This week I’m going to paddle along the dangerous shoreline of that sea known as “skin colour”. I’ll also write about hair and eye colour. Well aware of the undercurrents that could pull me under, as well as the potholes and quicksands into which I might slip, I’m gonna play it nice ‘n’ easy, slip along gently without stirring up any gnarly seaweed or stinging jellyfish.
I shall begin by stating that I have a huge interest in prehistory, archaeology, archaeogenetics and, specifically, how the genes I carry around got to where they’re at. Show me a book or research paper about the Bell Beaker people or what language was associated with Urnfield culture and you’ve nabbed me. I want to know where my ancestors came from, what language they spoke, words they used, clothes they wore, what gods they believed in — and what they looked like. Thus, a paper I came across recently (“Direct evidence for positive selection of skin, hair, and eye pigmentation in Europeans during the last 5,000 y“) has really sparked my imagination, as well as settling a few questions I had and triggering a dozen more.
The paper’s conclusions are:
. . . a combination of selective pressures associated with living in northern latitudes, the adoption of an agriculturalist diet, and assortative mating may sufficiently explain the observed change from a darker phenotype during the Eneolithic/Early Bronze age to a generally lighter one in modern Eastern Europeans, although other selective factors cannot be discounted. The selection coefficients inferred directly from serially sampled data at these pigmentation loci range from 2 to 10% and areamong the strongest signals of recent selection in humans.
In other words, the ancestors of people who are today grouped under the term Caucasian started developing lighter coloured skin, hair and eyes around the time the Copper Age was transitioning into the Bronze Age. The possible reasons for this lightening of pigmentation include: adaptation to life at higher latitudes; the move away from a hunter-gather to a farming diet; and non-random mating patterns. The selective pressure behind the move from dark to light colouring in Europe is among the strongest found for human genes.
The three genes the authors looked at are responsible for melanin production (TYR), control of melanin production and the pigment’s distribution (MATP), and blue/brown eye colour (HERC2). The ancestors of every one of us had fully functioning versions of these genes, giving them dark skin, brown eyes and black hair, but at some point after human beings began to settle northern Europe, mutations arose in these genes, giving their carriers lighter coloured eyes, hair and skin. Now if these mutations had been deleterious, their carriers would have disappeared from the population, taking the mutations out with them. If the mutations had proven neutral, yielding neither competitive advantage nor disadvantage, they would have persisted in our common genome’s background — present at low levels, but not particularly prevalent, not gaining any ground over competing versions. If, however, the mutations proved advantageous in some way, over the course of the millennia the new genes’ prevalence would slowly increase in the population, until nearly everyone became a carrier. What the authors set out to prove was this latter case, and their attempts to do so involved them taking a snapshot of the northern European genome as it was five thousand years ago.
The authors got their hands on sixty-three samples of ancient DNA from the skeletal remains of Copper Age/Bronze Age inhabitants of the Pontic-Caspian steppe, centred on modern-day Ukraine. They then sequenced the DNA and characterised the status of each individual vis-à-vis the genes of interest. Next of all, they compared mutated gene frequencies among our Copper Age/Bronze Age ancestors with those of modern-day Ukrainians, Europeans, Africans and Asians. Frequencies of the mutated alleles are essentially zero among Africans and Asians, meaning that these populations retain the pigmentation of all humans’ common ancestors — the first (African) humans. Mutated gene frequencies among the Copper Age/Bronze Age northern Europeans lie in between those of modern Europeans and Ukrainians and their African ancestors. And so, the authors have captured a moment in human evolution where the prevalence of new traits was in between the ground state of the ancestral population (0.00%) and where they are at today (97% in the case of the MATP mutation).
All well and good, you might say, but why this recent (the first mutations appeared about 20,000 years ago but the process really began in the Copper Age) change in pigmentation? How does being light-coloured help your average high-latitude-living Homo sapiens? The mutations in the three genes only bedded down in Europe, so we must look for clues in comparing the environment in which the change happened (cold, no sun, agriculture) with the environment from which the ancestral population came (hot, sunny, hunter-gatherer/nomadic). Here’s what the authors have to say:
The samples in our study were from between 42°N and 54°N, a latitudinal belt in which yearly average UVR is insufficient for vitamin D3 photosynthesis in highly melanized skin. Constraints on the ability to photosynthesize vitamin D3 imposed by low incident UVR intensity may have provided significant selective pressure favoring lighter pigmentation populations in high-latitude regions such as the northern Pontic steppe belt. The need to admitUVB radiation to catalyze the synthesis of vitamin D3, together with the decreased danger of folate photolysis at higher latitudes, may account for the observed skin depigmentation from prehistoric to modern times in this region.
The changeover from a hunter-gatherer diet, where large quantities of vitamin D-rich oily fish were consumed, to a diet based on agricultural produce could also have increased the need for sunlight-derived vitamin D synthesis.
So environment and diet can be invoked to explain the lightening of skin tone, but what about eye and hair colour? Again I’ll quote the authors:
Given that intraspecific pigmentation variability in other taxa, particularly avians, has been attributed to signaling and other factors associated with mate choice it is possible that depigmented irises and the various hair colors observed in Europeans arose through sexual selection. Frequency-dependent sexual selection in favor of rare variants has been observed in vertebrates, and such selection favoring rare pigmentation morphs could have driven alleles associated with lighter hair and eye colors to higher frequency.
In other words, it’s all about sex! We are programmed by evolution to find those with novel traits attractive, as these novel traits represent a re-jigging of the genome that could signify increased chances of survival for our offspring. Back in the Bronze Age your blue- or green-eyed blond(e) was a living, breathing example of new and possibly fitter genes and gene rearrangements, and your discerning tribal chief or matriarch just had to have at least one of these is their harem! It may have been noticed that lighter coloured kids were less predisposed to rickets, and the penny may have dropped that it was a safer bet to grab yourself a blond(e) husband or wife.
Studies such as the paper discussed here have done much to shed light on the origins of human beings and the changes that have occurred in populations as they journeyed through time and space. The tools available to geneticists today allow them to extract DNA from remains that may be up to hundreds of thousands of years old, sequence this DNA and compare it to that of contemporaries or more modern humans. The sheer power and resolution of the new DNA and bioinformatics technologies means that archaeogenetics is just going to keep on providing answers to questions that have intrigued archaeologists and historians for centuries. We’re finding out ever more about human beings’ origins in and journey out of Africa, about how agriculture spread, about how the Americas were peopled from Asia. We’ve learned that the Basques and Sardinians have more or less stayed genetically isolated since the Neolithic, when farmers spread along the Mediterranean from the Balkans, that we humans interbred with our Neanderthal cousins, that the Irish, Welsh, Scots and north-western Iberians are very closely related.
While choosing to focus on skin, eye and hair colour may seem close to the bone for some, and invite criticisms that their research could be used by racists or white supremacists, Wilde et al., have given us some important clues as to why Europeans look the way they do and given those of us whose knuckles do not scrape off the ground a wonderful riposte to anybody who claims superiority for Caucasians over their more pigmented brethren.
“You’re only talking about three genes, my friend,” we can say. “Three mutated genes and an evolutionary adaptation to weak sunlight and a move away from eating salmon and trout!”