[slowking2]

This doesn't have that much to do with the genetic heritability of intelligence, which is a measurement in proportion to environmental variability. Civilizations with farming, metallurgy, etc. showed up independently in many places although at highly varying times due to human dispersal. You can also see that intelligence isn't all its cracked up to be from an evolutionary point of view since it sometimes goes down. Has it gone up in most places over the last few thousand or ten thousand years overall. I think so, read the recent Reich paper (EDIT: this Reich https://reich.hms.harvard.edu/publications). But it doesn't only go up all the time.

Disease factors were probably the biggest factor in selection until very recently (and maybe now is just a temporary blip before a return to the historical pattern). Disease almost wiped out the Native Americans and strongly held sub-Saharan Africa back.