If Arabia is indeed the only plausible route out of Africa for terrestrial mammals during the Plio-Pleistocene, it remains difficult to judge the relative contributions of the Sinai versus the Bab-el-Mandeb as gateways. In a recent paper, Petraglia and Alsharekh (2004) outline some of the problems that beset understanding the Middle Paleolithic or later Middle Pleistocene hominin occupation of Arabia, and the relative paucity of evidence for Lower Paleolithic occupation at the small number of known localities. However, as Petraglia (2003) also showed, both Oldowan and Acheulean assemblages are known, and the eastern side of the Bab-el-Mandeb Straits in particular appears to have been occupied by hominins with this technology. Unfortunately, the absence of good chronological control remains a major obstacle to assessing the pattern oflithic assemblage distribution, while research focus and the difficulty of fieldwork add to problems in overcoming our lack of knowledge of the area. But while these obstacles to understanding are clearly real, it remains hard to believe that much of the Peninsula was exploitable for much of the time by latest Pliocene or Early Pleistocene hominins.
As Petraglia and Alsharekh (2004) show, while movement across Sinai offers the possibility of movement along the Levant and then perhaps south into Northern Arabia either along the eastern Red Sea Coast or inland behind the highlands of the Hejaz Asir, movement across Bab-el-Mandeb confronts any dispersing population directly with the highlands. These would tend to restrict movement to the coastal strips, north along the eastern Red Sea Coast or east along the Arabian Sea Coast. While annual rainfall today in the Hejaz Asir or in the Oman Mountains at the easternmost corner of the Peninsula can reach well over 100 mm, much of the southeastern portion, the Rub Al Khalie or empty quarter, may have no more than 50 mm with temperatures that exceed 50° C (Glennie and Singhvi 2002). The fact that Lower and Middle Paleolithic occupation did occur means of course that conditions were not always unfavorable, and as Glennie and Singhvi show, the presence of substantial alluvial fans suggests that some earlier interglacials may have been more humid than today, although again the lack of chronological control hampers interpretation. We also know little of the Plio-Pleistocene fauna of Arabia, which is even more sparsely represented than that of the Miocene. The sole exceptions are the small assemblages from An Nefud in northern Saudi Arabia, thought to be of Early Pleistocene age (Thomas et al. 1998) and which, with spotted hyena, hexaprotodont hippo, horse, elephant (cf. Elephas recki), several bovids including a species of Pelorovis as well as crocodile and fish, are of distinctly African stamp. The range of species implies good grassland and standing water in the vicinity, an interpretation supported by isotopic analyses of herbivore teeth.
However, as Glennie and Singhvi also point out, increased aridity is indicated during glacial periods, beyond even that seen today despite the fact that temperatures may have averaged somewhat lower, and this factor presumably played a large part in determining the extent to which mammals, including early hominins, could maintain any occupation throughout the Pleistocene. Glacial periods with their massive falls in sea level are of course precisely the point at which the Bab-el-Mandeb crossing is likely to have been at its most obvious and navigable to early hominins (Rohling et al. 1998; Cachel and Harris 1998), so that the easiest and most attractive access by that route is likely to have been at a point when conditions in southern Arabia, and for that matter on the corresponding coastal area of Africa, are likely to have been least appealing. Taken overall, the Bab-el-Mandeb Straits do not seem likely to have offered a likely gateway out of Africa for terrestrial mammals during the Lower and Middle Pleistocene, suggesting that movement across Sinai and then northward along the Levant, southward into Arabia, or eastward and beyond is the most plausible route. However, movement across the Bab-el-Mandeb during the later Pliocene, before the Straits had fully formed, and thus before the crossing into Arabia was dependent on sea-level fall, may have been an entirely different matter, as previously pointed out (Turner 1999b). The importance of the Afar region of Africa to the south and west of Bab-el-Mandeb as an area of attractive resources for mammals, including hominins, following rivers into the developing depression as rifting progressed from Miocene times onward was highlighted by Kalb (1995, p 366), who stressed "the step-by-step process of animal migrations into and dispersal across intercontinental areas prior to complete plate separation.''
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