The following is mostly based on chapter seven of “Lone Survivors: How We Came to Be the Only Humans on Earth” (2012) by Chris Stringer of the Natural History Museum in London:
Huge advances in reading DNA since the 1980s means we can now work out a rough history like this:
- 6,000,000 years ago: the common ancestor of chimpanzees and humans (and their head lice)
- 3,000,000: the naked ape: early man is now mostly hairless (head lice and pubic hair lice are now different)
- 500,000: Denisovans
- 350,000: Neanderthals
- 135,000: humans (Homo sapiens). Mitochondrial Eve in East Africa – the woman that all living humans come from
- 115,000: Africans and (future) Eurasians split.
- 100,000: huge increase in human head lice (presumably from a huge increase in humans)
- 60,000: humans in the Middle East
- 55,000: Eurasians split into Europeans and Asians. They are already light-skinned but will get even lighter
- 50,000: humans in South East Asia
- 45,000: humans in Europe
- 40,000: In Africa, Pygmies and Khoisan split.
- 30,000: Neanderthals die out
- 20,000: blue eyes start to become common in Europe
- 15,000: humans in North America
- 11,000: Europeans get whiter
A million years ago our direct ancestor was Homo heidelbergensis. They became Neanderthals in Europe, Denisovans in Asia and, in Africa, they became us. We have a good genetic read on all three.
The humans who left Africa seemed to have mated with Neanderthals so that they are now about 2% Neanderthal. But it is not the same 2% everywhere.
In addition, humans in Melanesia (New Guinea and islands east) are about 5% Denisovan.
No one is sure what these Neanderthal or Denisovan genes do. Probably they fight diseases found outside Africa. The Neanderthal genes that whites got did not make their skin lighter or make them fit for the cold north, so it is likely they mixed with Neanderthals in North Africa or the Middle East, not in Europe.
The farther away you get from Africa on foot the less genetically diverse Homo sapiens become. That means there was very little mating overall with Denisovans, Neanderthals and whoever else was out there.
There is one exception: Melanesia. It is the most genetically diverse place outside of Africa. Possibly because humans did mate with those who were there before – the Denisovans at least, maybe others.
The genes that make us different from Neanderthals handle stuff like brain function, skin, hair, sweat glands, sperm activity and the structure of the skull and skeleton. The genes we know about that directly affect language are the same in both.
The known genetic differences between humans in different parts of the world mainly concern junk DNA, disease, body shape and looks. Some of it is naturally selected by climate, like skin colour. Some by sexual selection, like eye colour. And some by mere chance (genetic drift, the founder effect), like the thickness of lips.
Intelligence: 50,000 years is long enough for genetic differences in intelligence to appear, but IQ tests hardly prove it. If IQs were mostly genetic you would expect to see huge differences within Africa and, apart from Melanesia, smaller differences as you move away from Africa.