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The secret history of Africa

Gene ExpressionBy Razib KhanApril 17, 2009 6:40 PM


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In light of the

relatively recent interaction of Bantu farmers and Pygmies in Central Africa, this paper is of note, Genetic and demographic implications of the Bantu expansion: insights from human paternal lineages:

The expansion of Bantu languages, which started around 5,000 years before present (YBP) in west/central Africa and spread all throughout sub-Saharan Africa, may represent one of the major and most rapid demographic movements in the history of the human species. Although the genetic footprints of this expansion have been unmasked through the analyses of the maternally-inherited mitochondrial (mtDNA) lineages, information on the genetic impact of this massive movement and on the genetic composition of pre-Bantu populations is still scarce. Here we analyze an extensive collection of Y-chromosome markers - 41 SNPs and 18 STRs - in 883 individuals from 22 Bantu-speaking agriculturalist populations and 3 Pygmy hunter-gatherer populations from Gabon and Cameroon. Our data reveal a recent origin for most paternal lineages in west Central African populations most likely resulting from the expansion of Bantu-speaking farmers that erased the more ancient Y-chromosome diversity found in this area. However, some traces of ancient paternal lineages are observed in these populations, mainly among hunter-gatherers. These results are at odds with those obtained from mtDNA analyses, where high frequencies of ancient maternal lineages are observed, and substantial maternal gene flow from hunter-gatherers to Bantu farmers has been suggested. These differences are most likely explained by socio-cultural factors such as patrilocality. We also find the intriguing presence of paternal lineages belonging to Eurasian haplogroup R1b1*, which might represent footprints of demographic expansions in central Africa not directly related to the Bantu expansion.

I've heard about the R1b1* in Africa before, but hadn't thought much about it. In short it looks like the most plausible models is one of back migration from Western Eurasia within the last 10,000, to the point where derived haplotypes are extant at relatively high frequencies in some regions of West-Central Africa. In light of the likelihood that the phylogeography of Africa has been assembled into its recognizable present form very recently these data are of more interest. Dienekes notes from the paper:

A remarkable finding of our study is the substantial number of individuals belonging to haplogroup R1b1* (5.2%). Surprisingly, it has been previously observed in northern Cameroon (40%) at high frequencies (Cruciani et al. 2002), and at lower frequencies in southern Cameroon (1.12%) (Cruciani et al. 2002), Oman (1%), Egypt (2%), Hutu from Rwanda (1%) (Luis et al. 2004). The presence of this lineage in Africa has been claimed to be a genetic signature of a possible backflow migration from west Asia into Africa (Cruciani et al. 2002). Here we observe R1b1* in 12 Bantu agriculturalist populations (ranging from 2% to 20%) and in two Pygmy individuals. A network of R1b1* haplotypes performed using STR-data (Figure 2) shows two main clusters, without any population structure. Interestingly, the estimated expansion time for these haplotypes - 7,000 years (SD 8,100) - precedes the time at which the Bantu expansion occurred. ... It is noteworthy that the Fang population is the Bantu agriculturalist group presenting the highest frequency of R1b1*. The presence of the Fang in west Central Africa appears to be recent and they 20 are thought to have entered the region from the north-eastern open grassland plateau during the 17th and 18th centuries (Perrois 2006).

The standard deviation is big, so I wouldn't too much faith the estimates aside from the likelihood that the expansion didn't occur during the height of the Ice Age. One might remember that

Eurasian variants of LCT

which confer lactase persistence are found in West Africa, among the peoples of he Sahel. Though this is likely a more recent instance of gene flow (with more certitude as to its time interval than the R1b1* back migration), it shows that despite the Sahara there are ways that genes can seep back into Africa from the outside. One of Dienekes readers has posted a list of links on the topic for those who want to dig deeper....

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