About 160,000 years ago the Clever Primate (homo sapien) began expanding beyond his equatorial homeland to exploit the virgin north country. I mean, what with population pressures, depleting savannah fauna and the like, what’s a fella to do?…you get the picture.
Except for a few southerly migratory aberrations – mostly colonial in nature – and occasional periods of relative population intransience, the northward trek has carried on through the ages…I see no reason why it would abate now.