An open question in the history of human migration is the identity of the earliest Eurasian populations that have left contemporary descendants. The Arabian Peninsula was the initial site of the out-of-Africa migrations that occurred between 125,000 and 60,000 yr ago, leading to the hypothesis that the first Eurasian populations were established on the Peninsula and that contemporary indigenous Arabs are direct descendants of these ancient peoples. To assess this hypothesis, we sequenced the entire genomes of 104 unrelated natives of the Arabian Peninsula at high coverage, including 56 of indigenous Arab ancestry. The indigenous Arab genomes defined a cluster distinct from other ancestral groups, and these genomes showed clear hallmarks of an ancient out-of-Africa bottleneck. Similar to other Middle Eastern populations, the indigenous Arabs had higher levels of Neanderthal admixture compared to Africans but had lower levels than Europeans and Asians. These levels of Neanderthal admixture are consistent with an early divergence of Arab ancestors after the out-of-Africa bottleneck but before the major Neanderthal admixture events in Europe and other regions of Eurasia. When compared to worldwide populations sampled in the 1000 Genomes Project, although the indigenous Arabs had a signal of admixture with Europeans, they clustered in a basal, outgroup position to all 1000 Genomes non-Africans when considering pairwise similarity across the entire genome. These results place indigenous Arabs as the most distant relatives of all other contemporary non-Africans and identify these people as direct descendants of the first Eurasian populations established by the out-of-Africa migrations.
This is a good paper. They’ve taken a stab at it, and are very circumspect. But in the end they state that “these two conclusions therefore point to the Bedouins being direct descendants of the earliest split after the out-of-Africa migration events that established a basal Eurasian population.”
To catch everyone up, Lazaridis et al. suggested based on results from ancient DNA that many West Eurasian populations have an ancestry which derives from a lineage basal to all other non-Africans unmixed with this population. That means that the genetic distance of this group to Pleistocene European hunter-gatherers and Pleistocene Australians is the same, while the genetic distance between these two groups is smaller than between them and this population. Therefore they are termed “basal Eurasians,” or bEu.
But it is also important to note that they are a construct. The ancient DNA has not found any unmixed basal Eurasians. This is in contrast to other groups which are used as donor populations: European hunter-gatherers and Siberian hunter-gatherers. About ~50% or so of the ancestry of the Anatolian farmers who were the precursor of the first agriculturalists in Europe derive from bEu ancestry, with the balance consisting of a heritage similar to to European hunter-gatherers. The hunter-gatherers recently discovered in the Caucasus also have this bEu ancestry. Ergo, almost all West Eurasian and South Asian populations have bEu ancestry.
In the paper above, which is open access, the authors found a group of Qatari Bedouin, who seem to have low admixture from Africans or other Middle Eastern groups. Though some preliminary analysis was done with SNP-chips, they went whole genome for most of the work (allowing them to look for rare variants, etc.). I would have been convinced to a great extent if they put a TreeMix graph out which showed that their indigenous Arab population was a good donor to ancient Anatolians along with European hunter-gatherers. But I did not see that. Or they could have done an F4 ratio test showing that the Bedouin were more basal Eurasian than any other modern population. I did not see that.
I did see an F4 ratio test for Neanderthal admixture. I am not confident that their assertions hold. Take a look at the pattern of Neanderthal admixture in the supplements; it’s all over the place. It isn’t in line with the broad patterns found in the latest work out of David Reich’s lab.
There are also some assumptions within the paper which I think are untenable. They seem to be positing a continuity of these Qatari Bedouin within the Arabian peninsula for tens of thousands of years. The divergence of the bEu population, putatively ancestral to these Bedouin, occurred from other non-Africans even before the settlement of Australia, over 50,000 years ago! I don’t think it is likely that the Bedouin were resident in or around the Arabian peninsula for that long.
Finally, there’s some reference to effective population sizes vs. X and autosome. This isn’t a major part of the paper, but I would be skeptical of these sorts of claims. There is a lot of work in this area, and it turns out everything is way more clouded than you might think on first blush.
Overall, good paper. But there’s still a mystery here. The only solution is clearly more ancient DNA from this region.