Sunday, February 10, 2008

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’.

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13 comments:

Jim Bowery said...

There is a lot to comment on here but the most important thing that hits me upon reading this is that there is no mention of the pre-agricultural environment of subsahran Africa being relatively supportive of women's independence due to higher carrying capacity provided by the natural, uncultivated, ecosystems there. The pygmies and Khosa may be a reflection of their own paleolithic ecologies selecting for monogamy rather than of paleolithic ecologies in general.

If so, there could have been areas in Africa, supporting polygyny for a very long time.

Admittedly, if polygyny has such a profound impact on physique, one should expect to see skeletal remains reflecting this prior to the 20kybp mentioned, but if the climate was much different then, can we be reasonably sure the digs are in the right place?

Peter Frost said...

I've heard variations of this criticism before, i.e., Khoisans now live in resource-poor environments and thus inaccurately represent the hunter-gatherer lifestyle of more normal environments.

Pygmies, however, live in relatively resource-rich environments, and their polygyny rates are equally low (less than 10% of all sexual unions). So I think we're looking at a real difference between tropical hunter-gatherers and tropical agriculturalists.

This being said, I agree with your main point: among hunter-gatherers, the polygyny rate is higher in the tropical zone than in the temperate and arctic zones. Since the polygyny rate correlates positively with female self-sufficiency in food procurement, women become less self-suffient the further away they live from the equator (because they cannot gather as much food during the winter).

We don't have as many skeletal remains from sub-Saharan Africa as we do from Europe, although Khoisan-like remains have been found as far north as Sudan. As far as I know, "Asselar Man" (6,500 BP) is the oldest skeleton that approximates the physical characteristics of present-day black Africans. Do you know of anything older?

Jim Bowery said...

Perhaps the metric we're looking at needs to be refined for higher correlation with polygyny as Alexander et al found with male to female body length ratio among primates.

I must confess ignorance of the skeletal remains. I am not at all surprised by the amplification of polygyny by agriculture. This is the most under-studied phenomenon in all of human knowledge given its profound implications for human social organization. That may sound hyperbolic but reflection will show it is quite rational. Its very gratifying to scholars grappling with it. It somewhat remedies my jaundiced view of academia.

n/a said...

Perhaps the metric we're looking at needs to be refined for higher correlation with polygyny as Alexander et al found with male to female body length ratio among primates.

But negroids show less sexual dimorphism than Europeans. In the U.S., black men tend to be slightly shorter than white men, and black women are as tall as or slightly taller than white women.

Guessedworker said...

I am not a scientist, so please forgive my layman's language.

Peter, you argue for a cascade of change .. that high West African ST follows follows from intensification of male sexual selection, which follows from polygyny, which follows from food self-reliance of the female, which follows from the advent of farming.

Rushton postulated that the unmitigatable evolutionary pressures of disease and drought, not changes to skills and social mores, were the determinants of African ST. This is a simple mechanism, avoiding the requirement for cascades.

Its principal deficiency appears to be the gap of 290,000 of the 300,000 years of Homo sapiens sub-Saharan existence while we waited for the high ST Bantu to appear. However, do we know that older peoples like the Khoisan and the Pygmie exhibit a low ST by present global standards? Or is a somewhat higher than average ST loading akready apparent among the descendents of older African populations?

Further, Lynn has suggested that a substantial component of the enhancement to the IQ of African-Americans reflects the available improvement through a Western diet.

If Lynn is right, nutrition is a rapid producer of a significant advantage in mate selection. One is bound to ask, therefore, why during the 12,000 years max of the agricultural revolution in West Africa, there has been no selection of the more intelligent agriculturalist - and no intelligent agriculture, of course.

I'd appreciate your comments.

Anonymous said...

What does ST mean in your layman's language?

tnx

Peter Frost said...

I'm not sure what ST stands for. My argument is that high male robustness and high testosterone/DHT point to a higher level of male-male competition for access to women, such as we see in many non-human species.

I don't see how disease and drought can explain any of the above. Nor do I see why disease and drought would have been any worse in sub-Saharan Africa than elsewhere. (The current drought in much of Africa is recent and its causes are rooted in current political and demographic conditions). As a rule, environments become less stable and more resource-poor the further away you go from the Tropics.

I respect Phil Rushton, but much of his writing on this subject is simply wrong. Tropical environments are more conducive to k-type reproductive strategies, and the oldest African populations (Khoisans and pygmies) tend to be very-'k' (i.e., long intervals between births, relatively low fecundity, etc.). Tropical environments, especially agricultural ones, also offer the most stable food supply with the highest yields per square kilometer.

If you're looking for populations that 'crash' frequently and need to replenish their numbers just as often, you should look in the temperate and arctic zones, not in the tropics

Anonymous said...

This is interesting, do I read correctly that the so-called "Hamitic" people living south of Egypt in ancient times were originally a lot lighter than they are today?

Peter Frost said...

This is what Junker (1921) states. Before 2700 BC, the Egyptians did not depict the inhabitants of Nubia as being different in appearance. During the same period, skeletal remains from Nubia do not look much different from those of Egypt.

The word 'Nubian' itself seems to have been originally a geographic designation. Only later did it come to mean 'black African'.

Anonymous said...

You seem to have a limited definition of "black african". Why is the bantu group the only group considered black. I am a black east african and my ancestors never practiced agriculture. We have been keeping cattle for an extremely long time. How are the Nilo-Saharan groups considered non-black. I think you have a dangerous racial agenda (leaning towards Nazis).

Anonymous said...

There's no need to play the race card. Mr. Frost's argument is sound, and perfectly in synch with all the available mainstream scientific data on Black Africans. Blacks all really do originate from a relatively small West African and possibly pygmy population within the last 20,000 years. The peoples of the Horn of Africa were already in East Africa long before blacks first arrived there in the Bantu and Nilotic expansions. They therefore do not, racially-speaking, share the same origin as blacks. Ancient Horn Africans also did use to practice agriculture. In his "Clines and Clusters" paper, the anthropologist Loring Brace makes this clear by pointing out that Horn Africans have a considerably smaller tooth size than blacks -- a fact already observed in other studies e.g. Hanihara et al. 2005. Horn Africans cluster with Eurasians in this respect (and in many others too). Brace continues that "evidently the ancestors of the Somalis have long been associated with food preparation practices that reduced the selective force intensity maintaining tooth size. This is consistent with the possibility that the Ethiopian highlands were the locale of one of the ancient and semi-independent centers of plant domestication." Brace's conclusion is also consistent with climactic data which show that the Horn of Africa used to be, as recently as 5,500 years ago, a fully fertile area with arable land, plants, lakes, etc. and not the semi-desert it largely is now: http://www.guardian.co.uk/science/2005/sep/16/highereducation.climatechange

Anonymous said...

Anonymous said...
"There's no need to play the race card. Mr. Frost's argument is sound, and perfectly in synch with all the available mainstream scientific data on Black Africans. Blacks all really do originate from a relatively small West African and possibly pygmy population within the last 20,000 years. The peoples of the Horn of Africa were already in East Africa long before blacks first arrived there in the Bantu and Nilotic expansions. They therefore do not, racially-speaking, share the same origin as blacks. Ancient Horn Africans also did use to practice agriculture. In his "Clines and Clusters" paper, the anthropologist Loring Brace makes this clear by pointing out that Horn Africans have a considerably smaller tooth size than blacks -- a fact already observed in other studies e.g. Hanihara et al. 2005. Horn Africans cluster with Eurasians in this respect (and in many others too). Brace continues that "evidently the ancestors of the Somalis have long been associated with food preparation practices that reduced the selective force intensity maintaining tooth size. This is consistent with the possibility that the Ethiopian highlands were the locale of one of the ancient and semi-independent centers of plant domestication." Brace's conclusion is also consistent with climactic data which show that the Horn of Africa used to be, as recently as 5,500 years ago, a fully fertile area with arable land, plants, lakes, etc. and not the semi-desert it largely is now: http://www.guardian.co.uk/science/2005/sep/16/highereducation.climatechange"



The Black Nilotics of your post are a one of the "Nilosaharan groups" the previous poster mentions. There is no evidence that they originated in west Africa. Some linguists eg:Blench posit a common origin of Niger congo and Nilosaharan possibly in Chad with the former a branch of the latter. The Nilotic negroid type (or something similar) may be earlier than many western variants. The remains at Ishango have been described similar to Nilotids.
see :
Barbed Bone Points: Tradition and Continuity in Saharan and Sub-Saharan Africa
John E. Yellen

Anonymous said...

Professor Frost,


Could you clarify what you think the people of Nubia looked like prior to c. 2000BC?

Is it your contention that:

A) The original population of both Nubia and the Horn resembled present-day Egyptians, and that both Nubians and Horners were subsequently modified by black migrations? Or

B)that Nubians originally looked like Ethiopians/Somalis (i.e. aquiline-featured but dark-skinned) and that only Sudan was significantly altered by the West African influx?

If you're arguing for A), this would seem to be contradicted by genetic evidence. The studies I've seen show very little West African ancestry in Ethiopians or Somalis - apart from minority groups that have recent admixture from the slave trade. As a rule, Horners seem to descend from a very ancient, indigenous "East African" population (closer to Khoisan than West Africans) that was subsequently altered by waves of Southwest Asian migration/gene flow.

If your hypothesis allows for the presence of this older, generalized African stratum in the Northeast African population prior to the black expansion, then it may well be in accord with the genetic data.