Those ancient humans were B-U-S-Y! They apparently hunted all these animals to extinction: mammoth, mastodon, giant cave bear, short-faced bear, giant bison, giant ground sloth, north american horse, north american camel, giant beaver, stag-moose, dire wolf and american lion. And that's just a partial list!
Fortunately, their descendants learned from this Pleistocene holocaust and they came to live in perfect harmony with nature - as depicted in "Dances with Wolves."
Speaking of giant bison; that raises a question. Or two. Where did modern bison come from if ancient man killed all the giant bison? Why didn't more recent hunters eliminate the much smaller modern bison? Wouldn't they have been easier to kill with the improved spears and arrows (archaeology bears that out) and the smaller size of the bison? Not to mention the fact that all those other huge animals were gone leaving little else to hunt.
Ancient man must have been like ants covering the whole of the earth.