This observation is important because when looking at modern populations we cannot a priori assume the survival of the most ancient inhabitants. Nonetheless, it can be well argued that Homo sapiens is an extremely adaptable species: not only did it spread throughout the world in a geological blink of an eye over the last 50 thousand years or so, but also persisted throughout most of the world, coming to occupy nearly every corner of the planet.
So, even though hyper-arid periods may have driven away most people from desert areas, perhaps they did not drive away everybody. There may yet be relics of ancient populations to be found. This is exactly what a new paper proposes: that Arabia possesses extremely old mtDNA lineages within the major macro-haplogroup N, dating to about 60,000 years ago. This is quite close to the estimates time depth of haplogroup L3 which unites many Africans with the Eurasians belonging to macrohaplogroups M and N.
The mainstream understanding of what happened -according to most geneticists- is that modern humans began spreading from Africa at around that time, about 60-70 thousand years ago. On the contrary, archaeologists have found indisputable evidence (palaeoanthropological or archaeological) of modern humans in Asia from before 100 thousand years, stretching from the Levant to the southern parts of Arabia.
There are two possibilities:
- The pre-70ka modern humans in Asia left absolutely no traces of mtDNA, and all of the extant mtDNA in Asia is derived from post-70ka Africans. Hence, the pre-70ka modern humans in Asia were the descendants of failed exodi.
- The people who expanded post-70ka in Asia were descended from people who lived in Asia before 100ka, descendants of successful exodi perhaps associated with the Mount Carmel hominins or the recently discovered Nubian Complex.
However, if the second hypothesis is true, there is a problem: haplogroup L3 is dated to 70ka, so if the expansion associated with it started in Asia, that means that there must have been substantial back-migration of L3-related lineages back to Africa. I don't see any major problem with that hypothesis, but it is true that many scientists are reluctant to feature extensive back-migration to Africa into their models. At present it has not been possible to determine to what extent genetic diversity in Africa is due to great antiquity vs. admixture of divergent human populations, which I have called Afrasian (related to Eurasians) and Palaeoafrican. If L3 did originate in Africa, then the concusion of a recent African exodus is inescapable.
The major contribution of the current paper is that it fixes a major human expansion Out-of-Arabia at very close to 60ka. Whether this expansion originated from transient Out-of-Africans who had recently exited Africa, or from long settled populations of Asia (prior to 100ka) remains to be seen.
From the paper:
The presence of archaeological sites in the Gulf basin demonstrates a long tradition of human occupation.9 However, neither direct cultural influences from the Levant nor any African influence has been detected in the Upper Palaeolithic (Late Pleistocene) lithics observed in eastern Arabia, pointing to a local development of cultural techniques.9,47 Curiously, however, the fact that some of the branches studied here include deep lineages in eastern Africa (haplogroups I, N1a, and N1f) shows that migration back to Africa occurred a number of times between 15 and 40 ka ago.
The hypothesized Gulf Oasis9 appears to be the most likely locus of the earliest branching of haplogroup N, including the three relict basal N(xR) haplogroups studied here, as well as the major Eurasian haplogroup R. Time estimates, frequencies, and genetic diversities reported here for these haplogroups are often similar between the Levant and Arabia, challenging the hypothesis of longterm isolation between these two regions. The other two refugia identified in the south and southwest of the Peninsula might have acted as a corridor for migrations west, back toward eastern Africa. Y chromosome microsatellite diversity in the Arabian Peninsula has suggested that Dubai and Oman share genetic affinities with other Near Eastern populations, whereas Saudi Arabia and Yemen show signs of greater isolation (although for fast-evolving microsatellites, these differences might reflect more recent events).4
The American Journal of Human Genetics, 26 January 2012 doi:10.1016/j.ajhg.2011.12.010
The Arabian Cradle: Mitochondrial Relicts of the First Steps along the Southern Route out of Africa
Verónica Fernandes et al.
A major unanswered question regarding the dispersal of modern humans around the world concerns the geographical site of the first human steps outside of Africa. The “southern coastal route” model predicts that the early stages of the dispersal took place when people crossed the Red Sea to southern Arabia, but genetic evidence has hitherto been tenuous. We have addressed this question by analyzing the three minor west-Eurasian haplogroups, N1, N2, and X. These lineages branch directly from the first non-African founder node, the root of haplogroup N, and coalesce to the time of the first successful movement of modern humans out of Africa, ∼60 thousand years (ka) ago. We sequenced complete mtDNA genomes from 85 Southwest Asian samples carrying these haplogroups and compared them with a database of 300 European examples. The results show that these minor haplogroups have a relict distribution that suggests an ancient ancestry within the Arabian Peninsula, and they most likely spread from the Gulf Oasis region toward the Near East and Europe during the pluvial period 55–24 ka ago. This pattern suggests that Arabia was indeed the first staging post in the spread of modern humans around the world.