Mostly, it was probably climate: the last (current) Ice Age, the "Quaternary glaciation" took a breather about 15kya. Things warmed nicely for about two millen

Mostly, it was probably climate: the last (current) Ice Age, the "Quaternary glaciation" took a breather about 15kya. Things warmed nicely for about two millennia, then the Younger Dryas, a cold spell of about a millennium, ended about 11-12 kya. Before this, the "Last Glacial Maximum" (just what it sounds like) was glacier city. Ice was maxed ~26,500 years ago, ending 16-13kya, depending on whom you talk to. Even far from the ice, the climate was radically different from today. Along with the serious climate swings, our culture changed. How much is straight-up the result of less ice is debatable, (I don't want to sound like Jared Diamond), but the fact remains: glaciers went, we developed agriculture. And we've been on the treadmill ever since.