There’s a story that many scientists tell about the evolution of human skin color.
The story goes roughly like this:
In the beginning, our ancestors had dark fur and lightly pigmented skin. This was perhaps six million years ago? Over time, our ancestors lost their fur and needed darkly pigmented skin to protect themselves from the harsh light of the sun.
Later, some people left their ancestral homeland. Migratory humans covered the globe. As humans traveled farther from the equator, they evolved light skin again – otherwise they’d have too little vitamin D.
In Joanne Cole (author) & Bruce Degen (illustrator)’s The Magic School Bus Explores Human Evolution (which is surprisingly good! You can read my review here), this story is told in a single panel.
Variants on this story percolated through the scientific literature for years, but the version above is derived largely from the work of anthropologists Nina Jablonski & George Chaplin. In their article “The Evolution of Skin Coloration,” they write that “As hominins migrated outside of the tropics, varying degrees of depigmentation evolved to permit ultraviolet-light-induced synthesis of vitamin D.”
This story is often treated as accepted science, even by researchers who describe human evolution from an explicitly anti-racist perspective. For example, in A Brief History of Everyone Who Ever Lived, Adam Rutherford writes that “The unglamorous truth is that there are but a handful of uniquely human traits that we have clearly demonstrated are adaptations evolved to thrive in specific geographical regions. Skin color is one. The ability to digest milk is another, which fits perfectly with the emergence of dairy farming.”
However, this story about the evolution of human skin color isn’t supported by the actual data. Instead, it’s based on Eurocentric misconceptions about what sort of environment and lifestyle are “normal” for human beings.
Unquestionably, darkly pigmented skin can protect humans from sunlight. And sunlight is dangerous! You should wear sunscreen. (I’m sure that somebody has told you this already.)
But the benefits of light skin have been vastly overstated by (light-skinned) researchers. And a quick glance at the data is enough to demonstrate the major flaws in the evolutionary story I described above.
That same page of The Magic School Bus Explores Human Evolution includes a world map with a (again, surprisingly accurate!) depiction of the paths that ancient humans took to populate the planet.
Looking at those red arrows, you’ll see several occasions when groups of humans migrated farther from the equator. The people who settled in France, Korea, and Patagonia had all reached similar latitudes. (As did the humans who settled in New Zealand, but they only arrived about 800 years ago, which probably isn’t enough time to expect dramatic shifts in skin color. Especially given the likelihood of continued gene flux across latitudes – by the time anyone reached New Zealand, people were probably traveling to and fro by boat often, rather than forming an isolated community.)
If the above story about the evolution of human skin color were correct, we’d expect that indigenous people from France, Korea, and Patagonia would all have similar skin color. Indeed, artist Gail McCormick worked closely with Jablonski & Chaplin to create a cut-paper map depicting the indigenous skin color that their story predicts for various regions.
But this map doesn’t match the skin color we actually see from humans across the globe. The indigenous people of France evolved lightly pigmented skin. The indigenous people of Korea, Patagonia, and North America did not.
Jablonski & Chaplin arrived at their conclusion because they considered very few human populations; Figure 4 from their paper, which I’ve included below, depicts in white all the regions of the globe that they left out of their data set.
Each human migration was another natural experiment: Does migration away from the equator result in lighter skin?
For the people migrating into Europe, the answer is pretty unambiguously “yes.” We have evidence of dramatic, rapid selection for genes that result in lighter skin among these people. Many of the gene variants responsible for lightly pigmented skin in Europeans had been long present among ancient humans living in Africa (as documented by Crawford & colleagues in “Loci Associated with Skin Pigmentation Identified in African Populations”), but then spread rapidly among Europeans approximately 4,000 years ago (as documented by Mathieson & colleagues in “Genome-Wide Patterns of Selection in 230 Ancient Eurasians”).
The dramatic selection for genes associated with lightly pigmented skin in Europe occurred within the span of about a thousand years, and occurred about 30,000 or 40,000 years after Homo sapiens first populated that region.
Among the various groups of ancient humans who migrated toward similar latitudes, only the indigenous people of Northern Europe evolved lightly pigmented skin. This trait spread rapidly (by evolutionary standards) about 4,000 years ago.
This timing is similar to the spread of lactose tolerance genes among the people of Northern Europe. Most animals, including most humans, can’t digest milk in adulthood. Even among humans who live in cultures where cows’ milk is a major component of the diet, many people can’t digest it and will experience routine gastrointestinal distress and diarrhea. (Which is serious! Although a few bottles of Gatorade would save their lives, diarrhea still kills about 2 million people per year. Among ancient humans, diarrhea could easily cause deaths by malnutrition, dehydration, or increased susceptibility to disease.)
For their 2022 study “Dairying, Diseases, and the Evolution of Lactase Persistence in Europe,” Evershed & colleagues looked at food residues stuck to ancient pottery and found that cows’ milk has been a major part of European diets for approximately 9,000 years. But these people couldn’t digest milk well. For their 2020 study “Low Prevalence of Lactase Persistence in Bronze Age Europe Indicates Ongoing Strong Selection over the Last 3,000 Years,” Burger & colleagues found that most of the dead warriors from an ancient European battleground did not have the genes for lactose tolerance.
And yet, just before the Europeans’ vast spree of kidnapping, abduction, and resource extraction led to massive amounts of human migration (which began approximately 500 years ago), nearly 95% of the people living in Europe had the genes for lactose tolerance.
That’s a huge change, and really fast! Which should make us realize that something strange might be going on with this group of people – they must’ve had particularly atrocious diets. Which helps explain why they’d need lighter skin.
After all, vitamin D is a dietary nutrient. If you get enough vitamin D from your food, there’s no downside to darkly pigmented skin. And, as David Graeber & David Wengrow describe comically in The Dawn of Everything (“We might call this the ‘all the bad spots are taken!’ argument”), most ancient humans chose to live in places where they could find food, water, and shelter. Otherwise they’d migrate.
Yet, in a savage twist of fate, the same culture that generally resulted in low-quality diets – farming – also made migration more difficult. People stayed near their farms, with their insufficient amounts of low-quality food, because that way they’d at least have something.
I’ve written previously about the social and environmental repercussions of ancient farming – a lovely essay, in my opinion! – but in order to understand the evolution of skin color, all we really need to know is the impact of farming on human health. As James Scott writes in Against the Grain,
“Evidence for the relative restriction and impoverishment of early farmers’ diets comes largely from comparisons of skeletal remains of farmers with those of hunter-gatherers living nearby at the same time. The hunter-gatherers were several inches taller on average. This presumably reflected their more varied and abundant diet. It would be hard, as we have explained, to exaggerate that variety. Not only might it span several food webs – marine, wetland, forest, savanna, arid – each with its seasonal variation, but even when it came to plant foods, the diversity was, by agricultural standards, staggering. The archaeological site of Abu Hureyra, for example, in its hunter-gatherer phase, yielded remains from 192 different plants, of which 142 could be identified, and of which 118 are known to be consumed by contemporary hunter-gatherers.”
The crops and livestock raised by farmers in Northern Europe provide very little vitamin D. But ancient humans often settled in areas where they could catch fish, which provides plenty of dietary vitamin D (as measured by Schmid & colleagues for their study “Natural Vitamin D Content in Animal Products”).
As it happens, if the picture from The Magic School Bus Explores Human Evolution were an accurate depiction of those people’s diet (not to mention their clothes, exposing quite a bit of skin!), they’d probably experience very little selective pressure for lighter skin.
Whenever we discuss evolution, it’s important to remember that natural selection doesn’t enrich for traits that are “better.” There’s rarely any such thing as “better.” Consider: the ancestors of starfish had brains! But – given their particular environment – their lineage was more successful after evolving to be brainless. Or: the ancestors of penguins could fly! But – given their particular environment – their lineage was more successful after evolving to be flightless.
We humans have long legs and arched feet that are great for running, but these same long legs and stubby toes make us so much worse at climbing trees than a chimpanzee. It’s a trade-off. (And a trade-off that I’m pretty happy with, given that I love to run and am afraid of heights.)
Lightly pigmented skin carries a very clear cost – UV penetration with its attendant folate degradation, skin cancers, and discomfort – and only carries a compensatory benefit at extreme northern or southern latitudes among ancestral populations with diets low in vitamin D.
We do ourselves a major disservice – and perpetuate Eurocentric racism – if we consider the selective pressures encountered by one particular group of Homo sapiens to be the default against which all others are measured.