Did Early Humans go North or
Where Out-of-Africa humans entered Asia
by Peter Forster and Shuichi Matsumura
Published 2005 as "Did Early Humans go North or South?", Science, 308:965-966, 13 May 2005.
The authors are at the McDonald Institute for
Archaeological Research, University of Cambridge CB2 3ER,
By analyzing the DNA of living humans from different locations, geneticists are able to assemble a detailed reconstruction of prehistoric human colonization of the world. This research endeavor was championed by the late Allan Wilson and his colleagues (refs. 1, 2), who led the way with their studies of maternally inherited mitochondrial DNA (mtDNA). Their work led to the proposal of a recent African origin for modern humans, some 5000 generations ago. Anthropologists and geneticists have since joined forces to create a broad framework ofpossible prehistoric human migration routes and time scales (refs. 3-6). The two latest additions to this framework are described by Thangaraj et al. (ref. 7) on page 996 and Macaulay et al. (ref. 8; on page 1034 of issue 308 of Science).
Our current understanding is that modern humans arose - 150,000 years ago, possibly in East Africa, where human genetic diversity is particularly high. Subsequent early colonization within Africa is supported by old genetic mtDNA and Y chromosome branches (often called "haplogroups") in the Bushmen or Khoisan of the Kalahari Desert, and in certain pygmy tribes in the central African rainforest. Early humans even ventured out of Africa briefly, as indicated by the 90,000-year-old Skhul and Qafzeh fossils found in Israel. The next event clearly visible in the mitochondrial evolutionary tree is an expansion signature of so-called L2 and L3 mtDNA types in Africa about 85,000 years ago, which now represent more than two-thirds of female lineages throughout most of Africa. The reason for this remarkable expansion is unclear, but it led directly to the only successful migration out of Africa, and is genetically dated by mtDNA to have occurred some time between 55,000 and 85,000 years ago. Studies of the paternally inherited Y chromosome yield time estimates for the African exodus that are in broad agreement with those derived from mtDNA.
It is at this point in the narrative that the studies by Thangaraj et al. (ref. 7) and Macaulay et al. (ref. 8) come into the picture. Which route did the first Eurasians take out of Africa? Most obvious, perhaps, is the route along the Nile and across the Sinai Peninsula leading into the rest ofthe world (see the map below). But if that were so, why was adjacent Europe settled thousands of years later than distant Australia? In Europe, Neanderthals were replaced by modern humans only about 30,000 to 40,000 years ago, whereas southern Australia was definitely inhabited 46,000 years ago and northern Australia and Southeast Asia necessarily even earlier (refs. 9, 10). Or did our ancestors instead depart from East Africa, crossing the Red Sea and then following the coast of the Indian Ocean (ref. 11)? A purely coastal "express train" would conveniently explain the early dates for human presence in Australia, but would require that humans were capable of crossing the mouth of the Red Sea some 60,000 years ago. Why, then, was this feat not repeated by any later African emigrants, particularly when the Red Sea level dropped to a minimum about 20,000 years ago?
Map below: How did they get there? Hypothetical routes along the Indian Ocean coastline that could have been taken by early humans emigrating out of Africa. The oldest human traces outside of Africa and the Levant are at Lake Mungo in Australia (>46,000 years old) and in the Niah Cave of Borneo (>45,000 years ago). New mtDNA data, from Malaysians and aboriginal Andaman islanders, suggest that human settlements appeared along the Indian Ocean coastline 60,000 years ago (ref. 7, 8).
red arrows: the two
A. Bab-el-Mandeb strait (southern route)
Ideally, these questions would be answered by investigating ancient fossils and DNA from the Arabian Peninsula. But because this option is currently not available, Thangaraj et al. and Macaulay et al. have centered their investigation on the other side of the Indian Ocean, in the Andaman Islands and Malaysian Peninsula. Both groups used genetic studies of relict populations known to differ substantially from their Asian neighbors to estimate the arrival time of the first humans in these locations. Thangaraj and colleagues sampled the Andamanese, who were decimated in the 19th century by diseases imported by the British and then suffered displacement by modem Indian immigration (ref. 12). Macaulay and co-workers sampled the native tribal people of Malaysia, called the Orang Asli ("original people").
Fortunately, the two teams arrived at compatible conclusions. In the Andaman Islands, Thangaraj et al. identified the M31 and M32 mtDNA types among indigenous Andamanese. These two mtDNA types branched directly from M mtDNA, which arose as a founder 65,000 years ago. This time estimate for the arrival of M founder mtDNA is matched by that of Macaulay and co-workers. These investigators found mtDNA types M2l and M22 in their Malaysian data set. These M types are geographically specific branches of M that branched off from other Asian mtDNA lineages around 60,000 years ago. Thus, the first Eurasians appear to have reached the coast of the Indian Ocean soon after leaving Africa, regardless of whether they took the northern or the southern route. Interestingly, the adjacent Nicobar Islands do not harbor any old mtDNA branches specific to the islands. Instead, their mtDNA has a close and hence recent genetic relationship (on the order of 15,000 years or less) with the mtDNA of other Southeast Asian populations. This is not unexpected given the more Asian appearance of the Nicobar islanders.
Macaulay and colleagues go two steps further and estimate the prehistoric migration speed of early humans along the coast of the Indian Ocean; they also estimate the likely population size ofthe emigrant population. Comparing genetic dates of founder types between India and Australia, and assuming a 12,000-km journey along the Indian Ocean coastline, they suggest a migration speed for the first Eurasians of 0.7 to 4 km per year. This value is of the same order of magnitude as genetically dated inland journeys of migrant populations during the last Ice Age, 60,000 to 10,000 years ago (ref. 6).
One intriguing question is the number of women who originally emigrated out of Africa. Only one is required, theoretically. Such a single female founder would have had to carry the African L3 mtDNA type, and her descendants would have carried those mtDNA types (M, N, and R) that populate Eurasia today. Macaulay et al. use population modeling to obtain a rough upper estimate of the number of women who left Africa 60,000 years ago. From their model, they calculate this number to be about 600. Using published conversion factors, we can translate this estimate into a number between 500 and 2,000 actual women. The authors' preferred estimate is several hundred female founders. All such estimations are influenced by the choice of parameters and by statistical uncertainty; hence, it is understood that the true number could have been considerably larger or smaller. Improved estimates will involve computer simulations based on informed scenarios using additional genetic loci.
Time is short if researchers wish to secure data on dwindling indigenous populations such as the Andamanese and the Orang Asli. The studies by Macaulay et al. and Thangaraj et al., which are devoted to the peoples inhabiting the "southern route" along the Indian Ocean, are therefore very welcome. We hope that the new findings will inspire archaeological exploration between the Arabian Peninsula and Southeast Asia in search of the remains of the first Eurasians 50,000 to 100,000 years ago.
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Last changed 30 March 2006