The most state-of-the-art evolutionary genetics suggests that depigmentation in Europeans is a very recent feature of human biogeographic variation (though, the same techniques tell us that Europeans as we understand them as a genetic cluster are also a very recent feature of human biogeographic variation!). A new paper in Nature Communications, A genome-wide association scan in admixed Latin Americans identifies loci influencing facial and scalp hair features, suggests that the same is true of straight hair. That is, straight hair is a derived characteristic which emerged independently several times outside of Africa, with curly hair being the ancestral state. This is an open access paper, so I invite you to peruse the list of SNPs. I checked the markers in my own pedigree and we’re surprisingly monomorphic, as in no variation (also, many of them are not in the SNP-chips, but this is where DNA Land can help by imputing a VCF).
This gets at one of the major findings it strikes me in this paper. Researchers have long known that there are large effect variants that impact hair color; it segregates within families in a quasi-Mendelian manner, albeit not as clearly as eye color. Though it is polygenic, big explainers of the variation like EDAR above lurk in the genome. But, as highlighted in the paper there are more genes under heaven than EDAR, and even within EDAR there may be different SNPs under selection in different regions. This resembles another complex human trait subject to normal variation: skin color.
And yet there are complexities within complexities. True, the selective sweep we see in EDAR in East Asians and Amerindians is relatively recent. But it is almost certainly older than the Holocene, because most of the ancestors (>90%) of Amerindians diverged from the ancestors of modern East Asians ~15,000 years ago. Probably after the LGM ~18,000 years ago, but definitely before the end of the Ice Age. To my knowledge it is the same haplotype acrosss the two regions. A similar sequence of markers which are hallmarks of commmon genealogical descent from an original copy. Second, Mathieson et al. discovered several copies of this same EDAR haplotype in ancient Swedish hunter-gatherers who flourisheed ~8,000 years ago. If you check in modern Europeans this variant is totally absent from all Europeans except Finnns, and the proportion of admixture is very easily explained by relatively recent Siberian ancestry equivalent to the fraction of the derived EDAR haplotype (very much the same can be said of South Asians, as the fraction in the Bangladeshi sample can be explained by the allele frequency of derived EDAR segregating in Southeast Asian populations). What I’m trying to get at is that the emergence of straight hair, and definitely very straight hair, is just not a good fit with the model of recent changes during the Holocene due to shifts in economic modes of production.
Additionally, if you look at the frequency distribution of the derived EDAR variation you note that it often does not fix. To me that is suspiciously reminiscent of some sort of balancing selection going on. Possibly frequency dependent, or, more likely in my opinion, the putative target of selection being dominantly expressed (since the phenotype which selected against then has the frequency of a recessive trait as the q allele > 0.10 against it almost ceases). That means that it is highly unlikely to be hair form itself which being under selection, as opposed to that trait just being swept along for the right because of pleiotropy. EDAR has many effects. No one knows which one, or ones, may have been the target of selection.
Which goes back to my original title. Is straight hair recent in evolutionary history? I’ll make a prediction that it isn’t. Rather, modifications in hair form have emerged repeatedly in hominin evolutionary history, often as a side effect of altering the functions of developmental genes such as EDAR. Many of the salient morphological characteristics of humans which we have co-opted for culturally easy identification in racial groups may have a similar origin. Basically, outputs from the G-matrix have become the basis of whole industries!
Addendum: This post is not an invitation for a particular reader to engage in a “core dump” on sexual selection.