Wow, and here I thought if it weren't for us humans, the earth would go merrily along in pristine perfection for all eternity.
Just one small problem, though. Humans were on the North American Continent long before the Clovis types which are generally dated ca 12,000 years ago. Seems they would have gnawed the last of the big critters before Clovis or Folsum cultures got their spears sharpened...
Ordinary climate change and diminishing habitat got the megafauna, and climate has been changing and will continue to change regardless of humans actions short of full tilt nuclear war. I really doubt we have that much global influence.
Most of this crap was started as a Communist Bloc effort (their seed money plus a lot of useful idiots) to shut down West Bloc industry during the Cold War.