4,000 - 5,000 years ago? Wasn’t that around the time of the Noahic Flood?
Traditionally dated about 4,200 years ago.
This 4,000 years ago event occurs at the same time as the beginning of the Monsoon Winds into India ~ which allowed the vast central massif forests to dry out and allow human settlement.
It is also coincident with a little known event ~ Lake Erie shrank to its present size. Before this time it extended nearly all the way to the Western boundary of present day Indiana. Something happened and the water level dropped, the lake shrank, and bingo it took on its current appearance.
It's probably safe to say that there was a lengthy period of droughts around the world. Most likely seasons got shorter and the world cooled.
The genetic transformations were what they always are during those events ~ the guys from the North moved South!