The Pliocene-Pleistocene boundary presents a period of transformation in human evolution. This is a period after which it is clear that early humans are using subsistence strategies that can be characterized as hunting and gathering, assisted with the advent of stone tool technologies, and clearly dispersing from East African refugia at places like Oldovai, Koobi Fora, Daka and Bodo to destinations outside of Africa ((Stringer, C. Â (1995). â€œThe evolution and distribution of later Pleistocene human populations,â€ in E. S. Vrba, G. H. Denton, T. C. Partridge, and L. H. Bickele, (Eds.), Paleoclimate and Evolution with an Emphasis on Human Origins, 524-532. New Haven: Yale University Press, 524-532)) ((Dennell, R. (2003). Dispersal and colonisation, long and short chronologies: how continuous is the Early Pleistocene record for hominids outside East Africa? Journal of Human Evolution, 45, 421-440)) ((Derricourt, R. (2005). Getting â€œout of Africaâ€: sea crossings, land crossings and culture in the hominin migrations. Journal of World Prehistory, 19, 119-132)).
And hominid physical morphology begins at this point to take on characters that can be more easily associated with anatomically modern humans: increases in brain sizes and stature as well as body proportions that more closely resemble that of modern humans than of our Australopithecus and Paranthropus predecessors ((AntÃ³n, S.C. and Swisher, C.C. (2004). Early dispersals of Homo from Africa. Annual Review of Anthropology, 33, 271-296)). That modern humans evolved and dispersed, eventually around the planet, from hominid ancestors who very probably and ultimately originated from the Rift Valley of eastern Africa is a widely accepted supposition.
Physical remains of Homo erectus/ergaster showing up a places that are far and wide from their probable African origins: Java and the Indonesian archipelago ((Wolpoff, M., A. G. Thorne, J. Jelinek, and Z. Yinyun (1994). The case for sinking Homo erectus. 100 years of Pithecanthropus is enough!, in J.L. Franzen Â (Ed.), 100 Years of Pithecanthropus: The Homo Erectus Problem. Frankfort am Main: Courier Forschungs-Institut Senckenberg)) ((Stinger, C. (2002). Modern human origins: progress and prospects. Philosophical Transactions of the Royal Society of London Biological Science, 357 (1420), 563-579)) ((AntÃ³n, S.C. (2003). Natural history of Homo erectus. Yearbook of Physical Anthropology, 46, 126-170)) ((ref:4)) ((Zhu, R.X., et al (2008). Early evidence of the genus Homo in East Asia. Journal of Human Evolution, 55(6), 1075-1085)) the Yuanmou Basin of China, dating to about 1.7 Ma ((ref:9)); Dmanisi, Georgia, also dating to about 1.7Â Ma ((Stringer, C. (2000). â€œHuman evolution: how an African primate became global,â€ in Â S. J. Culver and P.F. Rawson. (Eds.), Biotic Response to Global Change: The Last 145 Million Years, 379-390. London: Cambridge University Press)) ((Petraglia, M.D. (2003). The Lower Paleolithic of the Arabian Peninsula: Occupations, Adaptations and Dispersals. Journal of World Prehistory, 17 (2), 141-179)) ((ref:4)); and in the Levant ((ref:4)).
At what geographic point or points did Homo erectus depart the African continent?
The answers to this question invariably include one or more of four possible locations: the Sinai Peninsula through the Levant; the strait of Bab el-Mandab near the Horn of Africa; the Strait of Gibraltar; or across the Mediterranean through Sicily ((ref:3)).
Several models of dispersal have been developed, examining the possible routes that early humans could have used to migrate out of the African continent. Whether those models work largely depends upon the period of time for a given migration event since factors to include are climate and resulting sea-levels as well as the capabilities of the population that is migrating. Crossing the Mediterranean at both a Morocco to Spain route or from Tunisia to Sicily would still require that Homo erectus navigate several kilometers of open water ((ref:3)). The channel that passes through the Strait of Gibraltar, even accounting for lowered sea-levels of early Pleistocene glacial periods would have still been at least 5 km wide and hundreds of meters deep with a probable easterly current of 2 knots. The current at a Tunisia-Sicily crossing would likely have been less but the distance would have been far greater and still deep enough to have required a technological advantage not previously attributed to H. erectus. Nearly as challenging, though only slightly less so, would have been the strait at Bab el-Mandab at the southern end of the Red Sea where the Horn of Africa nearly joins with the Arabian Peninsula. Like the Strait of Gibraltar, Bab el-Mandab would have only been about 5 km wide though it’s been suggested that during the Early Pleistocene there may have been glacial periods in which sea-levels and the central channel may have been completely exposed ((ref:12)), though this hasn’t been substantiated. If it were the case, however, archaeological data would support a southern Red Sea crossing since there is significant evidence of both Oldowan and Acheulean industries at several undated strata on the Arabian Peninsula ((ref:12)) as well as off-shore at paleoshorelines as deep as 40 km ((Flemming, N.C. (2004). Submarine prehistoric archaeology of the Indian continental shelf: a potential resource. Current Science 86, 1225â€“1230)).
However, each of these three dispersal models are unnecessary to explain the dispersal of Homo erectus in the early Pleistocene. The most plausible and parsimonious and, therefore, the most probable dispersal point for H. erectus out of Africa into Eurasia and the Arabian Peninsula is via the Sinai Peninsula where the Nile river delta, the Red Sea and Asia meet. Even without the lowered sea-levels of glacial periods, migrations back and forth through the approximately 70 km wide strip of land that separates the Gulf of Suez from the Mediterranean Sea would have been possible. From there, H. erectus could have traveled to Dmanisi, the Arabian Peninsula, and further east into Asia as well as up through the Bosperus Strait region and into Europe then down into Italy and Spain. While it may have been more direct to leave Africa through one of the other routes, even the furthest European and Arabian destinations are less distance when traveled through the Sinai Peninsula than Java and China where it is clear Homo erectus was willing to travel. A dispersal through the Sinai Peninsula invokes the fewest new assumptions about the technology and culture of Homo erectus and eliminates any need for a water crossing.
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