Modern intelligence needed the hot islands in the south and the cold and protein rich north to thrive and develop culturallyKlevius simplified human evolution formula for dummies: If you have one liter of brain capacity (e.g. erectus) and get stuck on a jungle island for thousands of years then the brain will shrink to half a liter. However, if selection favors those who could keep their IQ unchanged the result would have been something like Homo floresiensis who had a brain size of a chimp but a cultural level on pair with Homo erectus. Now, when sea level fell again the island became mainland and opened up for re-mixing with erectus. And sooner or later the new brain setup came to reach the really big brained dummies in the north.
Do note that we don't know whether Ust-Ishim man was archaic or already affected by the new brain setup from SE Asia. Do also note that in Klevius theory there is no necessary correspondence between brain size and intelligence - other than the very core of Klevius theory, namely that some children to those individuals who had the genetic setup for a better brain of course became even more intelligent if they had a bigger vessel to expand it in.
However, hybridization and due back migration out of Siberia also meant that only some individuals in every kinship group got the new brain setup. But this was quite enough in the beginning in the north where the easy access to proteins and fat favored longevity and multi-generational knowledge transfer/conservation.
It's quite hard to find Eurocentrism in the 1984 map below. But much easier to find genetic etc correspondence with what we know today - yet some racist people can't really swallow.
Human variation in cranial capacity. Black, 1,450 cc and over; checkerboard, 1,400-49 cc; crosshatching, 1,350-99 cc; horizontal striping, 1,300-49 cc; diagonal striping, 1,250-99 cc; dots, 1,200-49 cc; white areas, under 1,200 cc (Beals et al., 1984).
Leading to the Caucasian "bastard" belt mix with archaics to the West/SW and the strengthening of Mongoloid characteristics to the East/SE.
For those who are really dumb (or fanatic?!), e.g. Chris Stringer etc: Today you can find idiots and geniuses with whatever racial characteristics. However, because culture has long since replaced biological intelligence (which has been falling since early out of Siberia) it is now possible for idiots to sit in professor chairs.
And if you are politically correct and funded from Saudi Arabia etc, then you probably suffer less morally if you are really dumb.