid or comet always would soon hit in the same exact location wiping out the local megafauna.
To believe man was responsible you would have to believe that small groups of hunter-gatherers in N. America were responsible for the destruction of mammoths, mastodons, giant bison, horses, camels, harrington's mt. goat, american cheetahs, american lions, american mt. deer, short-faced bears, smilodons, the giant cave bear, the woodland muskox, the giant ground sloths, dire wolves, the Stag-moose, the giant armadillo, and on and on and on...
The extinction list is way too long and diverse (including birds and reptiles) to simply tack it onto the butcher's bill of humans. Correlation is not causation.