Showing posts with label Khoisans. Show all posts
Showing posts with label Khoisans. Show all posts

Monday, March 9, 2020

The ghosts of West Africa



Bushmen in the Kalahari (Wikicommons, Andy Maano). When recorded history began, in Sumer and Egypt, black Africans were absent from most of Africa, even from most of West Africa. The lands south of the Sahara were largely home to various hunter-gatherers who were small, almost childlike in build, and light reddish-brown in color. 



Most Americans think of native Africans as black and of white Africans as recent intruders; and when they think of Africa's racial history they think of European colonialism and slave trading. But very different types of peoples occupied much of Africa until as recently as a few thousand years ago.

When Jared Diamond penned those words, analysis of ancient DNA was years away. Even when it began, there was a feeling that such analysis would always be impractical in Africa or anywhere else in the tropics. The climate is too warm for that stuff to last thousands upon thousands of years.

Apparently not. DNA has been retrieved from the remains of four individuals at a site in Cameroon, two of them going back 8,000 years and the other two 3,000 years. The main finding? The individuals were most similar to Pygmies, who still exist as isolated groups of hunter-gatherers in the Congo basin. There was no genetic similarity to the Bantu peoples who now predominate throughout central, eastern, and southern Africa (Lipson et al. 2020).

This finding is no surprise. Linguistic evidence has shown that the Bantu are all descended from a group of farming peoples who, some two to three thousand years ago, began to expand eastward and southward from what is now the Cameroon-Nigeria border. 

More intriguing is the discovery of admixture from an extinct West African people. These were hunter-gatherers who shared common ancestry with the Pygmies of central Africa and the Khoisans of southern Africa; however, they had intermixed much more with an archaic hominin that had diverged from ancestral modern humans at about the same time as the Neanderthals:

The West African clade is distinguished by admixture from a deep source that can be modelled as a combination of modern human and archaic ancestry. The modern human component diverges at almost the same point as Central and southern African hunter-gatherers and is tentatively related to the deep source that contributes ancestry to the Mota individual, and the archaic component diverges close to the split between Neanderthals and modern humans (Lipson et al. 2020)

This suggests that the Bantu expansion was the second leg of an earlier expansion of farming peoples who had first replaced the hunter-gatherers of West Africa. This is in line with the thinking of George Murdock, an American anthropologist who argued that black Africans originated with the spread of agriculture from the Niger's headwaters, near the Mali-Guinea border. This region was the cradle of the Sudanic food complex: sorghum, pearl millet, cow pea, and other crops.

Murdock’s scenario is supported by linguistic evidence. Speakers of proto-Niger-Congo broke up around 10,000 years ago, and the oldest group appears to be proto-Mande speakers, whose descendants inhabit the Niger's headwaters (Blench 1984, pp. 128-129; Ehret 1984; Murdock 1959, pp. 44, 64-68). Farming itself seems to have begun later. According to Harris (1976, p. 352), “the problem of dating must be left in abeyance, but it is clear that some form of seed-crop cultivation was underway in the interior at least by the second millennium B.C.”

It looks like a stable population of hunter-gatherers took shape on the Niger’s headwaters around 10,000 years ago. They gradually became proto-agricultural, i.e., more sedentary and better able to manage their food sources. By 4,000 years ago, they had become true farmers and were entering a phase of sustained demographic expansion that would see them colonize the banks of the Niger farther and farther downstream until they reached the rain forest in southern Nigeria. As they adapted to this new environment, they reached a modus vivendi with the Pygmy inhabitants, at first as tenants and then as de facto landowners who took over more and more of the land. Meanwhile, the Pygmies were pushed back farther and farther into the forest until they were no more.

In sum, farming can support a much larger population, and it was this demographic advantage that enabled farming peoples to replace hunter-gatherers, first in West Africa and eventually throughout almost all of sub-Saharan Africa.


Memories of the first West Africans

Those hunter-gatherers are remembered in the traditions of West Africa: 

Pygmies may have been the first inhabitants of Côte d'Ivoire. In their oral tradition, most of the present-day peoples, in particular the Dan-Yacouba, recount that their ancestors, on arriving in the country, found "little red men" whom they pushed back into the forest. Others speak of "little brown men", who had supernatural powers and to whom presents were given to win them over. (Mantongouine 2012)

According to some authors like Allou and Gonnin, the presence of these mysterious beings appears in the oral traditions. They are presented as short beings about 1m 44 to 1m 55 according to J.N. Loucou, with reddish skin, abundant hair, and feet pointing backward. They appear in almost all of the regions of prehistoric Côte d’Ivoire in the sense that almost all of the oral traditions of Côte d’Ivoire’s ethnic groups affirm that they found pygmies in the area before they became established. (Afri 2013; see also Gonnin and Allou 2006; Loucou 1984, p. 18)

Everywhere, but mainly in the countries from which the Pygmies have long disappeared, the Blacks who are considered to be the oldest occupants of the land say that it does not really belong to them and that, when their distant ancestors, coming from the East, established themselves, they found it in the possession of little men with reddish complexions and large heads who were the real natives and who, in exchange for fulfilment of certain agreements, permitted the Negroes who first arrived on a piece of land to enjoy its use and cultivate it. Eventually, those little men disappeared, but the memory of them has persisted. (Delafosse 1922, p. 14)

The Mano of Liberia say that the forested area used to contain only “talking chimpanzees.” These small creatures, called Lam, inhabited the area when the Mano first came. A Lam and his family would live in a hole in the ground (Riddell 1970, p. 27).


Year-round farming, polygyny. and increased stature and robustness

In addition to their means of subsistence, this expanding population of farmers differed from the hunter-gatherers in another way: a much higher rate of polygyny. Farming, especially year-round farming, makes women more self-reliant in feeding themselves and their children, thus cutting the costs, for a man, of having a second wife (van den Berghe 1979, p. 65). The result is a high polygyny rate: 20-50% of all marriages in sub-Saharan farming societies (Bourguignon and Greenbaum 1973, p. 51; Goody 1973; Pebley and Mbugua 1989; Welch and Glick 1981; White 1988).

If some men have more wives, others have to do without. In general, men must compete more keenly with each other for access to women. When such rivalry intensifies in nonhuman species, there is selection for larger, stronger, and more muscular males. This may explain the physical robustness of polygynous farming peoples in sub-Saharan Africa.

This point was studied by Butovskaya et al. (2015) in their study of two East African peoples: the polygynous Datoga and the monogamous Hadza. Datoga men were larger and more robust than Hadza men. They also scored higher on measures of physical aggression, verbal aggression, anger, and hostility. In fact, the two groups differed fundamentally in their attitudes toward aggression:

There is a negative attitude toward aggression among the Hadza but not among the Datoga. In situations of potential aggression, the Hadza prefer to leave. In contrast, aggression is an instrument of social control — both within the family and in outgroup relations — in Datoga society. Datoga men are trained to compete with each other and to act aggressively in particular circumstances.  (Butovskaya et al. 2015).

The two groups also differed at the androgen receptor gene, with the polygynous Datoga more often having an allele that correlated in men with aggressiveness and number of children fathered. Thus, through a process of gene-culture coevolution, a highly polygynous culture has produced a different sort of man, both mentally and physically.

There are other explanations for the diminutive and less robust appearance of African hunter-gatherers. O'Dea (1994) has argued that Pygmies are smaller and less robust because they are less exposed to sunlight in the rain forest and thus less able to synthesize vitamin D and maintain a large and strong skeleton. But how would this theory explain the small, gracile appearance of the Khoisan hunter-gatherers of the Kalahari, who live in an open environment with high solar radiation?


Darker skin

The polygyny rate correlates with darkness of skin, even after you control for latitude (Manning et al. 2004). This is particularly so in sub-Saharan Africa, where highly polygynous farming peoples are noticeable darker than the largely monogamous Pygmy and Khoisan hunter-gatherers. The reason may be a widespread mental association between gender and skin color. Because women are naturally lighter-skinned than men, traditional cultures tend to associate light skin with femininity and dark skin with masculinity (van den Berghe and Frost 1986). There is thus a selective compromise between natural selection for darker skin as a protection against solar radiation and sexual selection for lighter skin as a criterion of femininity (or darker skin as a criterion of masculinity). 

Because unmated women of any kind are scarce in a polygynous society, there is weaker sexual selection for women with lighter skin. This may be why farming peoples are noticeably darker-skinned in sub-Saharan Africa (Frost 2008).


Archaic admixture in West Africa

The ancient DNA study is also consistent with evidence that a partially archaic population used to live in West Africa. One piece of evidence is a skull from a Nigerian site (Iwo Eleru), which is only about 16,300 years old and yet is intermediate in shape between the skulls of modern humans on the one hand and the skulls of Neanderthals and Homo erectus on the other (Harvati et al. 2011; Stojanowski 2014). Furthermore, genomic analysis shows an apparently higher level of Neanderthal ancestry in the Yoruba of Nigeria than in the Luhya of Kenya. This admixture seems to come from a Neanderthal-like population that once lived in West Africa (Hawks 2012).


Conclusion

In the fifteenth century, Europeans discovered a continent whose inhabitants would have looked quite different a millennium earlier. Two tenth-century Arab geographers reported that "in the outer reaches of the land of the Zanj there are cool highlands in which live white Zanj" (Lewis 1990, p. 121, n. 3). The Zanj are the dark-skinned peoples of east Africa and the “white Zanj” were probably the Khoisan hunter-gatherers who once inhabited the inland plateau of southern Africa.

If we could rewind history, we would see true black Africans retreating progressively to West Africa and then to the area of the Niger’s headwaters. This leads us to a strange conclusion. When recorded history began, in Sumer and Egypt, black Africans were absent from most of Africa, even from most of West Africa. Perhaps they didn’t yet exist anywhere. The lands south of the Sahara were largely home to various hunter-gatherers who were small, almost childlike in build, and light reddish-brown in color. 


References

Afri, A. (2013). Existait-il des peuples en Côte d’Ivoire avant le XVIIIème siècle ?
http://anicetafri.over-blog.com/existait-il-des-peuples-en-cote-d’ivoire-avant-le-xviiième-siÈcle

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

Butovskaya M.L., O.E. Lazebny, V.A. Vasilyev, D.A. Dronova, D.V. Karelin, A.Z.P. Mabulla, et al. (2015). Androgen receptor gene polymorphism, aggression, and reproduction in Tanzanian foragers and pastoralists. PLoS ONE 10(8): e0136208.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4546275/ 

Delafosse, M. (1922). Les Noirs de l’Afrique. Paris: Collection Payot. 
https://www.herodote.net/Textes/delafosse_noirs_afrique.pdf

Diamond, J. (1994). How Africa Became Black. Discover, February 1
https://www.discovermagazine.com/planet-earth/how-africa-became-black

Ehret, C. (1984). Historical/linguistic evidence for early African food production. In J.D. Clark and 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.

Frost, P. (2008). Origins of black Africans, Evo and Proud, February 10
http://evoandproud.blogspot.com/2008/02/origins-of-black-africans.html

Gonnin, G. and R.K. Allou. (2006). Côte-d’Ivoire : les premiers habitants. Abidjan: Les éditions du CERAP.

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.

Harris, D.R. (1976). Traditional systems of plant food production and the origins of agriculture in West Africa. In J.R. Harlan, J.M.J. De Wet, and A.B.L. Stemler. (ed.) Origins of African Plant Domestication, (pp. 311-356), The Hague: Moulton.
https://books.google.ca/books?id=tGOtFegfro4C&lr=&hl=fr&source=gbs_navlinks_s

Harvati, K., C. Stringer, R. Grün, M. Aubert, P. Allsworth-Jones, C.A. Folorunso. (2011). The Later Stone Age Calvaria from Iwo Eleru, Nigeria: Morphology and Chronology. PLoS ONE 6(9): e24024. doi:10.1371/journal.pone.0024024
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0024024  

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

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

Lipson, M., I. Ribot, S. Mallick, et al. (2020). Ancient West African foragers in the context of African population history. Nature 577: 665-670.
https://reich.hms.harvard.edu/sites/reich.hms.harvard.edu/files/inline-files/Shum_Laka_published_online_0.pdf

Loucou, J-N. (1984). Histoire de la Côte-d’Ivoire. Tome 1 : La formation des peuples. Abidjan: Centre d’édition et de diffusion africaine (CEDA).

Manning, J.T., P.E. Bundred, and F.M. Mather. (2004). Second to fourth digit ratio, sexual selection, and skin colour. Evolution and Human Behavior 25(1): 38-50.
https://www.sciencedirect.com/science/article/abs/pii/S1090513803000825

Mantongouine. (2012). L'histoire de la Côte d'ivoire 
http://mantongouine.free.fr/index.php?option=com_content&view=article&id=66:lhistoire-de-la-cote-divoire&catid=34:description

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

O'Dea, J.D. (1994). Possible contribution of low ultraviolet light under the rain-forest canopy to the small stature of Pygmies and Negritos. Homo 44(3): 284-7.

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.

Riddell, J.C. (1970). Labor Migration and Rural Agriculture among the Gbannah Mano of Liberia. Dissertation, Department of Anthropology, University of Oregon.
https://scholarsbank.uoregon.edu/xmlui/bitstream/handle/1794/22554/Riddell_Labor%20Migration%20and%20Rural%20Agriculture.pdf?sequence=1&isAllowed=y

Stojanowski, C.M. (2014). Iwo Eleru's place among Late Pleistocene and Early Holocene populations of North and East Africa. Journal of Human Evolution 75: 80-89.
http://www.sciencedirect.com/science/article/pii/S0047248414000876

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

van den Berghe, P.L., and P. Frost. (1986). Skin color preference, sexual dimorphism and sexual selection: A case of gene-culture co-evolution? Ethnic and Racial Studies 9(1): 87-113.
https://www.tandfonline.com/doi/abs/10.1080/01419870.1986.9993516

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.

Tuesday, September 26, 2017

Tales from old bones



Around three thousand years ago Bantu began to spread east and south from the Nigeria/Cameroun border, eventually replacing the original inhabitants of eastern and southern Africa. Those people no longer exist. Only the DNA in their skeletal remains are left to speak for them.


When scientists began to retrieve ancient DNA from human remains, they succeeded only at sites in the temperate and arctic zones. It seemed impossible to retrieve any at tropical sites, apparently because warm year-round temperatures soon reduce DNA to a meaningless molecular jumble.

This problem seems to be solved. Two years ago, DNA was successfully retrieved from 4,500 year old remains in Ethiopia. Now, we have ancient DNA from several sites across eastern and southern Africa over a range of dates from 10,000 to 400 years ago (Skoglund et al. 2017).

Vanished peoples

This new study shows that eastern and southern Africans have changed a lot since the time of the ancient Greeks. As far north as Tanzania, the continent was once home to peoples related to the Hottentots (now called Khoisans, Khoe-Sans, or simply San)—short in stature, gracile in body build, and light yellowish brown in color. From Zanzibar north, people were of mixed Middle Eastern and Cushitic origin—sort of like present-day Ethiopians but with more Arab ancestry.

What happened to these peoples? They were either replaced or absorbed by Bantus moving in from the west, although it now looks like they were replaced a lot more than they were absorbed. No trace of them remains in Malawi's gene pool:


Population replacement by incoming food producers appears to have been nearly complete in Malawi, where we detect little if any ancestry from the ancient individuals who lived ~8,100-2,500 BP. Instead, present-day Malawian individuals are consistent with deriving all their ancestry from the Bantu expansion of ultimate western African origin. (Skoglund et al. 2017)

The original inhabitants were related to present-day Khoisans but had significantly diverged from them:

Notably, the Khoe-San-related ancestry in ancient individuals from Malawi and Tanzania is symmetrically related to the two previously identified lineages present in the San [...], estimated to have diverged at least 20,000 years ago [...], implying that this was an ancient divergent branch of this group that lived in eastern Africa at least until 1,400 BP. (Skoglund et al. 2017)

This is in line with previous DNA findings from the Fwe (a Bantu group of southwestern Zambia), particularly the presence of Khoisan admixture that resembles nothing in present-day Khoisans:

[It is possible] that the Fwe intermarried with a Khoisan group whose genetic composition differed from that of the populations included in molecular anthropological investigations to date. [...] it is plausible that the Fwe ancestors interacted with a Khoisan community that differed genetically from those still settled in southern Africa today, which was ultimately replaced by the newcomers. (Barbieri et al. 2013)

Aside from these scattered fragments of DNA, we also have the testimony of ancient observers. Two tenth-century Arab geographers state that "in the outer reaches of the land of the Zanj there are cool highlands in which live white Zanj" (Lewis 1990, p. 121, n. 3). The Zanj are the dark-skinned peoples of east Africa and the term 'white' is better translated by 'lighter-skinned.' (The words 'black' and 'white' are often used in a relative sense in Arabic). The highlands might be the Drakensberg Escarpment of South Africa. 

Encounters with the archaic Other

Modern humans arose some 80,000 years ago in eastern Africa through a series of population expansions that culminated twenty thousand years later in a big bang that spread outward in Africa and then into the Middle East, Europe, and Asia (Watson et al. 1997). There, they encountered more archaic hominins: Neanderthals and, farther east, Denisovans. There was some intermixture. How much? Some have argued that modern Europeans and Asians are 3.4 to 7.9 percent admixed (Lohse and Frantz 2013). Most still opt for a lower figure of 1.5 to 2.1 percent (Prüfer et al. 2014).

But it wasn't only in Eurasia that modern humans encountered Neanderthal-like groups. Archaic hominins were present in Africa itself, some being relatively close to modern humans, and some more distantly related.

The latest DNA study has confirmed that modern humans intermixed with at least one archaic group as they expanded into western Africa:

The possible basal western African population lineage would represent the earliest known divergence of a modern human lineage that contributed a major proportion of ancestry to present-day humans. Such a lineage must have separated before the divergence of San ancestors, which is estimated to have begun on the order of 200-300 thousand years ago. (Skoglund et al. 2017)

This archaic ancestry is visible in human remains found at the Iwo Eleru rock shelter, in southwestern Nigeria, and dated to approximately 16,300 BP:

Our analysis indicates that Iwo Eleru possesses neurocranial morphology intermediate in shape between archaic hominins (Neanderthals and Homo erectus) and modern humans. This morphology is outside the range of modern human variability in the PCA and CVA analyses, and is most similar to that shown by LPA individuals from Africa and the early anatomically modern specimens from Skhul and Qafzeh. (Harvati et al., 2011)

Archaic ancestry is also visible in present-day West Africans, particularly in their teeth: 

[...] compared to other world populations, Africans south of the Sahara Desert are distinct dentally — especially in their expression of nine high- and two low-frequency morphological features. [...] the same nine high-frequency traits are also ubiquitous in the dentitions of extinct hominids and many extinct and extant non-human primates.  
[...] The presence and, indeed, prevalence (see next section), of high-frequency Sub-Saharan dental traits in fossil and recent hominoids—some of which are probably direct ancestors of modern humans, suggests they have been around for a long time.  
[...] A final ancestral feature found with some regularity in Sub-Saharan Africans, relative to other modern groups, is polydontia. Numerous cases of extra incisors, third premolars, and fourth molars have been noted [...] In one study (Watters, 1962) the incidence reached 2.5-3% in several hundred west Africans; many of the extra teeth were fully formed and erupted. "Typical" mammals exhibit three incisors and four premolars (Jordan et al., 1992). Polydontia is also found in living non-human primates. (Irish, 1998)

How much archaic ancestry do sub-Saharan Africans have today? The latest DNA study is silent on this point. Any answer can only be approximate, there being no reconstructed genome of this Neanderthal-like population. Moreover, there was probably more than one such population within Africa. Watson et al. (1997) attribute 13% of the sub-Saharan gene pool to a population that expanded some 111,000 years ago—when Skhul-Qafzeh hominins entered the Middle East from Africa. Those hominins were anatomically modern, or almost so, but culturally Neanderthal. Hammer et al. (2011) estimate that about 2% of the sub-Saharan African genome comes from a much more divergent population that split off from the ancestors of modern humans some 700,000 years ago. That admixture entered the sub-Saharan gene pool about 35,000 years ago, perhaps in Central Africa, since pygmy groups from that region have the most.

It looks like the proportion of archaic ancestry is higher in sub-Saharan Africans than in other modern humans. This is to be expected because of the broader range of archaic populations in Africa, including some that were almost modern anatomically and behaviorally. Admixture with them would have been likelier.

Admixture: good, bad, or neither?

Some alleles have successfully introgressed from archaic hominins, thus helping our ancestors adapt to new climates and new diets (Racimo et al. 2015). In general, however, we should not expect such alleles to perform as well in the body of a modern human as they did in the body of an archaic hominin. It's like taking a part from a Chevy and installing it on a Subaru. It might work, but I wouldn't count on it. 

If we look at Neanderthal admixture in the Eurasian genome, we see that natural selection has tended to remove functional genes, while leaving the non-functioning ones alone. 

Neanderthal ancestry decreases in proximity to functional elements in all populations [...] as does Denisovan ancestry in Oceanians [...] most likely reflecting greater selection against Neanderthal ancestry in low B statistic regions. Power to detect archaic ancestry is elevated close to regions of linked selection due to a reduction in the rates of incomplete lineage sorting caused by the lower effective population size in these regions, so these results are not artifacts of reduced power. Thus, similar processes appear to have worked to remove Neanderthal and Denisovan ancestry near genes. (Sankararaman et al. 2016)

Archaic admixture is also associated with reduced male fertility:

Our study provides new evidence in support of the hypothesis that reduced male fertility may be a common feature of admixture between human populations diverged by at least a half million years, a hypothesis that was previously suggested based on genetic patterns associated with the hybridization between Neanderthals and modern humans.

[...] One line of evidence for reduced fertility in male hybrids is that the proportion of archaic ancestry in modern humans is significantly reduced on chromosome X compared to the autosomes. This is suggestive of reduced male fertility as loci contributing to this phenotype are concentrated on chromosome X in hybrids of other species. We confirm an extreme reduction of Neanderthal ancestry on chromosome X (16%-34% of the autosomes depending on the population) and find a quantitatively similar reduction of Denisovan ancestry (21% of the autosomes in Oceanians).

The second line of evidence in support of the hypothesis of reduced fertility in hybrids is that there is a reduction of archaic ancestry in genes that are disproportionately expressed in testes, a known characteristic of male hybrid fertility (Sankararaman et al. 2016)

In sum, archaic admixture did provide modern humans with some ready-made alleles that have helped them adapt to new climates and new diets, but this advantage hardly applies to Africa. There, modern humans were already adapted to the local climate and diet. Archaic admixture couldn't have done much to help them adapt, since the new environments they faced were cultural ones of their own making.

References

Barbieri, C., A. Butthof, K. Bostoen, and B. Pakendorf. (2013). Genetic perspectives on the origin of clicks in Bantu languages from southwestern Zambia, European Journal of Human Genetics, 21(4), 430-436.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3598317/  

Hammer, M.F., A.E. Woerner, F.L. Mendez, J.C. Watkins, and J.D. Wall. (2011). Genetic evidence for archaic admixture in Africa, Proceedings of the National Academy of Science (USA), 108(37), 15123-15128, www.pnas.org/cgi/doi/10.1073/pnas.1109300108

Harvati, K., C. Stringer, R. Grün, M. Aubert, P. Allsworth-Jones, C.A. Folorunso. (2011). The Later Stone Age Calvaria from Iwo Eleru, Nigeria: Morphology and Chronology. PLoS ONE 6(9): e24024. doi:10.1371/journal.pone.0024024
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0024024  

Irish, J.D. (1998). Ancestral dental traits in recent Sub-Saharan Africans and the origins of modern humans, Journal of Human Evolution, 34, 81-98.
http://www.sciencedirect.com/science/article/pii/S0047248497901913  

Lohse, K., and L.A.F. Frantz. (2013). Maximum likelihood evidence for Neandertal admixture in Eurasian populations from three genomes, Populations and Evolution, 1307, 8263
http://www.integratedbiology.com/uploads/2/5/6/9/25695765/_1307.8263.pdf  

Prüfer, K., F. Racimo, N. Patterson, F. Jay; et.al. (2014). The complete genome sequence of a Neandertal from the Altai Mountains, Nature, 505(7481), 43-49.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4031459/

Racimo, F., S. Sankararaman, R. Nielsen, and E. Huerta-Sanchez. (2015). Evidence for archaic adaptive introgression in humans, Nature Reviews Genetics, 16(6), 359-371.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4478293/  

Sankararaman, S., S. Mallick, N. Patterson, D, Reich; et al. (2016). The combined landscape of Denisovan and Neanderthal ancestry in present-day humans, Current Biology, 26(9), 1241-1247.
http://www.sciencedirect.com/science/article/pii/S0960982216302470  

Skoglund, P., J.C. Thompson, M.E. Prendergast, A. Mittnik; et al. (2017). Reconstructing prehistoric African population structure, Cell, 171(1), 59-71
http://www.cell.com/cell/fulltext/S0092-8674(17)31008-5  

Watson, E., P. Forster, M. Richards, and H-J. Bandelt. (1997). Mitochondrial footprints of human expansions in Africa, American Journal of Human Genetics, 61, 691-704.
https://ac.els-cdn.com/S000292970764333X/1-s2.0-S000292970764333X-main.pdf?_tid=9bebd320-a127-11e7-9e7c-00000aacb360&acdnat=1506257895_6658dfe089335953bd04987696fa7444