Tătăroaice – Petre Iorgulescu-Yor (source). Today,
the steppes north of the Black Sea lie within the European world—politically,
culturally, and demographically. Not so long ago, they were home to nomads of
Central Asian origin.
A new study shows that Europeans underwent strong
selection for white skin, non-brown eyes, and non-black hair … during historic
times!
Here we present direct estimates of selection acting on functional alleles in three key genes known to be involved in human pigmentation pathways—HERC2, SLC45A2, and TYR—using allele frequency estimates from Eneolithic, Bronze Age, and modern Eastern European samples and forward simulations. Neutrality was overwhelmingly rejected for all alleles studied, with point estimates of selection ranging from around 2-10% per generation. Our results provide direct evidence that strong selection favoring lighter skin, hair, and eye pigmentation has been operating in European populations over the last 5,000 y. (Wilde et al., 2014)
If true, this finding would contradict other recent findings. Two studies have found a much earlier time frame for the whitening of European skin: 11,000 to 19,000 years ago (Beleza et al., 2013) and 7,600 to 19,200 years ago (Canfield et al., 2014). Two studies of ancient DNA indicate that non-brown eyes were already in existence 7,000 years ago in Spain (Olalde et al., 2014) and 8,000 years ago in Luxembourg (Lazaridis et al., 2013). Moreover, the genes responsible are the same as the ones in above quote.
So who is right and who is wrong? All of these
studies are probably right, but only for some early Europeans and not for all.
In the latest study, the samples come from a very small part of Europe—the
steppes north of the Black Sea:2
Ancient DNA was retrieved from 63 out of 150 Eneolithic (ca. 6,500-5,000 y ago) and Bronze Age (ca. 5,000-4,000 y ago) samples from the Pontic-Caspian steppe, mainly from modern-day Ukraine. […] We also genotyped the three pigmentation-associated SNPs in a sample of 60 modern Ukrainians (28) and observed an increase in frequency of all derived alleles between the ancient and modern samples from the same geographic region (Table 1 and Fig. S1). This implies that the pigmentation of the prehistoric population is likely to have differed from that of modern humans living in the same area.
[…] Inferring natural selection based on temporal differences in allele frequency requires the assumption of population continuity. To this end we compared the 60 mtDNA HVR1 sequences obtained from our ancient sample to 246 homologous modern sequences (29–31) from the same geographic region and found low genetic differentiation (FST = 0.00551; P = 0.0663) (32). Coalescent simulations based on the mtDNA data, accommodating uncertainty in the ancient sample age, failed to reject population continuity under a wide range of assumed ancestral population size combinations. (Wilde et al., 2014)
The authors are placing the burden of proof on the wrong null hypothesis when they state that their simulations “failed to reject population continuity.” The null hypothesis should be population discontinuity. For example, Swedes and Greeks differ in skin tone and eye color, and if we compare their autosomal DNA we get a comparable FST of 0.0084 (Genetic History of Europe, 2014). Admittedly, FST is different with mitochondrial DNA.
I suspect the authors ruled out population
discontinuity because their FST
seemed incompatible with a non-European population giving way to a European one.
If so, they forgot one thing. They were comparing a population of the present
with one that existed some 5,000 years ago. If you go farther and farther back
in time, any human population will look more and more ancestral to a
present-day population. This is especially so in northern Eurasia, where a
population ancestral to both Europeans and Amerindians existed some 20,000
years ago. Yes, the FST
does seem incompatible with a non-European population giving way to a European
one, but this is because the ancient DNA comes from a non-European population
that was closer to the time of common origin for all northern Eurasians.
This ancient DNA may come from a mixed
European/Central Asian population or an intermediate and now extinct population, perhaps similar
to the Lapps. If we look at the derived (European) alleles for the three genes
in question (HERC2, SLC45A2, TYR), the frequencies fall halfway between those of Europeans and
Asians (see Table 1 in the paper). In any case, this population does not have to be of non-European origin to be noticeably darker in skin color. As shown by the recent Mesolithic findings from Luxembourg and Spain, there used to be apparently native dark-skinned populations in the heart of Europe.
Historical background
The hypothesis of population discontinuity becomes
even more plausible if we look at the history of this region. Today, the
steppes north of the Black Sea lie within the European world—politically,
culturally, and demographically. Not so long ago, they were home to nomads of
Central Asian origin. The latest of them, the Tatars, held sway until the 18th
century.
The Tatars intermixed extensively with Slavic wives
and concubines, so much so that they now look almost as fair as other
Europeans. But they were originally quite swarthy, as attested by medieval
sources. In a 14th-century romance, The
King of Tars, a Tatar Khan converts to Christianity and turns white in the
baptismal water. Two other chronicles of the same period describe how a Tatar
Khan's Christian concubine bears him a son white on one side and black on the
other. When baptized, the child emerges from the water white on both sides
(Hornstein, 1941; Metlitzki, 1977, p. 137).
Medieval writers often noticed this difference in
skin color. Genoese notaries usually described Tatar slaves as olive-skinned
(Plazolles Guillen, 2012, p. 119). Florentine acts of sale give the following
numerical breakdown of Tatar slaves by skin color: black 2, brown 18, olive
161, fair 11, reddish 5, white 45 (Epstein, 2001, p. 108). During a trial, a
slave tried to regain her freedom by claiming to be Russian and, hence,
Christian. Her owner rebuked her, saying: “You’ve lied to me. You look more
like a Tatar, not at all like a Russian” (Plazolles Guillen, 2012, p. 119).
The Tatars were preceded by other nomads of Central
Asian origin. The Scythians (8th to 2nd century BC) were likewise described as
dark-skinned. Hippocrates wrote: “The Scythian race are tawny from the cold,
and not from the intense heat of the sun, for the whiteness of the skin is
parched by the cold, and becomes tawny” (Hippocrates).
One can find references to the contrary (Scythians, 2014). Keep in mind that the word “Scythian” was often used in the ancient
world to encompass all northern peoples:
To the ancient Greeks the Scythians, Sarmatians, Germans, and Goths were the remote northern races of antiquity. Geographically near to one another, they were often grouped together under the term “Scythians,” which by the third century B.C.E. no longer had an ethnic or national connotation and had come to designate the peoples of the remote north. (Goldenberg, 2003, p. 43)
The term “Scythian” may also have subsumed different peoples north of the Black Sea, some of whom came from Central Asia and others from areas farther north and west.
Conclusion
Because this region is on the periphery of the
European world and has been exposed to migrations from Central Asia, population
change is a likelier explanation for the findings of Wilde et al.
These findings are nonetheless interesting. Together
with the ancient DNA from Mesolithic hunter-gathers in Spain and Luxembourg, we
have further proof that many early Europeans were brown-skinned. Indeed, this
seems to have been the physical appearance of all Europeans during their first 20,000
years in Europe. Only later, within the time frame of 20,000 to 10,000 years
ago, did some of them become white.
This may seem surprising to those who believe that
white skin is an adaptation to weak sunlight at high latitudes. It was thought
that Europeans became white because their ancestors no longer needed dark pigmentation
to protect themselves against sunburn and skin cancer. Meanwhile, light
pigmentation became necessary to maintain synthesis of vitamin D. There was
admittedly the example of dark-skinned peoples who have long lived at similar
latitudes in Asia and North America, but that counterfactual was attributed to the
availability of vitamin D from a marine diet, such as among the Inuit of
northern Canada.
Wilkes et al. do, in fact, address the apparent
contradiction between their findings and the hypothesis that ancestral
Europeans became white to maintain adequate production of vitamin D in their
skin. In their Discussion section, they suggest that the shift from hunting and
gathering to farming led to a decrease in dietary vitamin D (from fatty
fish and animal liver). The main problem with this explanation is that farming
came late to many parts of Europe: about 2,000 to 3,000 years ago for East Baltic
peoples and less than 3,000 years ago for Finnish peoples (and
incompletely at that). This leaves a very narrow time frame for evolution from
brown skin to white skin. Ultimately, this question will be resolved with
retrieval of ancient DNA from these populations.
Notes
1. Although Wilde et al. mention hair color, they
did not study the main hair-color gene, MC1R.
2. Razib Khan has a great map of the ancient DNA
samples.
References
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view
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