Genetic Chaos

Tuesday, March 08, 2005

Genetic Evidence for the Expansion of Arabian Tribes into the Southern Levant and North Africa

In a recent publication, Bosch et al. (2001) reported on Y-chromosome variation in populations from northwestern (NW) Africa and the Iberian peninsula. They observed a high degree of genetic homogeneity among the NW African Y chromosomes of Moroccan Arabs, Moroccan Berbers, and Saharawis, leading the authors to hypothesize that "the Arabization and Islamization of NW Africa, starting during the 7th century AD, ... [were] cultural phenomena without extensive genetic replacement" (p. 1023). H71 (Eu10) was found to be the second-most-frequent haplogroup in that area. Following the hypothesis of Semino et al. (2000), the authors suggested that this haplogroup had spread out from the Middle East with the Neolithic wave of advance. Our recent findings (Nebel et al. 2000, 2001), however, suggest that the majority of Eu10 chromosomes in NW Africa are due to recent gene flow caused by the migration of Arabian tribes in the first millennium of the Common Era (CE).

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Ancient mtDNA analysis and the origin of the Guanches

The prehistoric colonisation of the Canary Islands by the Guanches (native Canarians) woke up great expectation about their origin, since the Europeans conquest of the Archipelago. Here, we report mitochondrial DNA analysis (HVRI sequences and RFLPs) of aborigine remains around 1000 years old. The sequences retrieved show that the Guanches possessed U6b1 lineages that are in the present day Canarian population, but not in Africans. In turn, U6b, the phylogenetically closest ancestor found in Africa, is not present in the Canary Islands. Comparisons with other populations relate the Guanches with the actual inhabitants of the Archipelago and with Moroccan Berbers. This shows that, despite the continuous changes suffered by the population (Spanish colonisation, slave trade), aboriginal mtDNA lineages constitute a considerable proportion of the Canarian gene pool. Although the Berbers are the most probable ancestors of the Guanches, it is deduced that important human movements have reshaped Northwest Africa after the migratory wave to the Canary Islands.

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Palaeolithic Populations and Waves of Advance

The wave-of-advance model has been previously applied to Neolithic human range expansions, yielding good agreement to the speeds inferred from archaeological data. Here, we apply it for the first time to Palaeolithic human expansions by using reproduction and mobility parameters appropriate to hunter-gatherers (instead of the corresponding values for preindustrial farmers). The order of magnitude of the predicted speed is in agreement with that implied by the AMS radiocarbon dating of the late glacial human recolonization of northern Europe (14.2–12.5 kyr BP). We argue that this makes it implausible for climate change to have limited the speed of the recolonization front. It is pointed out that a similar value for the speed can be tentatively inferred from the archaeological data on the expansion of modern humans into the Levant and Europe (42–36 kyr BP).

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A Predominantly Neolithic Origin for Y-Chromosomal DNA Variation in North Africa

We have typed 275 men from five populations in Algeria, Tunisia, and Egypt with a set of 119 binary markers and 15 microsatellites from the Y chromosome, and we have analyzed the results together with published data from Moroccan populations. North African Y-chromosomal diversity is geographically structured and fits the pattern expected under an isolation-by-distance model. Autocorrelation analyses reveal an east-west cline of genetic variation that extends into the Middle East and is compatible with a hypothesis of demic expansion. This expansion must have involved relatively small numbers of Y chromosomes to account for the reduction in gene diversity towards the West that accompanied the frequency increase of Y haplogroup E3b2, but gene flow must have been maintained to explain the observed pattern of isolation-by-distance. Since the estimates of the times to the most recent common ancestor (TMRCAs) of the most common haplogroups are quite recent, we suggest that the North African pattern of Y-chromosomal variation is largely of Neolithic origin. Thus, we propose that the Neolithic transition in this part of the world was accompanied by demic diffusion of Afro-Asiatic speaking pastoralists from the Middle East.

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