One of the great mysteries of recent evolution is why did the skin of Europeans and Northeast Asians become white? The traditional answer is that in colder climates, you need less protection from the sun so selection for dark skin becomes relaxed or even reversed because dark skin blocks too much vitamin D from the much less potent Northern sun.

One problem with this explanation is that arctic people, despite being further North, retain dark skin, and white skin didn’t become common in Europe until 12,000 to 6000 years ago, despite people living there for roughly 40,000 years.

One theory is that the less vitamin D rich diet imposed by agriculture meant Europeans suddenly needed to absorb vitamin D through their skin to compensate.

But I may have discovered another explanation. White skin appears to have emerged around the same time the human brain suddenly started shrinking (after 4 million years of growth). Now I’ve argued that most of this brain shrinkage was caused by the malnutrition of agriculture and we’ve recovered most of the lost brain mass with improved modern nutrients (giving rise to the Flynn effect).

However not all of our lost brain mass has come back, leading me towards a popular explanation pushed by several prominent scholars: Domestication. It’s well known that domesticated animals have smaller brains than their wild ancestors.

It seems that the high IQ required to survive the cold Northern Eurasian winters had driven European and Northeast Asian brain size to ridiculously huge levels, but they couldn’t make full use of all that brain mass because they were still wild animals who would rather fight and bite than work as a team. Thus evolution sacrificed a tiny bit of individual brain mass (mostly from regions unrelated to cognition) to create a cooperative collective intelligence so powerful it would one day split the atom.

What does this have to do with white skin? Well, just as the animals they were domesticating would have white patches, perhaps white patches started showing up on Northern Eurasians. This may have inspired sexual selection for white skin against which the white patches would be invisible.

I suspect an even higher level of domestication occurred in India, but the intense sun killed off people born with white patches so skin remained dark.