My guess would be that it happened gradually. Someone living in a place where the minimum temperature is 15 might be willing to move north to a place where the minimum temperature is 13 as long as other factors are better for him. Couple of generations later you have someone who’s living in a place where the minimum temperature is 13 willing to go north to a place where the minimum temperature is 10, so on and so forth.
It happened even more gradually.
Your kids moved out of the house when they were adults and built their own, half a mile over the next hill north, cause the other good spots were taken. Repeat x 10000 and you’ve “migrated” all over the globe.
My guess would be that it happened gradually. Someone living in a place where the minimum temperature is 15 might be willing to move north to a place where the minimum temperature is 13 as long as other factors are better for him. Couple of generations later you have someone who’s living in a place where the minimum temperature is 13 willing to go north to a place where the minimum temperature is 10, so on and so forth.
It happened even more gradually.
Your kids moved out of the house when they were adults and built their own, half a mile over the next hill north, cause the other good spots were taken. Repeat x 10000 and you’ve “migrated” all over the globe.