Origins of black Africans

It is often assumed that black Africans, out of all human populations, most closely resemble our common ancestral state. After all, is not Africa the cradle of humanity? And did not modern humans spread ‘out of Africa’ some 50,000 or so years ago?

Indeed, we are all offspring of Africa. What is less true is the assumption that evolution stood still there while continuing elsewhere. Yes, some African groups do approximate ancestral Homo sapiens in their mode of subsistence, family structure, and physical appearance. These are the Khoisan and pygmy peoples. They still live by hunting and gathering, are overwhelmingly monogamous, and have light-brown skin and gracile, almost childlike bodies.

But they now inhabit only a few marginal environments, essentially the Kalahari and patches of rain forest. As elsewhere, time has moved on. There have arisen new populations who differ as much from ancestral Homo sapiens as do Europeans and Asians. These are the ‘true’ black Africans.

On the basis of genetic and archaeological data, black Africans seem to have radiated from a relatively small West African and possibly pygmy population within the last 20,000 years (Coon, 1962, pp. 651-656; Spurdle et al., 1994; Watson et al., 1996). The time and place of origin can be further narrowed down with linguistic data. Speakers of proto-Niger-Congo broke up c. 10,000 BP and the oldest derived group appear to be proto-Mande speakers, whose descendants inhabit the Niger's headwaters near the Mali-Guinea border (Blench, 1984, pp. 128-129; Ehret, 1984; Murdock, 1959, pp. 44, 64-68).

Murdock (1959, pp. 44, 64-68) associates black Africans with the spread of agriculture in sub-Saharan Africa. He too locates their place of origin on the Niger’s headwaters because this region is the cradle of the Sudanic food complex—a wide range of native crops now found throughout the continent (sorghum, pearl millet, cow pea, etc.). Other authors, like Shaw (1980), postulate an earlier, proto-agricultural phase that initially covered much of West Africa. This transitional period may have begun among Sangoan hunter-gatherers, who probably resembled present-day pygmies in appearance and lifestyle. These people gradually moved toward agriculture by first protecting fields of wild grains and making clearings for wild yams and oil palms (Davies, 1968; Shaw 1980, pp. 111-114). They began using hoes c. 12,000 BP and tending pili nut trees c. 8,000-9,000 BP (Posnansky, 1984, p. 149; Stahl, 1995, p. 262). Some form of agriculture is furthermore indicated by certain reconstructed words of proto-Niger-Congo, probably spoken before 10,000 BP (Ehret, 1984).

At a certain point in time, some of these proto-agriculturalists—apparently the ones inhabiting the Niger’s headwaters—surpassed the others in the transition to agriculture. They formed a nucleus of farming populations that would ultimately spread throughout sub-Saharan Africa. Meanwhile, this transition had already triggered a cascade of changes that would have profound behavioral and morphological consequences.

Increase in polygyny

Agriculture, especially year-round agriculture, enables women to become more self-reliant in feeding themselves and their children, thus making it less costly for men to take second wives (van den Berghe, 1979, p. 65). As a result, the polygyny rate is 20-50% of all marriages in sub-Saharan agricultural societies (Bourguignon and Greenbaum, 1973, p. 51; Goody, 1973; Pebley and Mbugua, 1989; Welch and Glick, 1981; White, 1988).

These societies seem to have long been highly polygynous. The ratio of Y chromosome to X chromosome variability is much lower than in other populations, apparently because proportionately fewer men have contributed to the gene pool (Excoffier et al., 1996; Scozzari et al., 1997; Spurdle et al., 1994; Torroni et al., 1990). Generalized polygyny is also attested by reconstruction of proto-Bantu, which was spoken approximately 3,000 years ago and has a specific term for "taking a second wife" (Polome, 1977).

Intensified sexual selection of men

If some men have more wives, others will have to do without. In general, men will have to compete more keenly with each other for access to women. When such rivalry intensifies in non-human species, the result is an intensification of sexual selection for larger, stronger, and more muscular males. This may explain why the highly polygynous, agricultural peoples of sub-Saharan Africa are so physically robust. They and their African American descendants outclass European-descended subjects for weight, chest size, arm girth, leg girth, muscle fiber properties, and bone density (Ama et al., 1986; Ettinger et al., 1997; Himes, 1988; Hui et al., 2003; Pollitzer and Anderson, 1989; Todd and Lindala, 1928; Wolff and Steggerda, 1943; Wagner and Heyward, 2000; Wright et al., 1995).

This masculinization of body build may be hormonally mediated. When Winkler and Christiansen (1993) studied two Namibian peoples, the weakly polygynous hunter-gatherer !Kung and the highly polygynous agricultural Kavango, the latter were found to have markedly higher levels of both total testosterone and DHT. The authors suggest that lower levels of these hormones may account for the !Kung’s neotenous appearance, i.e., sparse body hair, small stature, pedomorphic morphology, and light yellowish skin.

High testosterone/DHT levels are widely attested among sub-Saharan agriculturalists and their New World descendents. Young black men have more circulating testosterone than do young white men whereas young East Asian men, though intermediate in testosterone levels, have less 5α-reductase—an enzyme that converts testosterone into the physiologically more active DHT (Pettaway, 1999; Ross et al., 1992). These three geographic groupings also exhibit analogous differences in androgen receptor receptivity (Kittles et al., 2001). Broadly speaking, lifetime exposure to testosterone/DHT correlates with the incidence of prostate cancer and the highest incidences in the world are among African American men (Brawley and Kramer, 1996). Other populations of black African descent (i.e., West Indians and sub-Saharan Africans) exhibit lower incidences, but these have been shown to reflect underreporting and are probably just as high (Glover et al., 1998; Ogunbiyi and Shittu, 1999; Osegbe, 1997).

Relaxed sexual selection of women

If male-male rivalry intensifies sexual selection of men, it also tends to relax sexual selection of women. Because fewer women remain unmated, men are less able to translate their aesthetic criteria into actual mate choice. Such relaxed selection is suggested by visible female-specific characteristics. African Americans girls have narrower hips, broader waists, and thinner deposition of subcutaneous fat than do Euro-American girls (Hrdlička, 1898; Meredith and Spurgeon, 1980; Nelson and Nelson, 1986). Even before birth, Euro-American fetuses show significantly more sexual dimorphism than do African American fetuses (Choi and Trotter, 1970).

Relaxed sexual selection of women may also explain why, among sub-Saharan Africans, skin color is visibly darker in high-polygyny agriculturalists than in low-polygyny hunter-gatherers (i.e., Khoisans, pygmies) even though both are equally indigenous (Bourguignon and Greenbaum, 1973, pp. 171-175; Cavalli-Sforza, 1986a; Cavalli-Sforza, 1986b; Manning et al., 2004; Weiner et al, 1964). Skin color does, in fact, influence mate choice in all human societies; generally speaking, men prefer women who are lighter-skinned than the population mean (van den Berghe and Frost, 1986). In sub-Saharan societies, the preference is for so-called 'red' or 'yellow' women (see earlier post). Wherever African men were less able to act on this preference, there would have been less selection for lighter-skinned women and thus less counterbalancing of selection for darker skin to protect against sunburn and skin cancer (Aoki, 2002; Frost, 2007; Frost, 1994).

This preference may also have been crowded out by other mate-choice criteria. Vilakazi (1962, pp. 59-60) states: "The traditional Zulu does not make physical beauty a first priority or even an important qualification in a wife; and the skin colour of the woman is of little importance." In a rating study, Dixson et al. (2006) examined mate-choice criteria among subsistence farmers in Bakossiland, Cameroon, including preferred skin color of a potential female partner. No consistent preference emerged. This ambivalence was noted by Ardener (1954, p. 72) among the Ibo of Nigeria:

In the choice of a wife, yellow-skinned girls are regarded as beauties, and, other things being equal, they command higher bride prices. On the other hand it is generally held, especially by dark-complexioned persons, that yellow-skinned people are not as strong as the dark and do not live as long. A 'black' girl is said to be a harder worker. … A Mission headmaster was of the opinion that the preference for yellow girls was greater nowadays than in his youth. He thought that the reason for this was that people formerly looked for strength rather than beauty and tended to marry black girls. He claimed that black people had greater powers of endurance, and he cited his own village where, he said, of the oldest six or seven people, only one was yellow.

In Kenya, McVicar (1969, p. 242) notes similar views on the merits of ‘black’ versus ‘brown’ wives: "Among these tribes black girls are usually regarded as hard workers, possibly because many consider themselves fortunate enough to be married." In traditional African societies, women had to produce enough food for the entire family, typically through hoe farming in the sun. There was thus a premium on darker women. Lighter women may have been preferred aesthetically, but this preference remained unexpressed.

Timeline of expansion out of West Africa

All of these physical and hormonal characteristics seem to have arisen within a narrow timeframe. In sub-Saharan Africa, the beginnings of proto-agriculture cannot be pushed back much further than 12,000 BP. A tall, clearly black African skeleton has been dated to 6,500 BP (Camp, 1974, p. 241; Coon, 1962, pp. 649-650). This leaves a window of barely six thousand years for the changes that differentiate black Africans from their hunter-gatherer ancestors, i.e., a shift from a gracile, almost childlike body to a much more robust one, with attendant increases in stature, weight, and muscle mass.

By 6,000 to 7,000 years ago, the transition to agriculture had been completed in West Africa and these early agriculturalists were able to support much higher population densities than they had as hunter-gatherers. Inevitably, this nucleus of farming populations began to spread outward at the expense of more sparsely distributed Khoisan and pygmy peoples. By about 4,000 BP, the expansion had reached as far east as the middle Nile, when black Africans first appear in paintings from Pharaonic Egypt and in skeletal remains from Nubia (Junker, 1921). About 3,000 BP, another wave of advance began along the Nigerian-Cameroon border and spread rapidly throughout central, eastern, and southern Africa (Cavalli-Sforza, 1986c, pp. 361-362; Diamond, 1997; Oliver, 1966). By 300 AD, pioneering groups had advanced as far south as KwaZulu-Natal (see Bantu Expansion – Wikipedia).

Thus, black Africans were still absent from most of sub-Saharan Africa even within historic times. When the Egyptians began to build their pyramids, the peoples living to the south were scarcely darker in color. They were simply seen as uncivilized Egyptians. Thus, the civilized world initially encountered a much narrower range of human phenotypes than it would later on. This context shaped the intellectual worldview in its early stages, including theorizing on universal brotherhood. To a degree not easy to assess, we are heirs to notions of human sameness that were first conceived ‘before Africa became black’.

References

Aoki, K. (2002). Sexual selection as a cause of human skin colour variation: Darwin’s hypothesis revisited. Annals of Human Biology, 29, 589-608.

Ama, P. F. M., Simoneau, J. A., Boulay, M. R., Serresse, O., Thériault, G., and Bouchard, C. (1986). Skeletal muscle characteristics in sedentary Black and Caucasian males. Journal of Applied Physiology, 61, 1758-1761.

Ardener, E.W. (1954). Some Ibo attitudes to skin pigmentation, Man, 54, 71-73.

Blench, R. (1995). Recent developments in African language classification and their implications for prehistory. In T. Shaw, P. Sinclair, B. Andah, & A. Okpoko (Eds.) The Archaeology of Africa (pp. 126-138). London: Routledge.

Bourguignon, E. and Greenbaum, L.S. (1973). Diversity and Homogeneity in World Societies, HRAF Press.

Brawley, O.W. and Kramer B.S. (1996). Epidemiology of prostate cancer. In Volgelsang, N.J., Scardino, P.T., Shipley, W.U., and Coffey, D.S. (eds). Comprehensive textbook of genitourinary oncology. Baltimore: Williams and Wilkins.

Camps, G. (1974). Les civilisations préhistoriques de l'Afrique du Nord et du Sahara. Paris: Doin.

Cavalli-Sforza, L.L. (1986a). Demographic data. In (L.L. Cavalli-Sforza ed.). African Pygmies, pp. 23-44. Orlando: Academic Press.

Cavalli-Sforza, L.L. (1986b). Anthropometric data. In (L.L. Cavalli-Sforza ed.). African Pygmies, pp. 81-93. Orlando: Academic Press.

Cavalli-Sforza, L.L. (1986c). African Pygmies: an evaluation of the state of research. In L.L. Cavalli-Sforza (Ed.) African Pygmies (pp. 361-426). Orlando: Academic Press.

Choi, S.C., and Trotter, M. A. (1970). Statistical study of the multivariate structure and race‑sex differences of American White and Negro fetal skeletons. American Journal of Physical Anthropology, 33, 307‑312.

Coon, C.S. (1962). The Origin of Races. New York: Alfred A. Knopf.

Davies, O. (1968). The origins of agriculture in West Africa. Current Anthropology, 9, 479-487.

Diamond, J. 1997. "How Africa Became Black" in Guns, Germs and Steel: The Fates of Human Societies, New York: W.W. Norton.

Dixson B.J., Dixson, A.F., Morgan, B., and Anderson, M.J. (2006). Human Physique and Sexual Attractiveness: Sexual Preferences of Men and Women in Bakossiland, Cameroon. Archives of Sexual Behavior, 36, 369-375.

Ehret, C. (1984). Historical/linguistic evidence for early African food production. In J.D. Clark & S.A. Brandt (Eds.) From Hunters to Farmers: The Causes and Consequences of Food Production in Africa (pp. 26-35). Berkeley: University of California Press.

Ettinger, B., Sidney S., Cummings, S.R., Libanati, C., Bikle, D.D., Tekawa, I.S., Tolan, K., and Steiger, P. (1997). Racial differences in bone density between young adult black and white subjects persist after adjustment for anthropometric, lifestyle, and biochemical differences. Journal of Clinical Endocrinology & Metabolism, 82, 429-434.

Excoffier, L., Poloni, E.S., Santachiara-Benerecetti, S., Semino, O., Langaney, A. (1996). The molecular diversity of the Niokholo Mandenkalu from Eastern Senegal: an insight into West Africa genetic history. In A.J. Boyce & C.G.N. Mascie-Taylor (Eds.) Molecular Biology and Human Diversity. Cambridge University Press: Cambridge, pp. 141-155.

Frost, P. (2007). Comment on Human skin-color sexual dimorphism: A test of the sexual selection hypothesis, American Journal of Physical Anthropology, 133, 779-781.

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

Glover, F., Coffey, D., et al. (1998). The epidemiology of prostate cancer in Jamaica. Journal of Urology, 159, 1984-1987.

Goody, J. (1973). Polygyny, Economy and the Role of Women, in J. Goody (Ed.) The Character of Kinship, Cambridge: Cambridge University Press, pp. 175-190.

Himes, J. H. (1988). Racial variation in physique and body composition. Canadian Journal of Sport Sciences, 13, 117-126.

Howell, N. (1979). Demography of the Dobe !Kung. New York: Academic Press.

Hrdlička, A. (1898). Physical differences between White and Colored children. American Anthropologist, 11, 347‑350.

Hui, S.L., Dimeglio, L.A., Longcope, C., Peacock, M., Mcclintock, R., Perkins, A.J., and Johnston Jr., C.C. (2003). Difference in bone mass between Black and White American children: Attributable to body build, sex hormone levels, or bone turnover? Journal of Clinical Endocrinology & Metabolism, 88, 642–649.

Junker, H. (1921). The first appearance of the Negroes in history. Journal of Egyptian Archaeology, 7, 121-132.

Kittles, R.A., Young, D., Weinrich, S., Hudson, J., Argyropoulos, G., Ukoli, F., Adams-Campbell, L., and Dunston, G.M. (2001). Extent of linkage disequilibrium between the androgen receptor gene CAG and GGC repeats in human populations: implications for prostate cancer risk. Human Genetics, 109, 253-261.

Maley, J. (1995). The climatic and vegetational history of the equatorial regions of Africa during the upper Quaternary. In T. Shaw, P. Sinclair, B. Andah, & A. Okpoko (Eds.) The Archaeology of Africa (pp. 43-52) London: Routledge.

Manning, J.T., Bundred, P.E., and Mather, F.M. (2004). Second to fourth digit ratio, sexual selection, and skin colour. Evolution and Human Behavior, 25, 38-50.

McVicar, K.G. (1969). Twilight of an East African Slum. Ann Arbor, University Microfilms (UCLA Dissertation 1968).

Meredith, H.V., and Spurgeon, J.H. (1980). Somatic comparisons at age 9 years for South Carolina White Girls and girls of other ethnic groups. Human Biology, 52, 401‑411.

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

Nelson, J.K., and Nelson, K.R. (1986). Skinfold profiles of Black and White boys and girls ages 11‑13. Human Biology, 58, 379‑390.

Ogunbiyi, J. and Shittu, O. (1999). Increased incidence of prostate cancer in Nigerians. Journal of the National Medical Association, 3, 159-164.

Oliver, R. (1966). The problem of the Bantu expansion. Journal of African History, 7, 361-376.

Osegbe, D. (1997). Prostate cancer in Nigerians: facts and non-facts. Journal of Urology, 157, 1340.

Pebley, A. R., and Mbugua, W. (1989). Polygyny and Fertility in Sub-Saharan Africa. In R. J. Lesthaeghe (Ed.), Reproduction and Social Organization in Sub-Saharan Africa, Berkeley: University of California Press, pp. 338-364.

Penny, D., Steel, M., Waddell, P.J., & Hendy, M.D. (1995). Improved analyses of human mtDNA sequences support a recent African origin for Homo sapiens. Molecular Biology and Evolution, 12, 863-882.

Pettaway, C.A. (1999). Racial differences in the androgen/androgen receptor pathway in prostate cancer. Journal of the National Medical Association, 91, 653-660

Pollitzer, W. S. and Anderson, J. JB. (1989). Ethnic and genetic differences in bone mass: a review with a hereditary vs environmental perspective. American Journal of Clinical Nutrition, 50, 1244-1259.

Polome, E.C. (1977). The reconstruction of Proto-Bantu culture from the lexicon. In L. Bouquiaux (Ed.) L'Expansion bantoue, 2, 779-791. Centre national de la recherche scientifique.

Posnansky, M. (1984). Early agricultural societies in Ghana. In J.D. Clark & S.A. Brandt (Eds.) From Hunters to Farmers: The Causes and Consequences of Food Production in Africa (pp. 147-151). Berkeley: University of California Press.

Ross, R.K., Bernstein, L., Lobo, R.A., Shimizu, H., Stanczyk, F.Z., Pike, M.C., and Henderson, B.E. (1992). 5-apha-reductase activity and risk of prostate cancer among Japanese and US white and black males. Lancet, 339, 887-889.

Scozzari, R., Cruciani, F., Malaspina, P., Santolamazza, P., Ciminelli, B.M., Torroni, A., Modiano, D., Wallace, D.C., Kidd, K.K. et al. (1997). Differential structuring of human populations for homologous X and Y microsatellite loci. American Journal of Human Genetics, 61, 719-733.

Shaw, T. (1980). Hunters, gatherers and first farmers in West Africa. In J.V.S. Megaw (Ed.) Hunters, Gatherers and First Farmers beyond Europe (pp. 69-125). Leicester: Leicester University Press.

Spurdle, A.B., Hammer, M.F., Jenkins,T. (1994) The Y Alu polymorphism in southern African populations and its relationship to other Y-specific polymorphisms. American Journal of Human Genetics, 54, 319-330.

Stahl, A.B. (1995). Intensification in the west African Late Stone Age: a view from central Ghana. In T. Shaw, P. Sinclair, B. Andah, & A. Okpoko (Eds.) The Archaeology of Africa (pp. 261-273). London: Routledge.

Todd, T.W. & Lindala, A. (1928). Dimensions of the body: Whites and American Negroes of both sexes. American Journal of Physical Anthropology, 12, 35-101.

Torroni, A., Semino, O., Scozzari, R., Sirugo, G., Spedini, G., Abbas, N., Fellous, M. et al. (1990). Y chromosome DNA polymorphisms in human populations: differences between Caucasoids and Africans detected by 49a and 49f probes. Annals of Human Genetics, 54, 287-296.

van den Berghe, P.L. (1979). Human Family Systems. An Evolutionary View. New York: Elsevier.

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

Vilakazi, A. (1962). Zulu Transformations, Pietermaritzburg: University of Natal Press.

Wagner, D.R., and Heyward, V.H. (2000). Measures of body composition in blacks and whites: a comparative review. American Journal of Clinical Nutrition, 71, 1392-1402.

Watson, E., Bauer, K., Aman, R., Weiss, G., von Haeseler, A., & Pääbo, S. (1996). mtDNA sequence diversity in Africa. American Journal of Human Genetics, 59, 437-444.

Weiner, J.S., Harrison, G.A., Singer, R., Harris R and Jopp, W. (1964). Skin colour in southern Africa. Human Biology, 36, 294-307.

Welch, C.E., and Glick, P.C. (1981). The incidence of polygamy in contemporary Africa: A research note. Journal of Marriage and the Family, 43, 191-193.

White, D. R. (1988). Rethinking polygyny. Co-wives, codes, and cultural systems. Current Anthropology, 29, 529-572.

Winkler, E-M., and Christiansen, K. (1993). Sex hormone levels and body hair growth in !Kung San and Kavango men from Namibia. American Journal of Physical Anthropology, 92, 155-164.
Wolff, G. & Steggerda, M. (1943). Female-male index of body build in Negroes and Whites: An interpretation of anatomical sex differences. Human Biology, 15, 127-152.

Wright, N.M., Renault, J., Willi, S., Veldhuis, J.D., Pandey, J.P., Gordon, L., Key, L.L., and Bell, N.H. (1995). Greater secretion of growth hormone in black than in white men: possible factor in greater bone mineral density—a clinical research center study. Journal of Clinical Endocrinology & Metabolism, 80, 2291-2297.

Beginnings of black slavery - II

In my last post, I argued against the view that black slaves were numerically insignificant outside sub-Saharan Africa until the rise of Islam in the 7th century. It looks like black slaves began to enter the Middle East in growing numbers some time before 0 AD, the result being a slow but steady increase in the region’s black population throughout the early Christian era and into the Islamic era.

I will now discuss the origins of the black slave trade in terms of its causation. This subject has generally been approached from the demand side, i.e., the Muslim world and later the European world. Here, I will focus on the supply side. What causes, internal to sub-Saharan Africa, tended to create slaves in excess of local demand?

The question may seem incomprehensible to many. It is widely assumed that sub-Saharan Africans were enslaved because they were politically and militarily helpless in the face of European or Arab outsiders. Yet before the 19th century most black slaves were purchased peacefully at trading posts on the periphery of sub-Saharan Africa, either on the coasts or in the Sahel. They were procured from indigenous middlemen, who in turn procured them from indigenous sellers.

A more nuancé view holds that these indigenous agents enslaved their fellow Africans in response to rising demand outside Africa for black slaves, particularly on New World plantations from the 17th century onward. Nonetheless, there is evidence from earlier periods that some sub-Saharan societies were already generating slaves well in excess of their own needs. When Portuguese traders reached the Niger delta in the late 15th century they were able to purchase large quantities of slaves from indigenous sellers. This trade, at least in its initial stages, seems to have been supply-driven rather than demand-driven.

Who were these slaves? Overwhelmingly, prisoners taken in war. Although historians agree that endemic warfare was linked to the slave trade, they are less sure about the direction of cause and effect. Did the slave trade provide an incentive for warfare—as a way to get the wherewithal with which to buy foreign goods? Or did warfare provide an incentive for the slave trade—as a way to get rid of unwanted prisoners of war? The former view was propounded by abolitionists and is still popular, witness this Wikipedia article:

Enslavement became a major by-product of war in Africa as nation states expanded through military conflicts in many cases through deliberate sponsorship of benefiting Western European nations.

Yet, further on, the same article states:

The practice of enslaving enemy combatants and their villages was widespread throughout Western and West Central Africa, although wars were rarely started to procure slaves. The slave trade was largely a by-product of tribal and state warfare as a way of removing potential dissidents after victory or financing future wars.

Clearly, European traders benefited from the warfare that occurred endemically throughout sub-Saharan Africa. But they were at most an auxiliary cause. If we exclude hunter-gatherers, all sub-Saharan societies had warrior castes that predated European contact and encompassed all unmarried men. Although militarization of single males has equivalents in most cultural areas, it had a greater impact in sub-Saharan Africa because enforced bachelorhood lasted much longer and affected virtually all young men.

African men stayed single longer because so many women were siphoned off by older, high-status males. Generally 20-50% of all marriages were (and often still are) polygynous in sub-Saharan agricultural societies (Bourguignon and Greenbaum, 1973, p. 51; Goody, 1973; Pebley and Mbugua, 1989; Welch and Glick, 1981; White, 1988; see Figure 1). Year-round agriculture made women largely self-reliant in feeding themselves and their children, thus letting more men take second wives. More polygyny for some, however, meant no wives at all for others. Usually the wife shortage was resolved by raising the age of marriage for men. For instance, among the Nyakyusa:

There is no evidence of any marked difference in the survival rate of males and females, but there is a difference of ten years or more in the average marriage-age of girls and men, and it is this differential marriage-age which makes polygyny possible. (Wilson, 1950, p. 112)

Raising the marriage age, however, simply concentrated celibacy in one age group. For these young men the only way to get a woman was to abduct one through warfare. Indeed, although the decision to wage war usually lay with older, married males, the warfare itself was more easily initiated and pursued because young males saw it as the best means to become sexual, reproducing beings, i.e., ‘real men.’ The stresses created by this situation are described by Pierre van den Berghe (1979, pp. 50-51):

Typically, the more men are polygynous in a given society, the greater the age difference between husbands and wives. … The temporary celibacy of young men in polygynous societies is rarely absolute, however. While it often postpones the establishment of a stable pair-bond and the procreation of children, it often does not preclude dalliance with unmarried girls, adultery with younger wives of older men, or the rape or seduction of women conquered in warfare. Thus, what sometimes looks like temporary celibacy is, in fact, temporary promiscuity. These young men often devote themselves to warfare during their unmarried years and sometimes homosexuality is tolerated during that period.

There is a large body of literature, called Youth Bulge Theory, that relates the probability of war to the proportion of young single males in the population (Mesquida and Wiener, 1996; see also Wikipedia –War). This proportion is at its highest where the polygyny rate exceeds 20% of all marriages—mostly in the agricultural societies of sub-Saharan Africa and Papua-New Guinea.

In such regions, this internal social contradiction could be resolved by externalizing it, i.e., by abducting women from adjacent peoples through warfare and by selling off any male captives.

References

Bourguignon, E. and Greenbaum, L.S. (1973). Diversity and Homogeneity in World Societies, HRAF Press.

Goody, J. (1973). Polygyny, Economy and the Role of Women, in J. Goody (Ed.) The Character of Kinship, Cambridge: Cambridge University Press, pp. 175-190.

Mesquida, C. G. and Wiener, N.I. (1996). Human Collective Aggression: A Behavioral Ecology Perspective, Ethology and Sociobiology, 17, 247-262

Pebley, A. R., and Mbugua, W. (1989). Polygyny and Fertility in Sub-Saharan Africa. In R. J. Lesthaeghe (Ed.), Reproduction and Social Organization in Sub-Saharan Africa, Berkeley: University of California Press, pp. 338-364.

van den Berghe, P.L. (1979). Human Family Systems. An Evolutionary View. New York: Elsevier.

Welch, C.E., and Glick, P.C. (1981). The incidence of polygamy in contemporary Africa: A research note. Journal of Marriage and the Family, 43, 191-193.

White, D. R. (1988). Rethinking polygyny. Co-wives, codes, and cultural systems. Current Anthropology, 29, 529-572.

Wilson, M. (1950). Nyakyusa kinship, in Radcliffe-Brown, A.R., and Forde, D. (Eds). African Systems of Kinship and Marriage. (pp. 111-139), London: Oxford University Press.

The beginnings of black slavery

Alexander the Great’s conquests, and later the Roman Empire’s expansion, transformed the cultural landscape by merging different peoples into a syncretic Greco-Roman culture. Religion too became universal through the Hellenization of non-Greek deities and ultimately the emergence of a single universal faith. Within this same context, local markets dissolved into a larger market that brought supply and demand together throughout the Mediterranean, and even beyond.

These broadening horizons affected slave markets as well. Slaves were initially taken in war or carried off by pirates. With the suppression of piracy after the battle of Actium (31 BC) and the stabilization of the empire’s borders under the emperor Hadrian (117-138 AD), people turned to other sources: condemned criminals, children sold by indebted parents, foreigners purchased from outside the Empire (Westermann, 1955, pp. 84-85). The last source supplied nearly one eighth of all slaves, including some from sub-Saharan Africa by way of Egypt (Westermann, 1955, pp. 96, 135).

How many? Although it is known that some black Africans were present in the ancient Mediterranean world, the magnitude of their presence is hard to quantify. It is widely believed, notably by the historian Bernard Lewis, that they were relatively few in number until the creation of the Muslim world in the 7th century, when the black slave trade presumably took off (Lewis, 1990, p. 41). But there are reasons for believing that their numbers had grown considerably even before Islam, having increased slowly but steadily throughout the early Christian era. During that era, 4% of people buried in Corinth seem to have been black African (Angel, 1972). In the Egyptian scenes of the Ashburnham Pentateuch (6th century AD), almost a quarter of the faces are black. Finally, early Christian literature often mentions Ethiopians, as black Africans were called in Greek and Latin. Many of these mentions indicate personal familiarity.

Meanwhile, pagan, Christian, and Jewish writings increasingly associated dark skin with slavery. In the 1st century, the Roman author, Petronius, represents very light skin by Gauls and very dark skin by Ethiopian slaves (Satyricon 102). In the 2nd century, the Greek satirist, Lucian, wrote: “In the first place, is he not generous in his proportions and pleasing in his complexion, neither dark nor fair of skin; for the one befits a woman, and the other a slave (De parasito 41).

This mental association intensified during the early Christian era. It is especially attested by the tendency from the 3rd to 5th centuries to reinterpret the ‘Curse of Ham’ in the book of Genesis. This is the original version: when Ham saw his father Noah naked and drunk, Noah angrily condemned to slavery all of Ham’s descendents through Ham’s son, Canaan (Genesis 9: 22-27). Initially, the curse served to justify enslavement of the former inhabitants of the land of Israel, the Canaanites. Then, during the first centuries of the Christian era, it tended to target the brown- and black-skinned peoples of Africa—Ham’s supposed descendents. Finally, it came to hang solely over the black Africans, who were said to be descended from another of Ham’s sons, Chusi, but also more generally from Ham or even from Canaan.

Thus, in the 3rd century, St. Origen invoked the Curse of Ham to explain the servility of the “discolored” Egyptians:

Pharaoh easily reduced the Egyptian people to servitude, nor is it written that he did so by force. For the Egyptians are prone to a degenerate life and quickly sink to every slavery of the vices. Look at their origin: you will discover that their father Ham, who had laughed at his father’s nakedness, deserved a judgment of this kind, that his son Canaan should be a servant to his brothers, so that his condition of servitude would testify to the wickedness of his conduct. Not without reason, therefore, does the discolored posterity imitate the ignobility of the race. (Hom. in Genesim 16.1)

Dark skin is explicitly attributed to this curse in a 3rd or 4th century Samaritan exegesis: “When Kush saw the nakedness of his father, he was cursed and he wore darkness—he and all his descendents forever” (Goldenberg, 2003, p. 100). Similarly, according to a 4th century Christian account, attributed to St. Ephrem of Nisibis (Syria), Noah said: “Accursed be Canaan, and may God make his face black” (Lewis, 1990, p. 124). Another work attributed to St. Ephrem, The Cave of Treasures, also seems to link this curse to dark-skinned peoples:

For by means of singing, and lewd play, and the mad lasciviousness of the children of Cain, Satan had cast down the mighty men, the "sons of God," into fornication. And through the music of reed pipes and harps sin had multiplied among the former generations until, at length, God became wroth and made the Flood. And Canaan was cursed because he had dared to do this, and his seed became a servant of servants, that is to say, to the Egyptians, and the Cushites, and the Mûsâyê (Mysians), [and the Indians, and all the Ethiopians, whose skins are black]. And because Ham had dared to make a mock of his father he was called "vile" (or "lascivious") all the days of his life. (Ephrem, 1927, fol. 19a, 19b)

Two late versions (8th century) add: “and other blacks” or “and all those whose skin color is black” (Goldenberg, 2003, p. 173). These peoples are thus seen as having especially dark skin, i.e., darker than that of early Christians in the Greco-Roman world.

The Egyptians hold first place in this list, probably because they were the best known “people of color” at that time. Thus, the Greco-Roman world tended to see them as encompassing Ethiopians, i.e., black Africans, viewing the latter as Egyptians with even darker skin. This tendency to lump the two peoples together also appears in ancient rabbinic literature, which states that Egypt and Kush (Nubia and more generally sub-Saharan Africa) are the lands of “dark men” and that Egypt is “a place of ugly and dark people” (Goldenberg, 2003, p. 107-109, 117-118). According to the historian David Goldenberg (2003, p. 109): “Both the Jewish and the Greek etiologies show that in regard to skin color, the Kushites/Ethiopians were not considered in a separate category but were seen as part of a larger class of dark-skinned peoples.” It is in this broader sense that the word Egyptian seems to be used further on in The Cave of Treasures:

Now the seed of Canaan, as I have already said, are the Egyptians, and behold, they are scattered over the whole earth, and have been made servants of servants. And of what kind is this slavery of slavery? Behold, the Egyptians go round about all over the earth carrying loads on their backs (literally, necks). Now, men who are not fettered under the yoke of slavery, when despatched by their masters on journeys, do not march on their feet and carry loads, but they ride upon beasts in an honourable manner, like their masters. The seed of Ham are the Egyptians who carry loads, and they march on the roads with their backs and necks breaking under their loads, and they wander round to the doors of the children of their brethren. The seed of Ham was reduced, through the folly of Canaan, to suffer this penalty, that is, to become servants even to servants. (Ephrem, 1927, fol. 19b, 20a)

These “Egyptians” were probably black Africans. If so, this passage would testify, three centuries before Islam, to a sizeable diaspora of black slaves in the ancient world. In addition, these slaves were considered to be different from other slaves. This view is attested a century earlier when Origen wrote:

Thus, the divine laws provided that whoever has bought a Hebrew servant will not keep him indefinitely in servitude: he will serve six years and, the seventh, will go free. Nothing similar is decreed for the Egyptians: nowhere does the divine law have a provision for the freedom of the Egyptians, for they lost it willingly, and it abandons them to the eternal yoke of their fate and to perpetual servitude (Hom. in Genesim 16.1)

The Curse of Ham was similarly transformed in post-biblical Jewish literature. According to a 5th century text, Ham wished to keep his share of the inheritance intact and, to this end, tried to prevent his father from having another son. One day, seeing his father drunk and undressed, he seized the opportunity and castrated him. Noah awoke from his stupor and cursed Ham: “You have prevented me from doing something in the dark [i.e., sex], therefore your seed will be ugly and dark-skinned.” (Genesis Rabba 36:7). According to two Talmudic accounts from the 4th and 6th centuries, Ham violated a rule prohibiting sexual intercourse in Noah’s ark. Persuaded that the first child born after the flood would inherit the world, Ham defied the prohibition and copulated with his wife, whereupon his skin turned black (Tractate Sanhedrin 108b, Tractate Ta’an 1.6, 64d).

In the 6th century, a Christian philosopher from Alexandria, John Philoponus, named solely the black Africans as a people destined for slavery: “The Scythians and Ethiopians are distinguished from each other by black and white color, or by long and snubbed nose, or by slave and master, by ruler and ruled”; “The Ethiopian and Scythian … one is black, the other white; similarly slave and master” (Goldenberg, 2003, p. 135).

If at that time the Greco-Roman world already associated black skin with slavery, it is not because the slaves were almost all blacks but because the blacks were almost all slaves; this was the status of most foreigners. Indeed, it is precisely because black Africans formed a small visible minority that their servile status was easy to recognize (Goldenberg, 2003, pp. 135-136, 138). Furthermore, as Origen noted, they tended to retain this status longer, if not for life, thereby strengthening the mental association between dark skin and slavery.

It remains to be explained why this slave population became so numerically important even before the Islamic era. What was stimulating its growth? Perhaps supply and demand were reciprocally stimulating each other: since black Africans were less likely to lose their slave status, people increasingly came to see their tasks as “slave work” —unworthy of free people and treated accordingly. The supply thus favored its own demand by degrading the working conditions.

References

Angel, J.L. (1972). Review of Blacks in Antiquity, American Anthropologist, 74, 159-160.

Ephrem (1927). The Book of the Cave of Treasures, translated from the Syriac by E. A. Wallis Budge. London: The Religious Tract Society]

Goldenberg, D.M. (2003). The Curse of Ham. Race and Slavery in Early Judaism, Christianity, and Islam. Princeton: Princeton University Press.

Lewis, B. (1990). Race and Slavery in the Middle East. An Historical Enquiry. Oxford: Oxford University Press.

Westermann, W.L. (1955). The Slave Systems of Greek and Roman Antiquity. Philadelphia: Memoirs of the American Philosophical Society.

Skin color in early Christianity and Judaism

About 15 years ago, I received an e-mail from a specialist in Jewish studies, Dr. David Goldenberg, who had read an article of mine and wanted to know more about the subject. The article described how early Christians perceived Black Africans, or Ethiopians as they were then called, especially those who lived as a small visible minority in the Mediterranean world. I had come to this subject out of a desire to understand how differences in skin color were perceived in contexts that preceded the historical experiences of European colonialism and black slavery. This desire led me to such culture areas as the Mediterranean world of Late Antiquity, ancient India, and the contact zone between Melanesia and Polynesia. Ultimately, I wanted to isolate the patterns of perception and response that existed during the long period of human existence when, in any given society, skin color differed primarily between men and women.

David Goldenberg came to this subject from a very different angle. A number of African American authors were arguing that the Jews had invented anti-black racism, the “proof” being early rabbinical writings that had reinterpreted the Curse of Ham (originally pronounced on the Canaanites) as applying to the dark-skinned peoples of Africa. These writings certainly did exist. My article, however, showed that they were part of a larger Mediterranean tradition of attitudes to skin color that had originated as much with early Christians as with Jews.

For David, the situation was all the more worrisome because many Black Muslims were taking up the argument that “the Jews did it.” Ironically, this early Christian/Jewish ‘colorism’ had not disappeared from the Middle East with the rise of Islam; the Muslim world preserved it virtually intact, including the notion that God had condemned Black Africans to slavery and had blackened their skin as a mark of shame.

I provided him with more references, including other articles I had written. God knows what he thought of my other articles. Even fellow anthropologists think they’re weird—“But what does that have to do with racism?”

Some years after, in 2003, David Goldenberg came out with a book that pulls together all of his research: The Curse of Ham. Race and Slavery in Early Judaism, Christianity, and Islam. I have only just now started reading it and truly regret not having done so sooner. It provides a lot of material I was not aware of and is by far the most authoritative work on the subject. As such, it forms a companion piece to Bernard Lewis’ Race and Slavery in the Middle East.

Today, people routinely interpret antipathy to dark skin as being racially based. Dr. Goldenberg rises above this simplism, arguing that attitudes to skin color were much more fluid and less ethnically constructed in the ancient world. At that time, they were still largely aesthetic in nature and centered on the individual. It was only later, with the expansion of European societies into the non-European world, that these attitudes became almost wholly racialized and, as such, assumed a preponderant role in the modern worldview.

In short, what we call ‘racism’ did not develop historically from a blank slate. It arose from a transformation of earlier sentiments that were unrelated to race or ethnicity. This earlier pre-racial world is now half-forgotten, if not forgotten entirely.

As The Curse of Ham concludes on its last page:

Yet, what struck me as I read through hundreds of modern biblical commentaries and historical and cultural studies of ancient Judaism was how strongly the perspective of one’s own time and place shapes one’s view of another time and place. We today are heirs to centuries of anti-Black sentiment, which has greatly conditioned our perspective.
(Goldenberg, 2003, p. 200)

References

Frost, P. (1991). Attitudes towards Blacks in the early Christian era, The Second Century, 8(1), 1-11.

Goldenberg, D.M. (2003). The Curse of Ham. Race and Slavery in Early Judaism, Christianity, and Islam. Princeton: Princeton University Press.

Lewis, B. (1990). Race and Slavery in the Middle East. An Historical Enquiry. New York: Oxford University Press.

Why I have no answer

In my last two posts, I argued against two widespread truisms:

1. The human genome is 99.9% the same in all people.

2. If we look at the 0.1% that does vary, 85% of this variation exists only between individuals and not between populations.

Both truisms are at best superficially true. They don’t mean what many seem to think they mean. Moreover, they’ve been known to be misleading for some time; in the case of truism #1, from the moment it was first presented.

So Mr. Smarty Pants, how much do genes really differ within our species? And how much of this difference clusters into recognizable populations?

I don’t know. The problem is not simply lack of information. We’re dealing with a conceptual, even existential, problem. Genes differ in any number of ways—not only in the timing and magnitude of a particular trait, but also in countless qualitative aspects. If genes vary along a multitude of dimensions, how can we compress this multidimensional reality into one yardstick called “human difference”?

Let’s assume, as I suspect, that 10-15% of the human genome exhibits some variability. Can we say that humans differ by 10-15% from each other? I’m not so sure. For one thing, a lot of this variability is confined to remote, isolated populations that are close to extinction. Their variability is irrelevant to the overwhelming majority of humans.

For another thing, much of this variability has little or no selective value. How relevant are blood types to your daily existence? What about dormant DNA that might reactivate in one of your descendants?

Well, let’s stick to the 7% of the human genome that clearly varies because it has been selected differently in different environments (Hawks et al., 2007). How relevant are those selection pressures now? What’s the point of having a beard if you shave it off every day? And how important are hair and eye color? At one time, these color traits were under intense selection. Do they matter to us today? I suspect most people would say ‘no’ if asked. They would probably affirm that only ‘inner qualities’ matter, i.e., the soul, personality, intelligence, etc. Yet this viewpoint might change once they’re in a drugstore, especially the hair products section or the magazine counter …

Well, let’s stick to inner qualities. How do we weigh their relative importance? For instance, about 30 per cent of people have a gene variant that results in fewer dopamine receptors and, apparently, in stubborn behavior (Hall, 2008). How useful is this quality? For someone like Winston Churchill, it could have made the difference between losing and winning the war. For many elderly people, it might lead them to refuse ‘newfangled’ medication.

It’s hard to compare things that vary in value not only from one person to another but also from one situation to another. Ultimately, the single yardstick is survival and reproduction: does this trait help its bearer to survive and have children? If this is to be our yardstick, we must conclude that some people are ‘superior’ even though their behavior is widely deemed to be inferior, if not pathological. As anthropologist Henry Harpending points out:

Evolution is a double-edged sword. What evolution cares about is that I have more offspring. If you can do it by charming and manipulating, and I'm a hardworking farmer that's going to feed the kids ten years down the road, then you're going to win. Hit-and-run, irresponsible males are reproducing more. That isn't good for anyone except those males, but that's evolution. (Keim, 2007).

References

Hall, A. 2008. Why the British bulldog spirit is in the genes. Daily Mail, January 10.

Hawks, J., E.T. Wang, G.M. Cochran, H.C. Harpending, and R.K. Moyzis. (2007). Recent acceleration of human adaptive evolution. Proceedings of the National Academy of Sciences (USA) early view.

Keim, B. 2007. Humans Evolving More Rapidly Than Ever, Say Scientists. Wired Science, December 10, 2007.

The 85% truism

How much of the genome varies within our species? The question remained unanswered in my last post. Hawks et al (2007) have recently estimated that at least 7% of our genome has changed over the last 40,000 years—a period that has seen humans move into diverse environments with different selection pressures. Yet this is a minimal estimate that excludes much variation that may or may not be due to natural selection. The real figure could be higher. Much higher.

How is this genetic variation distributed among humans? Is it evenly scattered? Or does it form geographic clusters? Intuitively, the second answer seems more correct: This variation should be very unevenly distributed if it is due to humans settling in diverse environments with different selection pressures. It should occur primarily at the transition from one ecological zone to another or from one cultural zone to another (e.g., from agriculturalists to hunter-gatherers).

Yet this is not what we see in the data. If we look at genetic markers (blood types, serum proteins, enzymes, etc.), we consistently find far more variation within human populations than between them. And this is true not only for large ‘continental’ groups but also for smaller local populations. In a landmark paper, Richard Lewontin (1972, p. 397) concluded that 85% of human genetic variation exists only between individuals and not between populations:

It is clear that our perception of relatively large differences between human races and subgroups, as compared to the variation within these groups, is indeed a biased perception and that, based on randomly chosen genetic differences, human races and populations are remarkably similar to each other, with the largest part by far of human variation being accounted for by the differences between individuals.

This finding is true. Like many findings, however, it does not necessarily mean what we think it means. This became apparent when geneticists looked at genetic markers in other animals, such as dogs:

… genetic and biochemical methods … have shown domestic dogs to be virtually identical in many respects to other members of the genus. … Greater mtDNA differences appeared within the single breeds of Doberman pinscher or poodle than between dogs and wolves. Eighteen breeds, which included dachshunds, dingoes, and Great Danes, shared a common haplotype and were no closer to wolves than poodles and bulldogs. These data make wolves resemble another breed of dog.

… there is less mtDNA difference between dogs, wolves, and coyotes than there is between the various ethnic groups of human beings, which are recognized as a single species. (Coppinger & Schneider, 1995)

One could object that humans have created dog breeds using a limited set of criteria that reflect a limited set of genes. Therefore, all other criteria, especially those not visible to the eye, should vary independently of breed. The category ‘breed’ is thus an artificial construct that human selection, and not natural selection, has imposed on canine genetic variability.

This objection is not wholly true. Many breeds, such as dingoes, originated in prehistory long before kennel clubs. More to the point, if one argues that human selection acts on a limited set of genes, the implication is that natural selection acts on the entire genome. It doesn’t. Natural selection also acts on a limited set of genes, often a larger set than the one used by dog breeders, but still much smaller than the entire genome.

This point can be illustrated with non-canine examples. Considerable genetic overlap exists not only between breeds of dogs but also between many anatomically and behaviorally distinct species. In the deer family, genetic variability is greater within some species than between some genera (Cronin, 1991). Some masked shrew populations are genetically closer to prairie shrews than they are to other masked shrews (Stewart et al., 1993). Only a minority of mallards cluster together on an mtDNA tree, the rest being scattered among black ducks (Avise et al., 1990). All six species of Darwin’s ground finches seem to form a genetically homogeneous genus with very little concordance between mtDNA, nuclear DNA, and morphology (Freeland & Boag, 1999). In terms of genetic distance, redpoll finches from the same species are not significantly closer to each other than redpolls from different species (Seutin et al., 1995). Among the haplochromine cichlids of Lake Victoria, it is extremely difficult to find interspecies differences in either nuclear or mitochondrial genes, even though these fishes are well differentiated morphologically and behaviorally (Klein et al., 1998). Neither mtDNA nor allozyme alleles can distinguish the various species of Lycaedis butterflies, despite clear differences in morphology (Nice & Shapiro, 1999). An extreme example is a dog tumor that has developed the ability to spread to other dogs through sexual contact: canine transmissible venereal sarcoma (CTVS). It looks and acts like an infectious microbe, yet its genes would show it to be a canid and, conceivably, some beagles may be genetically more similar to it than they are to Great Danes (Cochran, 2001; Yang, 1996).

Does this seem paradoxical? Let’s review how organisms become different from each other through natural selection. This typically happens when a group buds off from its parent population and colonizes a new environment. The environment may be another ecosystem, another mode of subsistence or even, as with CTVS, another form of existence. As the group adapts to its new environment, it will begin to diverge anatomically and behaviorally from its parent population, in part because the environmental boundary hinders gene flow between them but more importantly because the pressures of natural selection are no longer the same. The two populations will evolve differently because what is useful in one environment may not be in the other. And vice versa.

Will these differences in selection affect the entire genome? No. For one thing, most genes have low selective value, some being little more than junk DNA. For another, many genes code for traits that are equally useful in a wide range of environments. The ‘building block’ proteins of human flesh and blood are largely identical to those of non-human primates and sometimes even non-primate mammals (King & Wilson, 1975).

Thus, only a fraction of the genome changes when one population differentiates from another in response to differences in natural selection. The rest remains unchanged, either because the genes have little selective value or because they handle adaptive problems that are common to both populations. Over most of the genome, then, variability is due not to adaptive differences created by different selection pressures but rather to non-adaptive variations that similar selection pressures have left in place.

Of course, once the two populations have become reproductively isolated, they will no longer accumulate the same non-adaptive variations and their entire genomes will drift steadily apart. But this takes time. Redpoll finches diverged into two species some 50,000 years ago and have distinct phenotypes, yet their mitochondrial DNA reveals a single undifferentiated gene pool (Seutin et al., 1995). It’s no surprise, then, that human populations exhibit so much genetic overlap. They began to move apart only 40,000 or so years ago (Pritchard et al., 1999).

References

Avise, J.C., C.D. Ankney, and W.S. Nelson. (1990). Mitochondrial gene trees and the evolutionary relationship of mallard and black ducks. Evolution, 44, 1109-1119.

Cochran, G. (2001). Personal communication.

Coppinger, R. and R. Schneider (1995). Evolution of working dogs. In J. Serpell (ed.), The Domestic Dog: Its Evolution, Behaviour and Interactions with People. Cambridge: Cambridge University Press, pp. 21-47.

Cronin, M. (1991). Mitochondrial-DNA phylogeny of deer (Cervidae). Journal of Mammalogy, 72, 533-566.

Freeland, J.R. and P.T. Boag. (1999). The mitochondrial and nuclear genetic homogeneity of the phenotypically diverse Darwin’s ground finches. Evolution, 53, 1553-1563.

Hawks, J., E.T. Wang, G.M. Cochran, H.C. Harpending, and R.K. Moyzis. (2007). Recent acceleration of human adaptive evolution. Proceedings of the National Academy of Sciences (USA) early view.

King, M-C. and A.C. Wilson. (1975). Evolution at two levels in humans and chimpanzees. Science, 188, 107-116.

Klein, J., A. Sato, S. Nagl, and C. O’hUigin. (1998). Molecular trans-species polymorphism. Annual Review of Ecology and Systematics, 29, 1-21.

Lewontin, R.C. (1972). The apportionment of human diversity. Evolutionary Biology, 6, 381-398.

Nice, C.C. and A.M. Shapiro. (1999). Molecular and morphological divergence in the butterfly genus Lycaeides (Lepidoptera: Lycaenidae) in North America: evidence of recent speciation. Journal of Evolutionary Biology, 12, 936-950.

Pritchard, J.K., M.T. Seielstad, A. Perez-Lezaun, and M.W. Feldman. (1999). Population growth of human Y chromosomes: A study of Y chromosome microsatellites.” Molecular Biology and Evolution, 16, 1791-1798.

Seutin, G., L.M. Ratcliffe, and P.T. Boag. (1995). Mitochondrial DNA homogeneity in the phenotypically diverse redpoll finch complex (Aves: Carduelinae: Carduelis flammea-hornemanni). Evolution, 49, 962-973.

Stewart, D.T., A.J. Baker, and S.P. Hindocha. (1993). Genetic differentiation and population structure in Sorex Haydeni and S. Cinereus. Journal of Mammalogy, 74, 21-32.

Yang, T.J. (1996). Parasitic protist of metazoan origin, Evolutionary Theory, 11, 99-103.

The 99.9% truism

There has been much comment on a recent finding that human evolution has accelerated over the past 40,000 years, i.e., the period during which our species has spread out of Africa and differentiated into the populations we see today (Hawks et al., 2007). There has been less comment on a related finding: at least 7% of the human genome has changed over the same 40,000 years.

This second finding seems to challenge a truism that has become widespread in academia and even in our political culture. In a speech earlier this year, Hillary Clinton cited genetic research showing that human populations are 99.9 percent the same and “that the differences in how we look -- in our skin color, our eye color, our height -- stem from just one-tenth of 1 percent of our genes.”

Isn’t there a contradiction here? How can human populations be 99.9% genetically identical if at least 7% of the genome has changed since they began moving apart some 40,000 years ago?

First, the 99.9% figure is not the number of genes that are the same. It’s the number of nucleotide sequences that are the same. A single gene is a long chain of nucleotides, often a very long one, and a single nucleotide mutation can significantly alter how the entire gene works. In theory, each and every human gene could differ by 0.1% from one population to another. And such a difference could make a big difference.

Second, the 99.9% estimate doesn’t capture higher-level nucleotide variation:

The technique originally used … could read the sequence of letters of a genetic code. But it couldn't detect repetitions of some parts of the code, which also occur. Differences in the number of these repetitions, called copy number variants, have since turned out to account for much of the variation in a species' DNA. Another type of variation recently found to be important is called insertion-deletion variants, snippets of code that are either extra or missing in some genomes compared to others. (World Science, 2007)

This higher-level variation has caused geneticist Craig Venter (of the Human Genome Project) to revise the 99.9% figure downward:

The find­ings re­veal “hu­man-to-hu­man varia­t­ion is more than sev­en-fold great­er than ear­li­er es­ti­mates, prov­ing that we are in fact very un­ique in­di­vid­u­als at the ge­net­ic lev­el,” Ven­ter said. The 99.9 fi­gure might need to be lowered to about 99, he added. (World Science, 2007) (also see original article: Redon et al, 2006)

So our nucleotide sequences may be closer to being 1% different, and not 0.1%. And don’t be fooled by small numbers. Whether it’s 1% or 0.1% the difference is still big in absolute terms. As John Hawks points out: “one-tenth of 1 percent of 3 billion is a heck of a large number -- 3 million nucleotide differences between two random genomes.”

Finally, there is a third reason why we should not read too much into any of these estimates. When the 99.9% figure first came out in the 1970s, geneticists had also discovered that nucleotide sequences were 98.9% the same between humans and chimpanzees (King & Wilson, 1975). And yet, humans and chimps exhibit a wide range of anatomical and behavioral differences. How come?

There is of course the aforementioned ‘small percentage fallacy’: a tiny sliver of the genome still amounts to a lot of DNA. More importantly, humans and chimps seem to differ the most in ‘regulatory genes’ whose effects are many times greater than those of ‘structural genes’ (the ones that code for the building block proteins of body tissues). A single regulatory gene has such a disproportionate impact because it can control the expression of many other genes.

These less numerous regulatory genes have gained importance as organisms have grown more and more complex. This has especially been so during human evolution. Whereas humans and chimpanzees are almost identical in the proteins that form their tissues, they differ radically in the way their brains and bodies develop. This point is summarized by King and Wilson (1975, p. 115):

The genetic distance between humans and chimpanzees, based on electrophoretic comparison of proteins encoded by 44 loci is very small, corresponding to the genetic distance between sibling species of fruit flies or mammals. Results obtained with other biochemical methods are consistent with this conclusion. However, the substantial anatomical and behavioral differences between humans and chimpanzees have led to their classification in separate families. … A relatively small number of genetic changes in systems controlling the expression of genes may account for the major organismal differences between humans and chimpanzees.

Interestingly, King and Wilson see this paradox as applying not only to human-chimpanzee genetic differences, but also to genetic differences within our species:

This [human-chimpanzee] distance is 25 to 60 times greater than the genetic distance between human races. In fact, the genetic distance between Caucasian, Black African, and Japanese populations is less than or equal to that between morphologically and behaviorally identical populations of other species. (King & Wilson, 1975, p. 113)

Yet human races are not identical populations, anymore than humans and chimpanzees are sibling species. These measures of genetic distance are not comparable because the nature of genetic change can vary dramatically. In one case, there is simply tinkering with an existing body plan through mutations in structural genes. In another, there is radical developmental change through mutations in regulatory genes.

Since the time that the ancestor of these two species lived, the chimpanzee lineage has evolved slowly relatively to the human lineage, in terms of anatomy and adaptive strategy. According to Simpson:

Pan is the terminus of a conservative lineage, retaining in a general way an anatomical and adaptive facies common to all recent hominoids except Homo. Homo is both anatomically and adaptively the most radically distinctive of all hominoids, divergent to a degree considered familial by all primatologists. (King & Wilson, 1975, p. 113)

This is the context in which the 99.9% statistic was initially presented to the academic community … way back in the 1970s. Even then, researchers thought it misleading and went to great pains to explain why it was misleading. Yet their caveats were to no avail. The 99.9% truism has taken on a life of its own, much like those stories we hear of alligators living in sewers or evil people sticking razor blades in Halloween apples. It seems to meet a deep-seated need to affirm our sameness and to give this affirmation a stamp of scientific approval.

But science it is not.

References

Anon. (2007). Finding said to show "race isn't real" scrapped http://www.world-science.net/othernews/070904_human-variation.htm

Elliott, P. (2007). Clinton tells grads only minor genetics make them different.
http://www.boston.com/news/nation/articles/2007/06/14/clinton_tells_grads_only_minor_genetics_make_them_different/

Hawks, J. (2007) Disagreeing with Hillary Clinton on human genetic differences.
http://johnhawks.net/weblog/topics/race/differences/clinton_2007_proportion_differences_speech.html

Hawks, J., E.T. Wang, G.M. Cochran, H.C. Harpending, and R.K. Moyzis. (2007). Recent acceleration of human adaptive evolution. Proceedings of the National Academy of Sciences (USA) early view.

King, M-C. and A.C. Wilson. (1975). Evolution at two levels in humans and chimpanzees. Science, 188, 107-116.

Redon, R., S. Ishikawa, K.R. Fitch, L. Feuk, G.H. Perry, T.D. Andrews, H. Fiegler, M.H. Shapero, A.R. Carson, W. Chen, E.K. Cho, S. Dallaire, J.L. Freeman, J.R. González, M. Gratacòs, J. Huang, D. Kalaitzopoulos, D. Komura, J.R. MacDonald, C.R. Marshall, R. Mei, L. Montgomery, K. Nishimura, K. Okamura, F. Shen, M.J. Somerville, J. Tchinda, A. Valsesia, C. Woodwark, F. Yang, J. Zhang, T. Zerjal, J. Zhang, L. Armengol, D.F. Conrad, X. Estivill, C. Tyler-Smith, N.P. Carter, H. Aburatani, C. Lee, K.W. Jones, S.W. Scherer & M.E. Hurles. (2006). Global variation in copy number in the human genome. Nature, 444, 444-454.