We reign supreme on this planet, but this mastery is relatively recent. In the process of becoming ever more knowledgeable, and commanding, of the different environments and geographies we found ourselves contending with over the past 6-8 million years of our evolution, it is known that at times we faced tremendous environmental challenges that appear to have wiped out entire regional populations, and subjected others to tremendous population bottlenecking -a term that refers to a severely reduced population that links periods of comparatively larger prior and subsequent populations. It is what happened during these bottlenecking periods that I propose explain certain phenotypic traits – epicanthic fold, among others – characteristic of oriental and oriental-derived groups of people, a small subset of the Northern European population, as well as bushmen tribes in Southern Africa.
Many species tend to have built-in inhibitions/mechanisms to avoid incest. It is a way of preventing deleterious recessive traits from expressing themselves through homozygosity. Incest avoidance also results in a greater genetic variability in a population’s gene pool, which increases its overall robustness in coping with environmental changes. The evolving human line, like their closest primate relatives, would have likewise had this aversion. Although incest-avoidance no longer likely had any inborn impetus after the human line lost its sexual instinct about 200,000 years ago (see www.humansexualevolution.com), incest would have nonetheless continued to be avoided based on the collective wisdom passed down over countless generations, that it tended to produce offspring with physical deformities, a host of health issues, and intellectual deficiencies. However, when populations declined to the point of near-regional extinction due to environmental conditions that took the form of biting cold, drought, or other effects, small isolated groups barely surviving amid such hardships would have had to forego any social taboo against incest. It is easy to see why: in the unforgiving ancient world of early humans, population stability of small groups was key to survival, because efforts, and perhaps resources, were communally pooled. Moreover, producing children that would one day be able to provide assistance and care to their parents in old age or infirmity, was of paramount importance to the future survival of every individual.
But the age group most severely culled by the harshest environmental conditions are the very young, and the very old, because both groups tend to have the smallest energy reserves, and the weakest immune systems to contend with environmental insults. For this reason, couples that remained childless due to significantly higher infant mortality during bottlenecking periods would have been desperate to produce surviving children, even as females got closer and closer to the age of menopause. But, it is known that the incidence of congenital disorders, most notably Down Syndrome, increases markedly with a mother’s age as she approaches menopause. Males with Down syndrome are almost uniformly infertile, and hence act as a dead-end for further progeny. Females with Down syndrome are significantly less fertile than unaffected individuals, and often have difficulties with miscarriage, premature birth, and difficult labor. There is approximately a fifty percent chance that the offspring of someone with Down syndrome will also have the syndrome themselves. So what we have is bleak circumstances turning ever more bleak within only a few generations, resulting in even greater population depletion. Therefore, when incest was the only way to beget children during periods of extreme environmental challenges, the forebears of humanity must have committed to it, time and again.
As is well known, engaging in incest in fraught with dangers. A large percentage of children – in the range of 50% – born out of primary incest (involving fathers/daughters, mothers/sons, siblings) have autosomal recessive disorders, congenital physical malformations, or some intellectual deficits. But importantly, healthy viable offspring can also be produced by incest. Among the physical traits commonly seen in children born out of incest is epicanthic fold, a skin fold of the upper eyelid covering the inner corner of the eye, giving a characteristic slant-eyed appearance. Of great significance, I believe, is that this feature is also seen in all oriental peoples (chinese, japanese, etc.), in peoples of ancestral oriental origin (Inuit, native americans, etc.), in bushmen tribes in southern Africa (!kung, koisan, etc.), people in some parts of the Pacific Islands, and also in people of Scandinavian descent. The leading explanation for epicanthic fold is that it is an evolutionary adaptation to extreme cold. However, thermal studies of the face show that none of the cranial features of mongoloids show any heightened resistance to cold. But equally damning, cold adaptation as an explanation for epicanthic fold clearly cannot account for its presence in the bushmen tribes of southern Africa, people who are not known to have ever contended with extremes of cold.
Clearly, a more consistent explanation is needed for epicanthic fold, and mongoloid features in general, which appear to varying degrees in the few aforementioned distinct groups of people. I hypothesize that the Mongoloid features associated with oriental peoples, including epicanthic fold, were a result of a marked increase in primary incest – possibly for a period lasting hundreds of years – within small isolated groups. All oriental people are thought to have descended largely from a small group of people that had made their way to the furthest reaches of northern Siberia about 25,000 years ago, likely following reindeer herds. But then, they found themselves confronted with the peak of the last global glaciation. The latest climate research reveals that temperatures dipped to -80 degrees Celcius in this region, and it became unimaginably dry as well, forcing this likely dwindling band of people to retreat to a slightly more hospitable location in eastern Siberia near the town of Mal’ta by about 22,000 years ago. Here temperatures still hovered around -40 degrees Celsius, and its inhabitants probably just barely survived with populations that remained marginal for perhaps centuries, leaving no option but to engage in fairly regular and sustained incest.
I propose that this same environmentally-induced population depletion – with its attendant obligatory incest – played a role in imparting the epicanthic fold and the suite of other mongoloid traits, in all populations exhibiting them. Did sexual selection also play a role in the evolution of mongoloid features? Others have previously speculated that it was sexual selection ALONE that resulted in oriental populations acquiring mongoloid features such as epicanthic fold, for example. One of the problems with this hypothesis is that it does not explain why these features would have been favored by only a subset of human populations. And is it only a coincidence that these preferences arose only among peoples living in harsh and precarious environments where extreme environmental conditions were present at times? And how did such features arise in the first place, for sexual selection to then act upon and magnify further? These should objectively be viewed as troubling holes in this hypothesis. Instead, I am proposing that only after mongoloid features became endemic as a result of primary incest during bottlenecking periods, did sexual selection probably begin to further refine those existing features. The second part of my hypothesis is that the longer the period of incest, ie. the more consecutive generations, or near-consecutive generations, that engaged in it, the more amplified the mongoloid features became – and the larger the suite of characteristic mongoloid features, some of which may be viewed as a reversion to more archaic ancestral phenotypes – and include traits like shovel-shaped incisors for example. And lastly, the more isolated these groups remained, the less outside genetic mixture there was over time to dilute the phenotypic signatures of incest, and the longer they retained them. Populations eventually mushrooming out of these small groups would have had the chance of retaining these distinct phenotypic traits, which then had the potential to characterize an entire race, or group of people.
This blog is part of my main site www.humansexualevolution.com, which showcases a theory of human sexual evolution I wrote and got published in the Journal Of The Gay And Lesbian Medical Association in 2000.