Thursday, May 28, 2009

Are we part-Neanderthal?

How did archaic humans evolve into the different populations of Homo sapiens we see today? The answer has long divided anthropologists. Some opt for the ‘out-of-Africa’ model; others for the multiregional model.

According to the out-of-Africa model, we all descend from a small group that existed some 100,000 to 80,000 years ago somewhere in eastern Africa. This group had an advantage over other humans, perhaps a superior ability to construct mental models in all four dimensions (Dubreuil, 2008; Klein & Edgar, 2002). They thus grew in numbers and progressively replaced their more archaic rivals, first in Africa and then elsewhere after 60,000-40,000 BP. Extinction was thus the common fate of all archaic humans throughout Europe and Asia, be they Neanderthals, Neanderthaloids, or mysterious Hobbits.

This claim is disputed by the multiregional model, which asserts that archaic Europeans and Asians did not die out. In fact, they went on to provide most of the gene pool of present-day Europeans and Asians. Modern humans thus have regional differences that can be traced to longstanding continental differences among earlier hominids. These continental groups evolved in parallel from the archaic evolutionary grade to the modern one, with gene flow gradually spreading advantageous alleles from one group to another.

The multiregional model no longer has many supporters, at least not in its pure form. With the recovery of DNA from Neanderthal remains, it has become clear that any genetic continuity with modern humans must be minor at best. There is nonetheless some support for a hybrid model, i.e., our current gene pool largely comes from a population that expanded out of Africa, but with significant admixtures from archaic Europeans and Asians. This view finds backing in a recent paper by Wall et al. (2009).

Wall and colleagues compared genetic polymorphisms in subjects of European, West African (Yoruba), and East Asian descent. They first looked for gene loci whose alleles show multiple mutational differences—a sign that the polymorphism either is very old or has undergone some kind of diversifying selection (i.e., a balanced polymorphism). Among these loci, they next looked for those that lay close to other polymorphic loci whose alleles likewise show multiple mutational differences. The authors reasoned that the more the apparently ancient alleles clustered by continental origin (West African, European, or East Asian), the greater the likelihood that these alleles had entered the gene pool from local archaic groups that modern humans had encountered while spreading out of Africa.

And the results? Wall and colleagues found significant archaic admixture: 14% in Europeans and 1.5% in East Asians. Curiously, no estimate was made for the West African subjects. The authors simply state: “Interestingly, we also find evidence for ancient admixture in the Yoruba.”

Well, how much? I’d like to know because there is another estimate based on a different methodology. When Watson et al. (1997) studied mtDNA lineages in sub-Saharan Africans, 87% of the lineages seemed to originate in a series of population expansions that began some 80,000 years ago. One cluster of lineages, dated to c. 60,000 BP, defines almost all non-African humans. The remaining 13% look much older and seem to be “the relics of a less dramatic and more ancient expansion event across Africa.

As for archaic admixture outside Africa, Wall and colleagues (like John Hawks) point to the Neanderthals and other local hominids. The admixture must have been specific to Europe and then East Asia because the European and East Asian subjects had different sets of apparently archaic alleles.

In contrast, Dienekes argues that the admixture occurred before modern humans began to spread out of Africa. The archaic alleles then persisted to varying degrees in the small founder groups that ‘budded off’ from the parent African population:

Thus, the expanding African population that eventually spilled over into Eurasia, would indeed be quite inbred and homogeneous, but its gene pool would also contain traces of the smaller, less successful African populations it had absorbed. Because of their low frequency, these traces would be more susceptible to extinction in the series of bottlenecks that led to Europeans on one side and East Asians on the other, with different sets of archaic genes preserved in either region.

I lean more to Dienekes’ explanation. If Europeans and East Asians have different archaic alleles because of separate admixture, their ancestors must have parted company before encountering the Neanderthals not long after leaving Africa (c. 40,000 BP). Yet this seems counterintuitive because the genetic distance between Europeans and East Asians (and hence their time of separation) is much less than that between Africans and non-Africans.

There is in fact a convergence of genetic, linguistic, and archeological evidence for a late split between ancestral Europeans and ancestral East Asians. A Y-chromosome study indicates that all North Eurasian peoples descend from a common ancestral population dated to about 15,000 BP (Stepanov & Puzyrev, 2000; see also Armour et al., 1996; Santos et al., 1999; Zerjal et al., 1997). The language families of northern Eurasia, particularly Uralic and Yukaghir and more generally Uralic-Yukaghir, Eskimo-Aleut, Chukotko-Kamchatkan and Altaic, share deep structural affinities that point to a common origin and not simply to word borrowing (Cavalli-Sforza, 1994, pp. 97-99; Fortescue, 1998; Rogers, 1986). Archeological evidence (characteristic lithic technology, grave goods with red ocher and sites with small shallow basins) also suggests a common cultural tradition throughout Europe and Siberia 20,000 to 15,000 years ago (Goebel, 1999; Haynes, 1980; Haynes, 1982). Finally, dental and cranial remains from southern Siberia (23,000-20,000 BP) indicate strong affinities with Upper Paleolithic Europeans (Alexeyev & Gokhman, 1994; Goebel, 1999).

Rogers (1986) places the European/East Asian split at the glacial maximum (20,000-15,000 BP). At that time, advancing glaciers and swollen glacial lakes created a barrier along the present-day Ob, thereby dividing a nomadic Eurasian population that hunted reindeer and other herbivores within a steppe-tundra belt stretching from Western Europe to Beringia.

A word of caution

Finally, a word of caution. Like any statistical analysis, the Wall et al. approach will yield a certain number of false positives. Many loci are highly polymorphic because selection has favored allele diversity, such as with balanced polymorphisms, and not because they are very old. And some will lie close on the genome to other balanced polymorphisms that likewise look older than they really are. And the alleles of these adjoining loci will often cluster by continental origin because they face different selection pressures in sub-Saharan Africans, Europeans, and East Asians—or just because of pure chance.

These false positives may also be more frequent among Europeans than among East Asians. If ancestral East Asians had budded off from an older European population, they would have taken less genetic diversity with them. They should have proportionately fewer polymorphisms and less diverse ones.

In conclusion, the jury is still out on this one. We won’t have a definite answer until we can match these continentally specific alleles with identical alleles on the Neanderthal genome now being reconstructed.


Alexeyev, V.P., & Gokhman, I.I. (1994). Skeletal remains of infants from a burial on the Mal'ta Upper Paleolithic site. Homo, 45, 119‑126.

Armour, J.A.L., Anttinen, T., May, C.A., Vega, E.E., Sajantila, A., Kidd, J.R., Kidd, K.K., Bertranpetit, J., Paabo, S., & Jeffreys, A.J. (1996). Minisatellite diversity supports a recent African origin for modern humans. Nature Genetics, 13, 154‑160.

Cavalli-Sforza, L.L., Menozzi, P. & Piazza, A. (1994). The History and Geography of Human Genes. Princeton: Princeton University Press.

Dienekes. (2009). Archaic admixture in modern humans? Dienekes’ Anthropology Blog.

Dubreuil, B. (2008). What do modern behaviors in Homo sapiens imply for the evolution of language?

Fortescue, M.D. (1998). Language Relations across Bering Strait. Reappraising the Archaeological and Linguistic Evidence. Cassell: London.

Goebel, T. (1999). Pleistocene human colonization of Siberia and peopling of the Americas: An ecological approach. Evolutionary Anthropology, 8, 208‑227.

Hawks, J. (2009). A new study of genetic introgression and human ancestry. John Hawks Weblog.

Haynes, C.V. (1982).Were Clovis progenitors in Beringia? In Paleoecology of Beringia, D.M. Hopkins (Ed.). New York: Academic Press, pp. 383‑398.

Haynes, C.V. (1980). The Clovis culture. Canadian Journal of Anthropology, 1, 115‑121.

Rogers, R.A. (1986). Language, human subspeciation, and Ice Age barriers in Northern Siberia. Canadian Journal of Anthropology, 5, 11‑22.

Santos, F.R., Pandya, A., Tyler‑Smith, C., Pena, S.D.J., Schanfield, M., Leonard, W.R., Osipova, L., Crawford, M.H., & Mitchell, R.J. (1999). The Central Siberian origin for Native American Y chromosomes. American Journal of Human Genetics, 64, 619‑628.

Stepanov, V.A., & Puzyrev, V.P. (2000). Evolution of Y‑chromosome haplotypes in populations of North Eurasia. American Journal of Human Genetics, 67, 220.

Wall JD, Lohmueller KE, and Plagnol V. (2009). Detecting ancient admixture and estimating demographic parameters in multiple human populations. Molecular Biology and Evolution (early online)

Watson, E., P. Forster, M. Richards, & H-J. Bandelt. (1997). Mitochondrial footprints of human expansions in Africa. American Journal of Human Genetics, 61, 691-704.

Zerjal, T., Dashnyam, B., Pandya, A., Kayser, M., Roewer, L., Santos, F.R., Scheifenhövel, W., Fretwell, N., Jobling, M.A., Harihara, S., Shimizu, K., Semjidmaa, D., Sajantila, A., Salo, P., Crawford, M.H., Ginter, E.K., Evgrafov, O.V., & Tyler‑Smith, C. (1997). Genetic relationships of Asians and Northern Europeans, revealed by Y‑chromosomal DNA analysis. American Journal of Human Genetics, 60, 1174‑1183.

Thursday, May 21, 2009

Latitude and sex ratio at birth

Does closeness to the equator make a woman likelier to have daughters? So says Kristen Navara (2009) in a recent article in Biology Letters:

Here, data collected from 202 countries over a decade show that latitude is a primary factor influencing the ratio of males and females produced at birth; countries at tropical latitudes produced significantly fewer boys (51.1% males) annually than those at temperate and subarctic latitudes (51.3%). This pattern remained strong despite enormous continental variation in lifestyle and socio-economic status, suggesting that latitudinal variables may act as overarching cues on which sex ratio variation in humans is based.

The article notes that this pattern results largely from low sex ratios at birth in sub-Saharan Africa. This is no surprise. Many other authors have noted relatively low numbers of male births in sub-Saharan Africa (Romaniuk 1968:278-281, 334; van de Walle 1968:38-43). The same observation has been made for African diaspora populations in the West Indies (Visaria 1967), Britain (James 1984), Latin America (Feitosa & Krieger 1993), and the United States (Ciocco 1938; Erickson 1976; Strandskov 1945; Teitelbaum 1970; Teitelbaum 1972). Sex ratios at birth are significantly lower in Black Americans than in White Americans even when birth order, socioeconomic status, paternal age, and paternal education are controlled (Erickson 1976; Teitelbaum 1972). In a review of the literature, Garenne (2008) states:

Much less work has been done in sub-Saharan Africa, primarily because of a lack of vital registration, the main source of information for studying sex ratios at birth. Ciocco (1938) noted that in 1917–1934 African Americans had a lower sex ratio (average 1.033) than Americans of European descent. Visaria (1967) also noted that the West Indies, where populations of African descent were in the majority, had low sex ratios (0.90–1.036). James (1984) concluded that the sex ratio of African populations was lower than the sex ratio of European populations, with an average value of 1.030.

Why are sex ratios at birth so low in populations of sub-Saharan descent? Most explanations point to polygyny, which reaches high levels (>20% of all sexual unions) in 85% of sub-Saharan African societies (Goody 1973, pp. 177-178). A study of seven different Kenyan ethnic groups found that polygynous relationships produce proportionately more daughters than monogamous ones (Whiting 1995). Whiting (1995) and Martin (1994) suggest that a woman will bear more daughters if she experiences sexual intercourse less frequently, as seems likely if her husband has other wives.

This facultative mechanism reduces the number of excess males later in life. In short, a low sex ratio at birth helps compensate for a higher operational sex ratio at reproductive ages. Over time, in a population with consistently high levels of polygyny, selection will tend to hardwire this adaptation by favoring alleles that decrease the sex ratio at birth.

Navara (2009), however, prefers to explain this phenomenon in terms of “climatic variables.”

… while genetic and artificial influences on the human sex ratio cannot be discounted, studies show that natal sex ratios among African countries are as diverse as in other parts of the world (Gerenne 2002) and show a positive correlation with latitude and its associated climatic variables just as we see globally (p>

The problem here is that polygyny likewise varies with latitude (Frost 2006; Frost 2008). Polygyny is more common among tropical hunter-gatherers and even more so in tropical agricultural societies. When women can gather or grow food year-round, they depend much less on men for food provisioning. It is thus much less costly for a man to take a second or third wife.

Navara (2009) cites Garenne (2002) to show that sex ratios at birth vary in sub-Saharan Africa in a way that cannot be explained by genetic factors. This is not quite Garenne’s conclusion:

The range of variation seems to go from below 1.00 to above 1.08, with possible values lower than 1 in some countries of Southern Africa. In particular, Bantu populations seem to have lower sex ratios (possibly around or below 1.00), whereas West African populations seem to have average sex ratios (close to 1.04). Some other populations, such as Nigeria and Ethiopia, could have sex ratios as high as 1.08 or higher.

The low values for Bantu populations are consistent with a genetic explanation, as are the high values for Ethiopians. But why do West Africans generally have average values and Nigerians high values? This is all the more puzzling because black diaspora populations in the United States and the West Indies have low sex ratios at birth and yet are largely descended from West Africans.

This issue is addressed by
Garenne (2008) in a later article:

Further investigations in Africa showed that the sex ratio also varied within population groups. For instance, in Nigeria the sex ratios were higher than elsewhere in Africa (>1.050) and were higher in northern Nigeria than in southern Nigeria (Ayeni 1975; Egwuatu 1984; Rehan 1982).

The difference between northern Nigeria (Muslim) and southern Nigeria (Christian/Muslim/animist) suggests that religion may be a factor. Muslims tend to have higher sex ratios at birth because of the patriarchal nature of their societies (less complete birth registration and/or higher infant mortality of daughters, tendency to postpone birth control until a son has been born, some cases of female infanticide).

Garenne (2008) did not directly study the relationship between polygyny and sex ratios at birth. He suggests that polygyny may depress sex ratio at birth in part through lower coital frequency (the husband has to satisfy more than one woman and also tends to be an older man):

Higher levels of male hormones (e.g., testosterone) seem to favor male births, and the higher levels also affect coital frequency, which indirectly affects the sex ratio. In fact, coital frequency was found repeatedly to have an effect on sex ratios, with higher frequency associated with higher sex ratios; coital frequency tends to decline with age, birth order, and marital duration (James 1996).

Polygyny may also depress sex ratios at birth through a maternal age effect, i.e., young brides are likelier to have daughters than older brides:

The effect of polygyny on sex ratios is controversial (Whiting 1993). It could be due to maternal and paternal ages, once genetic differences are controlled for. Indeed, the main consequences of polygyny are the low age at first marriage for women and the high age difference between the spouses, which is likely to increase the mean paternal age.

So if you want to have a daughter, would your chances be improved by moving to the equator? I doubt it. You’d have more luck being polygynous.


Ayeni, O. 1975. Sex ratio of live births in southwestern Nigeria. Annals of Human Biology, 2(2), 137–141.

Ciocco, A. (1938). Variation in the sex ratio at birth in the U.S. Human Biology, 10, 36-64.

Clark, S., E. Colson, J. Lee et al. 1995. Ten thousand Tonga: A longitudinal anthropological study from southern Zambia, 1956–1991. Population Studies, 49(1), 91–109.

Egwuatu, V. E. 1984. The sex ratio of Igbo births. International Journal of Gynecology and Obstetrics, 22(5), 399–402.

Erickson, D. (1976). The secondary sex ratio in the United States 1969-71: Association with race, parental ages, birth order, paternal education and legitimacy. Annals of Human Genetics, 40, 205-212.

Feitosa, M.F., Krieger, H. (1993). Some factors affecting the secondary sex ratio in a Latin American sample. Human Biology, 65, 273-278.

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), pp. 169-191.

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

Garenne, M. (2008). Poisson Variations of the Sex Ratio at Birth in African Demographic Surveys, Human Biology, 80 (5), October 2008, E-ISSN: 1534-6617 Print ISSN: 0018-7143 DOI: 10.1353/hub.0.0021

Garenne, M. (2002). Sex ratios at birth in African populations: A review of survey data, Human Biology, 74, 889-900.

Goody, J. (1973). The Character of Kinship. Cambridge: Cambridge University Press.

James, W.H. (1984). The sex ratio of black births. Annals of Human Biology, 11, 39-44.

Martin, J.F. (1994). Changing sex ratios. The history of Havasupai fertility and its implications for human sex ratio variation. Current Anthropology, 35, 255-280.

Navara, K.J. (2009). Humans at tropical latitudes produce more females, Biology Letters, published online 1 April 2009, doi: 10.1098/rsbl.2009.0069

Rehan, N. E. 1982. Sex ratio of live-born Hausa infants. British Journal of Obstetrics and Gynecology, 89(2), 136–141.

Romaniuk, A. (1968). The demography of the Democratic Republic of the Congo. In W. Brass, A.J. Coale, P. Demeny, D.F. Heisel, F. Lorimer, A. Romaniuk & E. van de Walle (Eds.) The Demography of Tropical Africa (pp. 241-341). Princeton: Princeton University Press.

Strandskov, H.H. (1945). Birth sex ratios in the total, the 'white' and the 'coloured' U.S. populations. American Journal of Physical Anthropology, 3,165-175.

Teitelbaum, M.S. (1970). Factors affecting the sex ratio in large populations. Journal of Biosocial Science (suppl.), 2, 61-71.

Teitelbaum, M.S. (1972). Factors associated with the sex ratio in human populations. In G.A. Harrison & A.J. Boyce (Eds.) The Structure of Human Populations (pp. 90-109). Oxford: Clarendon Press.

Van de Walle, E. (1968). Characteristics of African Demographic Data. In W. Brass, A.J. Coale, P. Demeny, D.F. Heisel, F. Lorimer, A. Romaniuk & E. van de Walle (Eds.) The Demography of Tropical Africa (pp. 12-87). Princeton: Princeton University Press.

Visaria, P.M. (1967). Sex ratio at birth in territories with a relatively complete registration. Eugenics Quarterly, 14, 132-142.

Whiting, J.W.M. (1993). The effect of polygyny on sex ratio at birth. American Anthropologist, 95, 435-442.

Thursday, May 14, 2009

The First European?

What did the first modern humans in Europe look like? The question comes up in a BBC2 series The Incredible Human Journey, which shows the reconstructed head of a man who lived in the Carpathian Mountains some 35,000 years ago. With its brown skin and broad nose, this ‘First European’ looks, well, very un-European.

The online article is followed by bitter comments. One observes: “It seems to me that this result was not modelled on actual human remains. But rather on the image the Ministry of Truth has on what a modern European OUGHT to look like.

And my comment? First of all, Europeans look European because they have physical traits that are rare or absent in other human populations. Since these traits are specific to Europe, they probably developed there. And they would have done so only after the arrival of modern humans 40,000-35,000 years ago. Therefore, a European living 35,000 years ago should have looked a lot less European than the ones around today.

So much for the theory. What about the facts? I should first point out that the reconstructed head is based not only on the Carpathian cranium but also on other remains from the same period and even on non-skeletal data. Obviously, skeletal remains don’t preserve skin color. We know that early modern Europeans were darker-skinned because the alleles for white skin arose much later in time—about 11,000 years ago at the SLC45A2 (AIM1) gene and 12,000–3,000 years ago at the SLC24A5 gene (Norton & Hammer, 2007; Soejima et al., 2005). As a Science journalist observed: “the implication is that our European ancestors were brown-skinned for tens of thousands of years” (Gibbons, 2007).

Does this seem counter-intuitive? How could Europeans have been brown for so long and so far north? Isn’t white skin an adaptation to northern latitudes and low levels of UV light? Well, human skin is brown among indigenous northern Asians and Amerindians who live just as far north with the same UV at ground level. As I’ve argued elsewhere, the extreme depigmentation of Europeans is probably due to sexual selection, as evidenced by other unique color traits, i.e., diversification of eye and hair color (Frost, 2006; Frost, 2008).

Early modern Europeans probably had broad noses too. We see this not only in the Carpathian cranium but also in other cranial remains from the same time period. We see this especially in a pair of skeletons from Grimaldi, northern Italy. The skeletons were initially dated to the early occupation of Europe by modern humans, c. 30,000 BP. Associated artifacts have since been radiocarbon dated to 14,000-19,000 BP but may come from later occupation layers (Bisson et al., 1996).

The Grimaldi skulls don’t look European. The face is wide but not high, the nose broad and flat, the upper jaw forward-projecting, and the chin weakly developed. The well-preserved dentition is not at all European. Among currently living populations, the ones who most closely resemble the Grimaldi humans seem to be the Khoisan peoples of southern Africa. The French physical anthropologists Boule and Vallois (1957, pp. 290-291) describe these early Europeans as having an almost African phenotype:

Comparisons which we have been able to make with the material at our disposal, in particular with the skeleton of the Hottentot Venus [a Khoisan individual], have led us to note, for instance, the same dolichocephalic character, the same prognathism, the same flattening of the nose, the same development of the breadth of the face, the same form of jaw, and the same great size of teeth. The only differences are to be found in the stature and perhaps in the height of the skull.

We know less about the soft-tissue characteristics. Alongside the skeletons were a number of female statuettes with big breasts, protruding bellies, full hips, and large buttocks. On the statuettes, the hair seems to be short and matted (Boule & Vallois, 1957, p. 311).

When did this original phenotype disappear? The data increasingly suggest that Europeans assumed their present-day appearance relatively late and over a relatively short time span. This transformation essentially took place during the last ice age (25,000 – 10,000 BP), with most of the changes probably occurring during the period after the glacial maximum (15,000 – 10,000 BP). As I’ve argued elsewhere, the cause was probably an intensification of sexual selection of women, i.e., too many women had to compete for too few men. On the one hand, male mortality increased in relation to female mortality because men had to cover much longer hunting distances. On the other, polygyny decreased because women depended much more on men for food provisioning (Frost, 2006; Frost, 2008).

This transformation took place on a vast expanse of steppe-tundra—today the plains of northern and eastern Europe—where highly mobile herds of reindeer and other herbivores were almost the sole food source for humans. The new phenotype must have then spread outward, via gene flow. Interestingly, the old phenotype may have persisted in some peripheral populations, perhaps into late prehistory and even after. As Boule and Vallois (1957, pp. pp. 291-292) note:

‘In Brittany, as well as in Switzerland and in the north of Italy, there lived in the Polished Stone period, in the Bronze Age and during the early Iron Age, a certain number of individuals who differed in certain characters from their contemporaries’, in particular in the dolichocephalic character of their skull, in possessing a prognathism that was sometimes extreme, and a large grooved nose. This is a matter of partial atavism which in certain cases, as in the Neolithic Breton skull from Conguel, may attain to complete atavism. Two Neolithic individuals from Chamblandes in Switzerland are Negroid not only as regards their skulls but also in the proportions of their limbs. Several Ligurian and Lombard tombs of the Metal Ages have also yielded evidences of a Negroid element.

Since the publication of Verneau’s memoir, discoveries of other Negroid skeletons in Neolithic levels in Illyria and the Balkans have been announced. The prehistoric statues, dating from the Copper Age, from Sultan Selo in Bulgaria are also thought to portray Negroids. In 1928 René Bailly found in one of the caverns of Moniat, near Dinant in Belgium, a human skeleton of whose age it is difficult to be certain, but which seems definitely prehistoric. It is remarkable for its Negroid characters, which give it a resemblance to the skeletons from both Grimaldi and Asselar.

It is not only in prehistoric times that the Grimaldi race seems to have made its influence felt. Verneau has been able to see, now in modern skulls and now in living subjects, in the Italian areas of Piedmont, Lombardy, Emilia, Tuscany, and the Rhone Valley, numerous characters of the old fossil race.

A similar observation is made by Fleure (1945):

In a few places in Sweden, Britain, and France people have been noticed who show characteristics of the skull and face that remind one of late-Paleolithic man: these people are usually darker, in hair and eyes, than their neighbors; sometimes they even have swarthy skins. Although this fact may not have great weight in argument, it does hint that there has been depigmentation in this region. The many stories of golden hair and blue eyes suggest that sexual selection may have helped the change.

An ancient Norse poem, the Rigsthula, describes how the god Rig created a class of thralls who were black-haired, swarthy, and flat-nosed (Jonassen, 1951). This theme comes up elsewhere in Old Norse literature (Karras, 1988). Thus, even in northern Europe, and as late as the proto-historic period, some Europeans may have retained a dark-skinned and broad-nosed phenotype. The ‘First European’ seems to have stayed around for a long time …


Bisson, M.S., Tisnerat, N., & White, R. (1996). Radiocarbon dates from the Upper Paleolithic of the Barma Grande. Current Anthropology, 37, 156–162.

Boule, M. & Vallois, H.V. (1957). Fossil Men. New York: Dryden Press.

Fleure, H.J. (1945). The distribution of types of skin color, Geographical Review, 35, 580-595.

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), pp. 169-191.

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

Gibbons, A. (2007). American Association Of Physical Anthropologists Meeting: European Skin Turned Pale Only Recently, Gene Suggests. Science 20 April 2007:Vol. 316. no. 5823, p. 364 DOI: 10.1126/science.316.5823.364a

Jonassen, C.T. (1951). Some historical and theoretical bases of racism in northwestern Europe, Social Forces, 30, 155-161.

Karras, R.M. (1988). Slavery and Society in Medieval Scandinavia. New Haven.

Norton, H.L. & Hammer, M.F. (2007). Sequence variation in the pigmentation candidate gene SLC24A5 and evidence for independent evolution of light skin in European and East Asian populations. Program of the 77th Annual Meeting of the American Association of Physical Anthropologists, p. 179.

Soejima, M., Tachida, H., Ishida, T., Sano, A., & Koda, Y. (2005). Evidence for recent positive selection at the human AIM1 locus in a European population. Molecular Biology and Evolution, 23, 179-188.

Thursday, May 7, 2009

More on paternal investment and agriculture

Many anthropologists have noted a correlation between the incidence of polygyny and the predominance of women in agriculture. The more women are responsible for producing food, the likelier men will have second or third wives. This correlation is especially evident in sub-Saharan Africa, where food is produced mostly by mothers hoeing garden plots and where fathers are highly polygynous (see Goody 1973 for a review of the literature).

This correlation has exceptions, even in sub-Saharan Africa. Goody (1973) points to the West African savannah where humans have adapted to this semi-arid zone by developing a very different system of food production. Here, women plant grain and help with the harvest but men do the rest of the farm work. Nonetheless, polygyny rates are as high as in other regions where women produce most of the food.

How do these men manage to feed all of their wives and children? Well, they don’t, at least not entirely. In this region, as in West Africa in general, women obtain food for themselves and their families through trade, i.e., by tending stalls in village markets and by gathering wood for sale. There is also food provisioning from unmarried sons. Since young men have to wait some fifteen years to get married (because so many women are taken by older polygynous males), the unmarried sons in a family are called upon to help their father provide for his wives and offspring.

This system is not without disadvantages. Ideally, a woman should specialize in activities that are compatible with infant care and transport, i.e., that can be done within a relatively small land area. This is why women tend to specialize in food gathering and hoe gardening. Collecting and selling wood requires much longer walking distances. The risks are higher not only for any infants being carried but also for the mother.

Nor does this system work very well for young men. Some of them die during the fifteen or so years they spend waiting for a bride. Their genetic contribution to the next generation is limited to kin selection (by helping out their polygynous fathers), illicit sex, and abduction of women during warfare.

If humans could design their societies, things might be done differently. The polygyny rate could be reduced to give all men early access to marriage and to equalize the burden of parental investment for men and women. This would make everything a lot fairer

Of course, societies are not designed. They come about ad hoc. And fairness has nothing to do with it either. When humans stumble into a new environment, they try to adapt while also trying to satisfy their existing inclinations and predispositions, these being psychological adaptations to the previous environment. In time, new adaptations will arise through natural selection, but again this takes time—all the more so if a way is found to sustain the existing ones.


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.