Journal of World Prehistory (C 2006) DOI: 10.1007/s10963-005-9000-6 Assessment of the Southern Dispersal: GIS-Based Analyses of Potential Routes at Oxygen Isotopic Stage 4 Julie S. Field1,2 and Marta Mirazon´ Lahr1 This paper explores the geographic and environmental context of the South- ern Dispersal Route, which has been proposed as a migratory route for Homo sapiens from East Africa to Australasia during oxygen isotope stage (OIS) 4 (71–59 kyr). A series of assumptions and constraints gar- nered from modern hunter-gatherer observations are used to build a model of coastal foragers, which is then integrated with high-resolution physio- graphic analyses to produce a potential dispersal route along the coastline of the Indian Ocean. Paleoenvironmental conditions that may have supplied critical resources or served as obstacles to human colonization are identified and discussed in regards to human subsistence, the speed of migration, and demographic expansion. These factors suggest that rapid dispersals along coastlines and river valleys would have occurred upon the initial expansion out of Africa, but slowed as populations expanded demographically into South Asia and the Sunda Shelf. This also suggests that archaeological sig- natures relating to the earliest modern Homo sapiens are more likely to be recovered in South Asia. KEY WORDS: Out-of-Africa; modern humans; dispersals; routes; coastlines. INTRODUCTION Research on modern human origins, behavior, and biological diversity suggests that modern Homo sapiens emerged in Africa in the late Middle Pleistocene (250,000–130,000 years ago – kyr) (Fleagle et al., 2002; Jorde 1Leverhulme Centre for Human Evolutionary Studies, University of Cambridge, Downing Street, Cambridge, CB2 3DZ, United Kingdom. 2Correspondence should be directed; e-mail: j.fi[email protected]. 0892-7537 C 2006 Springer Science+Business Media, Inc. Field and Lahr et al., 2000; Klein, 1999; Lahr, 1996; Relethford and Jorde, 1999; McBrearty and Brooks, 2000; Stringer, 1996; Watson et al., 1997;Whiteet al., 2003; Underhill et al., 2001). This localized origin contrasts with the widespread distribution of modern humans throughout the Old World by 60–40 kyr (Klein, 1999). This dichotomy has led to several distinct proposals as to the timing and geography of possible routes of dispersal (Lahr, 1996; Lahr and Foley, 1994, 1998). However, the identification of such potential dispersal events or processes remains a major challenge. Positive evidence of Afro- Eurasian movements in the Upper Pleistocene is still largely lacking, while uncertainties remain as to the chronology of first modern human occupation of different parts of Eurasia. MODERN HUMAN ORIGINS AND OUT-OF-AFRICA DISPERSALS Between 1984–1995, a major debate took place in palaeoanthropol- ogy over which of two profoundly different evolutionary processes best described the origins of modern humans and their diversity (Stringer and Andrews, 1988). One of these processes was formalized into a model which is widely, if not universally, accepted today—the Recent African Origin model (RAO). Originally based on the dating and morphology of the Pleis- tocene hominin fossil record (Stringer and Hublin, 1984), this model came to be strongly supported by the first molecular studies of genetic diversity in modern populations (Cann et al., 1987). Anthropological genetic studies of the last 15 years have generated a body of evidence for humans’ African ori- gins that most scholars now consider irrefutable in its basic points. During these discussions, however, the problem of how and when Eurasia was col- onized by Homo sapiens from Africa received comparatively less attention. Several authors have identified the barrier/corridor factor in analy- ses of habitat expansion and contraction that accompanied interglacial and glacial periods in Africa and Europe (Arribas and Palmqvist, 1999; Foley and Lahr, 1997; Lahr and Foley, 1998; Mithen and Reed, 2002; Tchernov 1992a, 1992b; Straus and Bar-Yosef, 2001). These authors note that equa- torial mammalian faunas experienced range expansions into the temper- ate northern and southern latitudes, often in association with wet mon- soon conditions that occurred following glacials (Overpeck et al., 1996), taking advantage of unoccupied regions that had previously been ice- bound or of limited biomass. Accordingly, interglacial Levantine faunas have been shown to partly trace the extension of savannah conditions and African faunas northwards from North Africa through the Sinai Peninsula (Tchernov, 1992a, 1992b). Thus, the environmental “corridor” that Assessment of the Southern Dispersal appeared through the Sinai Peninsula and the Levant during interglacials is thought by some to have been the primary route for hominin migrations from Africa into the rest of the world (Bar-Yosef, 1994; Klein, 1999). It also appears to have served as a significant environmental barrier to dispersals during glacial episodes, as desert conditions expanded across this isthmus (Foley and Lahr, 1997; Lahr and Foley, 1998; Rolland, 1998). Most stud- ies, therefore, assume that modern humans would have left Africa from the North, across the Levant. Indeed, the use of the Levant as a route into Eurasia very early in the record (120 kyr) is supported by the presence of an early modern popula- tion in the area (Bar-Yosef and Vandermeersch, 1993; McDermott et al., 1993), which, at least in the site of Djebel Qafzeh, was found associated with African faunas (Tchernov, 1992a, 1992b). Although the early modern human occupation of the Levant was not permanent, as shown by the re- colonization of the area by Neanderthals at least by 60 kyr (Stringer et al., 1989; Valladas et al., 1987), this route of dispersal is generally assumed to have been the one used 50,000 years later by the ancestors of living non- African populations. However, the lack of analyses of other potential routes out of Africa misleadingly implies that the Sinai Peninsula/Levantine corridor was the only way out of Africa. Besides this corridor, Africa has three other points of physical proximity with Eurasia—the straits of Gibraltar, the Tunisia- Sicily straits, and the straits of Bab al Mandab. All three have been pro- posed as potential routes of hominin migration (Alimen, 1957; Bonifay, 1995; Kingdom, 1993; Straus, 2001; Villa, 2001). However, it is the third of these—named the Southern Dispersal Route (Lahr and Foley, 1994), which has significant implications for our understanding of the structure of human global diversity. THE SOUTHERN DISPERSAL OUT OF AFRICA A route of human dispersal from Africa to southern Asia, independent of the movement of peoples across the Levantine corridor into Eurasia, was proposed in order to explain both the timing of colonization of Australia and the pattern of early Eurasian human biological and cultural diversity (Foley and Lahr, 1992; Lahr, 1996; Lahr and Foley, 1994, 1998; Kingdon, 1993; Stringer, 2000; Oppenheimer, 2003). This Southern Dispersal Route would have taken modern humans from the Horn of Africa towards the southern Arabian Peninsula and led to the first modern human coloniza- tion of India and Southeast Asia, and eventually that of Australia and New Guinea (Lahr and Foley, 1994). Field and Lahr In terms of the timing of the earliest permanent human expansions out of Africa, information currently derives from two very different areas. The earliest Upper Paleolithic assemblages in the Levant (Boker Tachtit and Ksar Akil) are dated to 47–32 kyr (Bar-Yosef, 1996). The latter is associated with a modern human skeleton (Bergman and Stringer, 1989), confirming the later widespread association of these industries with modern humans. The earliest occupation of Australia is conservatively dated to 45–42 kyr (O’Connell and Allen, 2004), placing it in the same age-range as the earliest Upper Paleolithic sites in the Levant. Earlier dates have been proposed, such as 50–46 kyr for Lake Mungo (Bowler et al., 2003; Thorne et al., 1999), 60–53 kyr for Nauwalabila (Bird et al., 2002), 47–44 kyr for Devil’s Lair (Turney et al., 2001) and 59–42 kyr for the Huon Peninsula of Papua New Guinea (Groube et al., 1986), which, if correct, would place modern humans at least 12,000 km from Africa up to 12,000 years before the earliest Upper Paleolithic sites in the Levant. However, paleoanthropological evidence in support of the Southern Dispersal route itself remains elusive. Archaeological assemblages with affinities towards the African Middle Stone Age are known from both the Arabian Peninsula (Petraglia, 2003) and India (James and Petraglia, 2005; Allchin, 1973). However, these lack good chronological control, and the fact that such industries in Africa are associated with late archaic and early modern hominins precludes associating their presence outside Africa with the dispersal of modern humans. Furthermore, the fact that possible early human archaeological sites were coastal, and thus subsumed by current sea- levels, makes the study even more difficult. Modern human fossils from the Arabian Peninsula are unknown, while the earliest from India (Badatomba- lena, Sri Lanka) postdate significantly the first colonization of Australia (Kennedy and Deraniyagala, 1989). Therefore, the timing of colonization of Australia remains the strongest paleoanthropological evidence that peo- ple were already dispersing across southern Asia before the chronologi- cally and culturally well-defined Upper Paleolithic expansion in northern Eurasia. In terms of the global structure of human diversity,