So maybe we're not to blame after all:

New findings suggest that an ongoing, epic whodunit may actually be a whatdunit. That is, climate change, not humans, may be what killed off Ice Age mammoths, horses, and other large animals in North America.
...
The upstart forests [which grew as a result of climate change] transferred the landscape's nutrients to the treetops, out of the reach of large mammals. Elks and bison, it seems, adapted better to the new landscape than mammoths and horses.

15,000 BC or no, I blame Karl Rove.