There is no history of this happening before, at least not on the time frames we are talking about here. This is thousands of years of change in decades. It isn't so much just the change but the pace that is the issue. Like how jumping off a building and taking the elevator achieve the same amount of change but on a very different time frame and thus very different results.

We weren't agrarian back then, we were still hunting stuff with sticks and simple stone tools (12,000 years ago/ 10,000 BCE).