The eye of the beholder?

Gender asymmetry in preferences for male and female faces (Lewis, 2012)

“Beauty is in the eye of the beholder.” This proverb is true in the sense that beauty is a mental judgment—an output of different algorithms within the human brain. Some of these algorithms have been formed by personal experience, but others are hardwired to varying degrees. There is thus an objective side to human beauty.

And yet human populations differ greatly in physical appearance. Does this mean that each population has evolved its own notions of beauty? This was Darwin’s view when he wrote The Descent of Man and Selection in relation to Sex. “The different races of man differ in their taste for the beautiful” (Darwin, 1936 [1888], p. 888).

Yet Darwin himself doubted this view:

Mr. Winwood Reade, however, who has had ample opportunities for observation, not only with the negroes of the West Coast of Africa, but with those of the interior who have never associated with Europeans, is convinced that their ideas of beauty are on the whole the same as ours; and Dr. Rohlfs writes to me to the same effect with respect to Bornu and the countries inhabited by the Pullo tribes. Mr. Reade found that he agreed with the negroes in their estimation of the beauty of the native girls; and that their appreciation of the beauty of European women corresponded with ours. They admire long hair, and use artificial means to make it appear abundant; they admire also a beard, though themselves very scantily provided. Mr. Reade feels doubtful what kind of nose is most appreciated; a girl has been heard to say, "I do not want to marry him, he has got no nose;" and this shows that a very flat nose is not admired (Darwin, 1936 [1888], p. 888).

In a footnote, Darwin added that the Amerindians of Tierra del Fuego consider European women to be extremely beautiful and that Sir Richard Burton "believes that a woman whom we consider beautiful is admired throughout the world" (Darwin, 1936 [1888], p. 888).

These opinions, which Darwin ultimately discounted, nonetheless have support from developmental psychology. Notions of human beauty seem to develop along similar lines in all humans. Children as young as 2-3 months old look longer at female faces that adults have rated as attractive, be they white infants looking at faces of black women rated by black men or black infants looking at faces of white women rated by white men (Langlois et al., 2000; Langlois et al., 1991; Langlois et al., 1987; Langlois and Stephen, 1977). Similar findings have been obtained with adults of various racial/ethnic origins (Bernstein et al., 1982; Cunningham et al., 1995; Maret, 1983; Miller, 1969; Perrett et al., 1994).

In the most comprehensive of these studies, Cunningham et al. (1995) assessed criteria of female beauty among men of different ethnic backgrounds: Taiwanese, White Americans, Black Americans, and recently arrived Asian and Hispanic students. All of them perceived a female face to be more attractive when possessing high eyebrows, widely spaced large eyes with dilated pupils, high cheekbones, small nose, narrow face with thin cheeks, large smile, full lower lip, small chin, and fuller hairstyle.

To be sure, the East Asian men tended to prefer more immature and inexpressive faces whereas the Black American men tended to prefer women with larger buttocks and a heavier body build. These differences in preference, however, are much smaller than the differences in physique that actually exist among human populations.

So what happens when physically different populations come into contact with each other? Are some judged to be better looking than others? And is there consensus on this judgment?

This question is not easy to answer. By “not easy” I don’t mean in an analytical way. I mean we normally view it through the lens of a certain paradigm, i.e., European dominance. Europeans have dominated the world for almost five centuries, and this geopolitical dominance has presumably shaped notions of human beauty throughout the world. Ask almost anyone, and you’re bound to hear this explanation.

There was of course a time when Europeans were weaklings on the world scene. Geopolitical power used to be centered on the Middle East, and on several occasions Arab or Turkish empires almost overran Europe. Yet throughout this time European women fetched high prices in the Middle East as concubines or wives, specifically because of their physical appearance. The term “white slavery”—now a synonym for prostitution—harks back to this largely forgotten trade in human flesh.

Today, the long period of European dominance is coming to an end. Perhaps this question can now be answered more objectively. A new attitude is being shown for example by psychologist Michael Lewis, who has sought to explain why interracial marriages are highly asymmetrical by gender:

A striking aspect of the data on interracial marriages is the size of the gender asymmetries. These asymmetries appear robust across time and culture. […] there are over twice as many marriages between Black men and White women than between White men and Black women in the US. An observed consequence of this pattern is a decline in marriage rates for Black women, which has been described in the US as the ‘marriage squeeze’. The asymmetry is smaller in the UK but still present.

The gender asymmetries are even larger for marriages that include Asian and White people. In this situation, however, it is the number of White men marrying Asian women that is over twice the number of White women marrying Asian men. The largest asymmetry shows that marriages between Black men and Asian women in the US outnumber those between Asian men and Black women by about five to one. (Lewis, 2012)

In the past, these asymmetries were explained as a tradeoff between “caste advantage” and “class advantage.” A wealthier Black man would exchange his class advantage for the caste advantage of a poorer White woman. As Lewis (2012) notes, however, “interracial marriages show the same degree of similarity between partners’ status as same-race marriages.”

Another explanation is that interracial marriage occurs when the man and the woman can afford its social cost (fewer career opportunities, weaker support network, etc.). This explanation likewise fails: interracial couples are not richer on average. If they were, we would also expect to see more White man /Black woman marriages, since White men are on average richer than Black men and can more easily pay the social cost.

Lewis then addresses racial differences in height as a possible explanation, i.e., women tend to seek taller mates and men tend to seek shorter mates. While this might explain the tendency of White men to marry East Asian women, it’s a poor fit for Black/White marriages. Black men and White men don’t differ enough in height to account for the gender asymmetry of such marriages.
Finally, Lewis addresses the possibility that this gender asymmetry may reflect an underlying asymmetry in sexual attractiveness: “If there are differences between the relative attractiveness of the genders between different races then asymmetries in interracial marriage will follow.” To this end, he asked male and female volunteers to rate the attractiveness of human faces that differed by ethnicity and gender. Of the male raters, 15 were White, 2 were Black, and 3 were Asian. Of the female raters, 14 were White, 3 were Black, and 3 were Asian.

The results are shown at the top of this post. Female raters gave the highest ratings to Black men, followed by White men and East Asian men. Male raters gave the highest ratings to East Asian women, followed by White women and Black women. There was no significant interaction between the race of the rater and the race of the face being rated.

The results replicate earlier findings that Black men are rated as more attractive than White men. It was further found that Asian men were rated as less attractive than either other race. For women the pattern was reversed with Asian women being rated as most attractive followed by White women and then Black women. The patterns observed occurred regardless of the race of the person doing the ratings.

The results are consistent with the patterns we see in interracial marriage. On the basis of census data, Lewis argues that the two asymmetries are an almost perfect match.

And my research?

These results are also somewhat consistent with my own studies. One of them indicated that the hormone estrogen orients women toward darker male skin. The young female participants were shown facial photos: three pairs of female faces and three pairs of male faces. Each pair was identical except for a slight difference in complexion, and the participant had to choose the face she liked the most. The choices, as it turned out, varied with the phase of the menstrual cycle. The darker male face was more strongly preferred by participants in the first two-thirds of the cycle (when estrogen levels are high in relation to progesterone levels) than by those in the last third (when estrogen levels are low in relation to progesterone levels). Menstrual cycle phase did not affect face preference if both faces were female or if the participants were taking oral contraceptives (Frost, 1994b).

There is also my cross-cultural study with Pierre van den Berghe. We found a strong tendency in traditional cultures, whether European or non-European, to associate lighter skin with women. This gender asymmetry seems to hold up over different historical periods and types of society. It seems, in fact, to reflect an innate sex difference in pigmentation. Women are paler and men browner and ruddier because of differing amounts of melanin and hemoglobin in the skin’s outer layers. This is a genuine sexual dimorphism that correlates with other aspects of sexual differentiation, e.g., digit ratio, thickness of subcutaneous fat, timing of puberty, etc. (Frost, 2011; van den Berghe & Frost, 1986).

Nonetheless, there are significant differences between my findings and Michael Lewis’. The cross-cultural study showed a general preference for lighter-skinned women, but only at the lighter end of the local range of skin color. We see this in folk terminology. Traditionally, a beautiful woman was ‘white’ in Europe and East Asia, ‘golden’ in Southeast Asia, and ‘red’ in sub-Saharan Africa.

As for my menstrual cycle study, the darker male face was indeed more strongly preferred by women in the first two-thirds of the menstrual cycle, i.e., when estrogen levels are high and not offset by progesterone. Yet, even in that group, there was still more preference for the lighter male face. In other words, estrogen seems to weaken a woman’s resistance to darker male skin, without reversing the direction of preference, at least not fully.

At the time, I attributed this result to my use of black-and-white photos. Had I used color photos, and thus accurately shown the ruddiness and brownness of male skin, more women might have opted for the darker male face. I also suspected that the darker male face was triggering negative mental associations in some participants (even though all of the faces looked ethnically ‘white’).

There is also the difference of twenty years between that study and Michael Lewis’. In the early 1990s, there were fewer images of blacks in popular culture, and those images tended to be male and female in equal proportions. Today, such images are not only more common but also overwhelmingly male. I’m not just thinking of the hip-hop scene. In general, eroticization of the black man has become much more mainstream in movies, popular entertainment, and poster advertising. Yes, black women also appear in this role, but they appear less often and tend to have European facial features and skin tone.

Finally, the ideological environment has changed over the past twenty years. In Lewis’ study, the White raters showed no tendency to prefer their own kind—an unusual finding in itself. Many of them may have thought long and hard before choosing a White face over a non-White one. Of course, this possible anti-White bias would not explain the gender asymmetry. It would simply shift all preferences towards the darker end of the color spectrum.

And that leads to another point. Perhaps some of the raters were unconsciously using East Asian preference as a proxy for White preference. In our current ideological environment, it is legitimate to admire East Asians for a wide range of good qualities: politeness, work ethic, self-discipline, attractive facial features, and so on. Such admiration incurs no social cost. So if you feel ashamed of your preference for White people, why not repackage it as East Asian preference?

Indeed, many social conservatives point to East Asians as a way of promoting values that once were mainstream in North American society. We see this, for instance, with the ‘tiger mom’ craze. We also see this in the frequent invidious comparisons that conservative economists make between the United States and China.

Sexual beauty and human physical differences

But how is it that humans look so different while sharing a similar sense of sexual beauty? Perhaps much of this physical variation is due to differences in sexual selection. Not differences in notions of beauty, as Darwin imagined, but rather differences in the intensity and direction of sexual selection.

In some populations, men competed against each other for access to women. This was especially so in tropical ‘horticulturalist’ societies where year-round farming enabled women to provide for themselves and their children with little male assistance. For men, the cost of taking a second wife was close to zero and may even have been negative. Such societies thus had a high polygyny rate and correspondingly intense male-male rivalry for mates. The pressure of sexual selection was therefore on men.

In other populations, women competed against each other for access to men. This was especially so in continental Arctic societies where men provided almost all the food and where long-distance hunting caused more deaths among young men than among young women. Such societies thus had a low polygyny rate and a surplus of women on the mate market. The pressure of sexual selection was therefore on women (Frost, 1994a, 2006, 2008).

References

Bernstein, I.H., Lin, T., and McClellan, P. (1982). Cross- vs. within-racial judgments of attractiveness. Perception & Psychophysics, 32, 495-503.

Cunningham, M.R., Roberts, A.R., Barbee, A.P., Druen, P.B., and Wu, C-H. (1995). "Their ideas of beauty are, on the whole, the same as ours": consistency and variability in the cross-cultural perception of female physical attractiveness. Journal of Personality and Social Psychology, 68, 261-279.

Darwin, C. (1936) [1888]. The Descent of Man and Selection in relation to Sex. reprint of 2nd ed., The Modern Library, New York: Random House.

Frost (2011). Hue and luminosity of human skin: a visual cue for gender recognition and other mental tasks, Human Ethology Bulletin, 26(2), 25-34.

http://media.anthro.univie.ac.at/ISHE/index.php/bulletin/bulletin-contents

Frost (2008). Sexual selection and human geographic variation, Special Issue: Proceedings of the 2nd Annual Meeting of the NorthEastern Evolutionary Psychology Society. Journal of Social, Evolutionary, and Cultural Psychology, 2(4),169-191. http://137.140.1.71/jsec/articles/volume2/issue4/NEEPSfrost.pdf

Frost (2006). European hair and eye color - A case of frequency-dependent sexual selection? Evolution and Human Behavior, 27, 85-103.

Frost (1994a). Geographic distribution of human skin colour: A selective compromise between natural selection and sexual selection? Human Evolution, 9, 141-153.

Frost, P. (1994b). Preference for darker faces in photographs at different phases of the menstrual cycle: Preliminary assessment of evidence for a hormonal relationship, Perceptual and Motor Skills, 79, 507-514.

Langlois, J.H., Kalakanis, L., Rubenstein, A.J., Larson, A., Hallam, M., and Smoot, M. (2000). Maxims or myths of beauty? A meta-analytic and theoretical review. Psychological Bulletin, 126, 390-423.

Langlois, J.H., Ritter, J.M., Roggman, L.A., and Vaughn, L.S. (1991). Facial diversity and infant preferences for attractive faces. Developmental Psychology, 27, 79-84.

Langlois, J.H., Roggman, L.A., Casey, R.J., and Ritter, J.M. (1987). Infant preferences for attractive faces: Rudiments of a stereotype? Developmental Psychology, 23, 363-369.

Langlois, J.H., and Stephan, C. (1977). The effects of physical attractiveness and ethnicity on children's behavioral attributions and peer preferences. Child Development, 48, 1694-1698.

Lewis, M.B. (2012). A Facial Attractiveness Account of Gender Asymmetries in Interracial Marriage, PLoS ONE 7(2), e31703
http://dx.plos.org/10.1371/journal.pone.0031703

Maret, S.M. (1983). Attractiveness ratings of photographs of Blacks by Cruzans and Americans. The Journal of Psychology, 115, 113-116.

Miller, E.L. (1969). Body image, physical beauty and colour among Jamaican adolescents. Social and Economic Studies, 18, 72-89.

Perrett, D.I., May, K.A., and Yoshikawa, S. (1994). Facial shape and judgements of female attractiveness. Nature, 368, 239-242.

van den Berghe, P.L. & P. Frost. (1986). Skin color preference, sexual dimorphism, and sexual selection: A case of gene-culture co-evolution?, Ethnic and Racial Studies, 9, 87-113.

Were there Neanderthals in Africa?

“Neanderthal” admixture seems to be higher in West Africans than in East Africans. How come? (Source)

When modern humans began their expansion from a small core somewhere in East Africa, the continent probably had several different archaic populations.

It now seems that one of them was related to the Neanderthals in Europe. In an ongoing study of Neanderthal admixture in present-day humans, John Hawks has found an apparently higher level of admixture in the Yoruba of Nigeria than in the Luhya of Kenya (see chart above).

This is counter-intuitive, as Hawks himself notes. The closest source of admixture would have been Neanderthals in what is now Israel (and probably elsewhere in the Middle East). Genes from that source should have first introgressed into northeastern Africans.

Perhaps this “Neanderthal” admixture actually came from a related archaic population that was already established in Africa. As modern humans spread west from East Africa after c. 60,000 BP, they would have partially intermixed with these archaic hominins before finally replacing them. There would thus be an east-to-west cline of increasing archaic admixture.

This would be consistent with the finding that dental traits are more “ancestral” in West Africa than in East Africa. When Irish (2011) compared dentitions from west, central, east, and south Africa, ranging in age from the late Pleistocene to the mid-1950s, the early Holocene Kenyans and Tanzanians were the ones that had the fewest ancestral traits of the Sub-Saharan African Dental Complex.

This may also explain why the level of Neanderthal admixture is almost the same throughout Eurasia (although John Hawks has found slightly higher levels among Europeans than among Asians). Perhaps this “Neanderthal” admixture was simply admixture with a Neanderthal-like population within Africa itself. Modern humans would have picked it up on their way out of the continent.

But this raises another question. Why is “Neanderthal” admixture lower in present-day sub-Saharan Africans than in present-day Eurasians? Perhaps there was another archaic population, very different from the Neanderthals, that modern humans encountered only in Africa. This might be the quasi Homo erectus represented by the Broken Hill (Kabwe) skull. It might also be the population that accounts for about 2% of the present-day African gene pool and that seems to have split away from the ancestors of modern humans some 700,000 years ago (Hammer et al., 2011). This secondary archaic source may have proportionately reduced the admixture from these “African Neanderthals.”

References

Hawks, J. (2012). Which population in the 1000 Genomes Project samples has the most Neandertal similarity? John Hawks Weblog, February 8
http://johnhawks.net/weblog/reviews/neandertals/neandertal_dna/1000-genomes-introgression-among-populations-2012.html

Hammer, M.F., A.E. Woerner, F.L. Mendez, J.C. Watkins, and J.D. Wall. (2011). Genetic evidence for archaic admixture in Africa, Proceedings of the National Academy of Science (USA), 108, 15123-15128, www.pnas.org/cgi/doi/10.1073/pnas.1109300108

Irish, J.D. (2011). Afridonty: the “Sub-Saharan African Dental Complex” revisited, American Journal of Physical Anthropology, 144(supp. 52), 174

Were they right after all?

Modern humans entered the Americas from northern Eurasia. As they entered tropical environments farther south, they had to evolve new genetic adaptations from scratch. They no longer had the ones their remote forbearers had back in Africa. (Source)

OK, so modern humans have archaic admixture, and the degree of admixture seems to be highest among sub-Saharan Africans and Melanesians. But what does this factoid mean? Does it mean anything at all?

One school of thought says it means nothing at all. Ernst Mayr held this view:

The claim has been made that species owe much of their genetic variability to introgressive hybridization. However, all the evidence contradicts this conclusion so far as animals is concerned. Not only are F1 hybrids between good species very rare, but where they occur the hybrids (even when not sterile) are demonstrably of inferior viability. The few genes that occasionally introgress into the parental species are not co-adapted […] and are selected against. Introgressive hybridization seems to be a negligible source of genetic variation in animals. (Mayr, 1970, p. 80)

“Co-adaptation” means that genes are selected for their ability to work with other genes. “On one genetic background a given gene may add to the fitness of the genotype; on another genetic background the same gene may create an unbalance and produce a severely deleterious effect” (Mayr, 1970, p. 169).

Yet some genes seem to be of the “stand alone” sort. We see this when genetically modified organisms are created, such as by transferring an antibiotic-producing gene from a microbe to a cultivated plant.

Mayr also overlooked the possibility that introgressing genes might benefit a species that is expanding into new territory and gradually displacing the native species. Such genes can circumvent the need to evolve genetic adaptations that already exist in the native population. This argument was used by Greg Cochran, John Hawkes, and Henry Harpending a few years back, when they thought that the most recent microcephalin variant had introgressed from the Neanderthals. Another case of Neanderthal introgression seemed to be the diversification of European hair colors, via the MC1R gene:

A few years ago, I was thinking about Out-of-Africa, and it occurred to me that we probably picked up a lot of favorable alleles from Neanderthals […] Looking at the new article in PNAS, I'd say that Bruce Lahn and company have probably found one.[…]

Now if the Neanderthals were really effectively isolated before we expanded into their territory, they'd have a lot of significantly different alleles. Some would have involved various kinds of regional adaptations, which might be a good thing to have in Eurasia. (MC1R?) But it's entirely possible that some alleles solved adaptive problems that had existed in Africa as well - but solved them better.

Brains had expanded over the last half-million years in both Africans and Neanderthals, but it seems likely that those changes in size and structure were driven by different mutations, just as light skin in Europe and East Asia was. The Neanderthals had slightly bigger brains than Africans: obviously those brains were useful for _something_.

[…] So when you think about the cultural explosion that occurred shortly after we overwhelmed the Neanderthals (cave paintings, sculptures, new tools and weapons, all that jazz) - well, you have to wonder if assimilating a passel of adaptive alleles in a few thousand years, way more than the typical number that would arise and become established over such a short time span, didn't give us a hell of a boost. There are signs of behavioral modernity a bit earlier in Africa - but those ostrich eggshells are dull as hell compared to Gravettian cave paintings. Expansion out of Africa must itself be a sign of new capabilities (I'd bet on sophisticated language) but you only see full-fledged behavioral modernity in the European Upper Paleolithic... (Cochran, 2006)

Since then, the Neanderthal genome has been fully reconstructed. The most recent microcephalin variant isn’t on it. Nor are the European MC1R alleles. It seems that the best laid theories of mice and men gang aft agley.

No one likes to be proved wrong. Especially in public. For some time yet, certain people will be busy flogging the dead horse of Neanderthal introgression. Will they find anything? Undoubtedly. Anything to do with intelligence or artistic creativity? Probably not.

In all fairness, we’re not giving the theory of adaptive introgression a decent chance by looking only at Neanderthal admixture in modern Europeans. That degree of admixture is only 1 to 4% (perhaps less if we adjust for archaic admixture in modern Africans). We might have more luck looking at sub-Saharan Africans, who have as much as 15% archaic admixture.

In sub-Saharan Africa, the main genetic change since the advent of modern humans has been the replacement of hunter-gatherers by farming peoples. This process began perhaps 6,000 to 7,000 years ago near the Niger’s headwaters. A small core population of hoe farmers progressively expanded westward and southward, eventually occupying almost the whole of sub-Saharan Africa, except for a few marginal environments in central and southern Africa inhabited by Pygmy and Khoisan hunter-gatherers (Murdock, 1959, pp. 44, 64-68).

This new means of subsistence set off a cascade of social and, ultimately, biological changes. Year-round farming enabled women to provide for themselves and their children with little male assistance. Because more men could afford a second wife, the polygyny rate soared from around 5% of all sexual unions to 20-40%. Men now had to compete more fiercely against each other for access to women, the result being selection for greater stature, muscle mass, and bone strength. Many of these changes were and still are testosterone-mediated, i.e., through higher testosterone levels in young adult males or through testosterone receptors that bind the target molecule more effectively. Other changes may have been initially testosterone-mediated and then gradually hardwired (Frost, 2001; Frost, 2008).

A similar process has affected Amerindian farming peoples in the tropical New World, but not to the same degree. Among the Yanomamö, an agricultural people of Amazonia, 10 to 20% of all men have more than one wife at any time (Hames, 1995). They are thus more polygynous than non-tropical Amerindians but not as polygynous as tropical Africans. Their bodies likewise don’t show the same degree of robustness. Despite living in the tropics for millennia, they still look remarkably Arctic-adapted. As Holliday (1997, pp. 425-426) notes:

Despite having as much as 18,000 years of selection in environments as diverse as those found in the Old World, body mass and proportion clines in the Americas are less steep than those in the Old World. […] In fact, as Hulse (1960) pointed out, Amerindians, even in the tropics, tend to possess some “arctic” adaptations.

In Mesoamerica, farming began about 6,000 to 7,000 years ago—the same time depth as in West Africa (although there is evidence of proto-agriculture going back to 12,000 years ago in West Africa). The difference between the two culture areas seems to have been more in the speed of co-evolution with this new means of subsistence, and especially with its social consequences. Briefly put, natural selection has had less raw material to work with among tropical Amerindians than among sub-Saharan Africans. Because modern humans entered the Americas from northern Eurasia, their genetic variability had been whittled down to the bare minimum for Arctic survival. They have thus evolved more slowly away from anatomical traits and behavioral predispositions that were originally meant for Arctic environments.

This is in contrast to the relatively rapid change in morphology that we see in sub-Saharan Africa—from a small, gracile, and almost childlike appearance, as typified by Pygmies and Khoisans, to a much larger and more robust body form. Just as radical has been the shift from a low rate of polygyny to a very high one.

Did evolution get a helping hand in sub-Saharan Africa? Was this change in physique facilitated by introgression of archaic genes? Perhaps Greg Cochran and John Hawkes were right after all. They just had the wrong example.

A final point

This leads to another point. It’s inexact to say that genetic introgression speeds up evolution. Rather, it speeds up evolution along a certain trajectory … to the detriment of other possible trajectories.

Take the example of tropical Amerindians. With the advent of year-round farming, they started down the same path that sub-Saharan Africans were taking on the other side of the Atlantic, i.e., more reproductive autonomy for women, lower cost of polygyny for men, higher incidence of polygyny, more male-male competition for access to women, etc.

But their progress down that path was slower. They had less of the relevant genetic variability for natural selection to act on. Meanwhile, another path opened up. This was the one leading to the formation of more complex societies, such as the Mayans, the Toltecs, and the Aztecs in Mesoamerica, and the Incas in South America. They were able to take that path because of their relatively low level of male-male rivalry. There was consequently less need for pacification—a key precondition for development of complex social relations (i.e., State formation, specialization of labor, creation of roads and other public infrastructures, etc.).

This was not the case in sub-Saharan Africa. The high rate of polygyny, and hence male-male competition, became a serious impediment to the creation—and survival—of complex societies.

References

Cochran, G. (2006). Neanderthal introgression & microcephalin, Gene Expression, November 8

http://www.gnxp.com/blog/2006/11/neanderthal-introgression.php

Frost, P. (2008). Sexual selection and human geographic variation, Special Issue: Proceedings of the 2nd Annual Meeting of the NorthEastern Evolutionary Psychology Society. Journal of Social, Evolutionary, and Cultural Psychology, 2(4),169-191. http://137.140.1.71/jsec/articles/volume2/issue4/NEEPSfrost.pdf

Frost, P. (2001). Polygyny and sex ratios, Encyclopedia of Birth Control, V.L. Bullough (ed.), Santa Barbara (Cal.): ABC-CLIO, pp. 218-223.

Hames, R. (1995). Yanomamö, Varying Adaptations of Foraging Horticulturalists, Just in Time Anthropology series, Prentice Hall and Simon & Schuster, supplemental readings for Ember and Ember's Anthropology, 8th edition.

Holliday, T.W. (1997). Body proportions in Late Pleistocene Europe and modern human origins, Journal of Human Evolution, 32, 423-447.

Murdock, G.P. (1959). Africa. Its Peoples and Their Culture History. New York: McGraw-Hill.

Encounters between modern humans and archaics in Africa

Broken Hill (Kabwe) skull. In Africa, very archaic hominins persisted into recent times.

Were archaic hominins still roaming over parts of Africa when farming villages began to form in the Middle East? I raised this question in my last post. But others are now raising it too:

The lesson is that Africa, a vast continent encompassing virtually all of our recent evolutionary ancestry, contains yet unknown degrees of diversity. At the same time that modern humans were emerging, they would have lived in landscapes alongside more archaic populations, possibly into very recent times. This implies also that at least some of the traces of archaic genetic markers still found in people today may have arisen as a result of intermixing of archaic and modern populations within Africa, rather than interbreeding of ‘pristine’ modern people with archaic populations only after leaving Africa. (Wells, 2012)

A new convert to this view is the paleontologist Chris Stringer, who only ten years ago was arguing that archaic hominins had long disappeared from Africa when modern humans began to spread into Europe and Asia. Today, the evidence cannot be easily ignored. There are too many skulls from western and southern Africa that look archaic and yet are surprisingly recent, like the Broken Hill cranium:

A relatively late date for the Broken Hill cranium suggests a long time span for the “rhodesiensis/heidelbergensis’’ group and warrants caution about inferring the presence of early modern humans from the presence of early Middle Stone Age artifacts (Stringer, 2011).

Other paleontologists, like Lily Malekfar, are similarly taking a second look at African cranial and skeletal remains that had previously been thought to be modern human:

Current research indicates that modern Homo sapiens originated in East Africa and then migrated across Africa as well as out of Africa, where they encountered archaic hominins. The Klasies River Main site (KRM) in South Africa is one location where there is evidence that modern and archaic Homo sapiens may have interacted. As Smith and other researchers have suggested, the KRM mandibular sample, in particular, exhibits significant size and morphological variability, which counters claims that the KRM specimens are fully modern.

[…] The results demonstrate that the KRM sample is markedly more variable than any of the comparative samples, which rejects the null hypothesis and is one possible indicator of an admixed sample at KRM. (Malekfar, 2012)

So just what is the story about modern humans and archaic hominins in Africa? Since DNA rapidly degrades in tropical climates, we will probably never retrieve DNA from those archaic skulls. But there is archaic DNA in living Africans, as Hammer et al (2011) discovered, so we may be able to piece together parts of the archaic African genome. Modern DNA can also tell us about ancient population movements within Africa, including the “big bang” that gave rise to modern humans some 60,000 years ago:

African mtDNA has three main lineages — L1, L2 and L3 — which have an estimated coalescence date of 126,000–165,000 yr BP. The L1 lineage is the most ancient and is present in the San population from South Africa and the Biaka Pygmies from the Central African Republic, which are two of the most genetically divergent populations in Africa18. L2 and L3 diverged from L1 ~60,000–103,000 yr BP. The L2 lineage is present in Mbuti Pygmies from the Democratic Republic of the Congo and in west African Bantu-speaking populations. The L3 lineage is widely dispersed throughout east Africa but is rare elsewhere in sub-Saharan Africa. Phylogenetic analysis indicates that the L3 haplogroup is the precursor of non-African mtDNA haplotypes and that a subset of this lineage (L3a) travelled out of Africa in the ancestors of modern Eurasians ~60,000–80,000 yr BP. The L3a lineage also occurs at a high frequency in Ethiopian mtDNA40, which supports the proposal that modern humans migrated out of Africa through Ethiopia. (Tishkoff & Williams, 2002)

Watson et al. (1997) elaborate further on this “big bang”:

It seems reasonable to speculate that a behavioral innovation appeared some 60,000-80,000 years ago in a subpopulation of anatomically modern humans, containing the ancestors of L3 (and possibly also L2), who had previously been living with a Middle Paleolithic/Middle Stone Age technology—and that this small subpopulation subsequently expanded as a result.

There seems to have been a series of expansions, beginning around 80,000 years ago. The last expansion, dated to c. 60,000 BP and associated with subcluster L3a, was the one that spread modern humans out of Africa and to the rest of the world (Watson et al., 1997).

In summary, the evolutionary sequence can be described as follows:

1. Half a million years ago, all of Africa was inhabited by an archaic population similar to the Neanderthals in Europe and the Denisovans in Asia

2. Over time, these African hominins differentiated into two populations: (a) an evolutionarily conservative one in western and southern Africa; and (b) a more evolving one in eastern Africa.

3. Around 120,000 years ago, the eastern hominins expanded north into the Levant (Skhul-Qafzeh). They were now almost like modern humans. Their material culture was like that of the Neanderthals but their anatomy was almost modern, albeit with some archaic features.

4. Beginning around 80,000 years ago, a sub-population of the eastern hominins underwent a series of expansions. Then, around 60,000 years ago, a final "big bang" eclipsed the preceding ones and gave rise to true modern humans.

5. Within Africa, these modern humans initially expanded into territory inhabited by “almost moderns,” i.e., people more or less modern in appearance while still largely archaic in behavior and material culture, like the Skhul-Qafzeh hominins of the Levant. Archaic admixture thus entered the gene pool of modern sub-Saharan Africans and today represents about 13% of the total.

6. As modern humans pushed farther into western and southern Africa, they encountered much more archaic hominins, almost like Homo erectus. There was less admixture from those archaics, only 2% of the total.

References

Hammer, M.F., A.E. Woerner, F.L. Mendez, J.C. Watkins, and J.D. Wall. (2011). Genetic evidence for archaic admixture in Africa, Proceedings of the National Academy of Science (USA), 108, 15123-15128.

Labuda, D., E. Zietkiewicz, & V. Yotova. (2000). Archaic lineages in the history of modern humans, Genetics, 156, 799–808.

Malekfar, L. (2012). An analysis of the Klasies River hominins using a hybrid model, American Journal of Physical Anthropology, Program of the 81st Annual Meeting of the American Association of Physical Anthropologists, p. 201.
http://physanth.org/annual-meeting/2012

Stringer, C. (2011). The chronological and evolutionary position of the Broken Hill cranium. American Journal of Physical Anthropology, 144(supp. 52), 287

Tishkoff, S.A., & S.M. Williams. (2002). Genetic analysis of African populations: human evolution and complex disease, Nature Reviews – Genetics, 3, 611-621.

Watson, E., P. Forster, M. Richards, and H-J. Bandelt. (1997). Mitochondrial footprints of human expansions in Africa, American Journal of Human Genetics, 61, 691-704. 0024024
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0024024

Wells, S. (2012). What made us human? Simon Wells reviews Chris Stringer’s ‘The origin of our species’, Red Mist, January 4
http://redmistreviews.com/?p=848