A widely accepted view of the departure from Africa of anatomically modern humans to colonise the rest of the habitable world is that it involved them crossing the Straits of Bab el Mandab in the southern Red Sea and following coastlines around Arabia and thence to the rest of Eurasia. That crossing would have become possible when sea level had fallen by more than 80m to expose much of the shelf between southern Eritrea and Yemen; a level that was reached during a glacial stadial from 60 to 70 ka as climate cooled erratically to reach the last glacial maximum. That hypothesis focused archaeologists on the narrow coastal fringe of Arabia in the search for remnants of human occupation. Indeed there have been discoveries of Palaeolithic stone tools in caves and rock shelters in southern and central Oman, and lately in the United Arab Emirates close to the Straits of Hormuz at the outlet of the Persian Gulf (Armitage, S.J. et al. 2011. The southern route ‘out of Africa’: evidence for an early expansion of modern humans into Arabia. Science, v. 331, p. 453-456). The trouble is that optically stimulated luminescence (OSL) dating of the UAE site (Jebel Faya) yielded ages of around 125, 95 and 40 ka for the tool-bearing layers; during the last (Eemian) interglacial, the early cooling in the succeeding glacial epoch and just before the last glacial maximum, respectively. For the two oldest ages sea level would have been high and the Bab el Mandab as wide as it is nowadays.
Armitage et al. focus on the stone tool kits at the site, finding them substantially different from any known Palaeolithic artifacts. The oldest tools are about the same age as those found at sites in the Levant (occupations at ~120 and 80 ka), but unlike them. The best match is with coeval tools from E and NE Africa. Accepting that view could point to a much earlier migration from Africa than currently accepted: probably during the previous glacial maximum (130-140 ka) as proposed by Armitage et al. when crossing the Red Sea would have been even easier because sea level had by then fallen 120 m. Alternatively, the anatomically modern human sites of the Levant may represent ‘waypoints’ along a northerly exodus. That has some geographic support as the narrow Nile flood plain would have provided continuous subsistence for gatherer hunters moving along it throughout even the most arid times. Yet before the hyperarid, and probably impassable desert would have separated the Levant from the Tigris Euphrates plains en route eastwards. Yet there is no evidence, other than their morphology, that the Jebel Faya tools were made by modern humans; skeletal remains are yet to be found and the tools could have been made by more archaic humans from a much earlier diaspora. Until tangible evidence of their association with anatomically modern humans emerges from Jebel Faya or other old Arabian sites, Neanderthals or, quite conceivably, H. erectus remain candidates. Perhaps, however, Jebel Faya presents a sign of a soon-to-come shift in ideas about human migration.
Morocco at the opposite side of the African continent also hosts a potentially revolutionizing discovery at the Grotte des Contrabandiers on the Atlantic coast (Balter, M. 2011. Was North Africa the launch pad for modern human migrations? Science, v. 331, p. 20-23). The cave revealed 108 ka remains of an 8 year-old child. Like other human fossils in Morocco and across North Africa, the child has much larger teeth than other contemporary Africans; a trait shared with some of the earliest anatomically modern human fossils outside the continent, including those found in the Levant. Merely following the Mediterranean coast would have brought migrants of this group into the Levant. Indeed there are old sites all along the Maghreb shore and in the Saharan interior that yield tool kits similar to those of the Grotte des Contrabandiers, which interestingly include triangular blades that may have been arrowheads or spear points. This surprisingly advanced culture, which also contains shell ornaments, has yielded ages up to 145ka. More archaic human remains on the Atlantic coast date to 160 ka suggest that modern-human occupation of North Africa may have been almost as prolonged as that of Ethiopia.
So, there are now two candidate groups of modern humans for populating the rest of the world: those of NE Africa (Nile to Levant and/or via Bab el Mandab to Yemen) and those of North Africa. Using records of past sea level and climate there is scope for hypothesizing multiple migrations. Since early migrants entered unknown territories they did not set out purposively to colonise them. But provided there were navigable and survivable routes simple diffusion could take people far and wide in radiometrically brief periods (order of 1-5 ka) as they followed similarly migrating prey species. As regards sea-level, it was low enough for the Bab el Mandab crossing (and that of the Straits of Hormuz) to be feasible during several stadials of the 240-130 ka glacial, and seashore resources would have sustained migrants hugging the coast during the aridity that accompanies low global mean surface temperatures. The desert stretching from northern Syria to Aqaba on the Red Sea, is passable now during periods of high rainfall, as it would have been during the Eemian interglacial. Yet there is every reason to believe it would have become far more arid in colder global climates; a major barrier to migration.
That humans reached India before crossing the Bab el Mandab was probably not feasible because of high sea level has been suggested from stone tools that occur below a 74 ka volcanic ash layer in Andhra Pradesh, India. The tools lie above sediments with a 77 ka date, and have Middle Palaeolithic characteristics, although that alone does not necessarily signify that they were made by modern humans. If they were then that suggests a route from the Levant eastwards. The search is on for anatomically human remains in Arabia and also in India, although whether they have been preserved in the acid tropical soils of southern India is less likely than in more arid regions.
See also: Petraglia, M.D. 2011. Trailblazers across Arabia. Nature, v. 470, p. 50-51