In March 2011 EPN reported in Human migration a puzzle relating to evidence for modern human occupation of Arabia on the southern shore of the Persian Gulf during the last Eemian interglacial at 125 and 95 ka. At that time sea level would have been as it is now, discouraging any attempt to cross the Red Sea via the Straits of Bab el Mandab; a widely suggested short-cut from East Africa to the rest of the world. Around 125 ka modern humans were making a living from coastal resources in Eritrea, leaving abundant stone tools in shoreline deposits at the head of the Gulf of Zula, and in the Sodmein Cave on Egypt’s Red Sea coast. They had also reached the famous Qafzeh and Skhul caves of Mount Carmel in today’s Israel around 100 thousand years ago. A route out of Africa through the Levant has not been widely favoured and the humans of Qafzeh and Skhul have been suggested to have reached a geographic cul-de-sac with no eastward exit because of the aridity of the Arabian Peninsula. Yet once in the Levant they could have skirted the desert interior by following the east coast of the Red Sea, and ‘strandloped’, as Jonathan Kingdon has dubbed following the coastline. But continuous access to fresh water would still have been essential.
The shores of the Red Sea preserve many examples of uplifted coral reefs, indeed signs of human presence in Eritrea occur in such a terrace. Being extremely porous, reef terraces are potential aquifers and a sign that they may have sourced freshwater springs is the conversion of the intricate coral skeletons from one form of calcium carbonate to another; original aragonite changes to calcite in the presence of fresh water, a complete replacement being estimated to take a thousand years of continual contact with fresh water. This change allowed Boaz Lazar and Mordechai Stein of the Hebrew University of Jerusalem and the Geological Survey of Israel to check for the presence of freshwater coastal springs in the past (Lazar, B. & Stein, M. 2011. Freshwater on the route of hominins out of Africa revealed by U-Th in Red Sea corals. Geology, v. 39, p. 1067-1070). Their test site was a series of uplifted reefs near Aqaba on the Red Sea coast of Jordan. The authors determined variations in the 230Th/238U ratio in the reefs relative to that of 234U/238U and showed open-system addition of 230Th and 234U during the aragonite to calcite recrystallization, that results in an isotopic compositional trend charting the timing of any alteration. Thus, the original age of reef terraces can be backtracked, revealing at Aqaba successively higher terraces formed recently and at 120, 142 and 190 ka. The oldest of the terraces seems to have been flooded with fresh water at the start of the Eemian interglacial (~140 ka), and may have been a source of springs that would have served the earliest human travellers well. It remains to use Lazar and Stein’s approach at other reef terraces along the postulated northern exit route for the earliest modern human emigrants from Africa and, more important, to find traces of their passage.
Added 21 December 2011. The likely route for leaving Africa got a push towards the Bab el Mandab with publication of evidence for a greener south Arabia at several times in the late Pleistocene (Rosenberg, T.M. and 8 others 2011. Humid periods in southern Arabia: Windows of opportunity for modern human dispersal. Geology, v. 39, p. 1115-1118). On the eastern edge of the now hyper-arid Rub al Khali are a series of former lakes with thin sediments. When first discovered they yielded radiocarbon ages of fossil molluscs of around 40 to 20 and 10.5 to 6 ka. However recent dating using optically stimulated luminescence (OSL) of the dune sands between which occur lacustrine muds and silts suggest that the lakes were water-filled for lengthy periods before those ages – radiocarbon dating can be reset to younger ages by precipitation of carbonates on older fossils. The OSL results show wet periods around 80, 100 and 125 ka, suggesting that around these times the Intertropical Convergence Zone was pulled northwards so taking seasonal monsoon rains well into the Arabian Peninsula. They tie in nicely with a variety of other parameters, including the timing of lowstands of the Red Sea. This created episodes a few thousand years long that would have been conducive to humans living there and passing through en route to Asia around eastern Arabia and perhaps to the Levant up the west side of the sub-continent. Potential occupancy was shut off by long arid periods, which might have allowed only pulses of migration. Had such episodic diffusion occurred it might have left a record in human DNA that ongoing and planned population genetic research may reveal.