Assessment of the Southern Dispersal: GIS-Based Analyses of Potential Routes at Oxygen Isotopic Stage 4

Journal of World Prehistory (C 2006) DOI: 10.1007/s10963-005-9000-6

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 signatures 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 origins that most scholars now consider irrefutable in its basic points. During these discussions, however, the problem of how and when Eurasia was colonized by Homo sapiens from Africa received comparatively less attention. Several authors have identified the barrier/corridor factor in analyses 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 temperate northern and southern latitudes, often in association with wet monsoon 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 studies, 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 population 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 proposed 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 colonization 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 people 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, two different lines of evidence suggest that Eurasia was not colonized through a single dispersal event. The first of these is the pattern of morphological differences among past and present European and Australasian populations, from patterns of cranial affinities, to body size and skin color (Kingdon, 1993; Lahr, 1996; Lahr and Foley, 1994; Stringer and McKie, 1996), which suggest that Australo-Melanesian and certain southern and southeast Asian populations may be more closely related to African groups than to potential Upper Paleolithic ancestors. The second biological line of evidence derives from genetic studies. These have shown that the “M” mtDNA Assessment of the Southern Dispersal lineages typical of Indian populations are found among contemporary East Africans (Kivisild et al., 1999; Passarino et al., 1996), where they most likely originated (Quintana-Murci et al., 1999a, 1999b); that some Asian and Australian groups have clades of Y-chromosome lineages (defined by the YAP/M145/M203 and RPS4Y/M216 markers) that are absent else- where (Underhill et al., 2001); and that Australo-Melanesian populations are as distinct in their pattern of Alu insertion polymorphisms as Africans (Macaulay et al., 2005; Stoneking et al., 1997). Therefore, the biological data indicate that the diversity of present-day populations in Eurasia and Ocea- nia is best explained by two different ancestral African groups, who in spite of subsequent substantial admixture, still can be identified to varying de- grees in the structure of human diversity outside Africa. The existence of a Southern Dispersal of modern humans out of Africa will be confirmed or refuted by paleoanthropological and genetic evidence. However, such studies will benefit from investigations of the potential physical and ecological constraints that ancestral peoples would have faced if and when they used this route to reach southern Asia from Africa. This paper aims at providing a first assessment of such constraints, in particular those imposed by the topography and paleoenvironment of this southern coastline during Oxygen Isotope Stage (OIS) 4 (71–59 kyr), the period during which the most dramatic ancestral expansions of human populations are thought to have occurred (Jorde et al., 1998, Sherry et al., 1997). This paper presents the results of a series of GIS-based analyses of potential routes along the perimeter of the Indian Ocean. These core analyses provide a foundation for the discussion of dispersals and the potential impact environments may have had on subsistence, the speed of migration, and demographic expansion.

THE PALEOENVIRONMENT OF OIS 4

The paleoenvironmental context of the Indian Ocean rim is a crucial aspect in the analysis of the Southern Dispersal. Geological research has established that the ratio of oxygen isotopes 18 and 16 from the strata of deep-sea ocean sediment cores provides an index of global glaciation and deglaciation in prehistory (Emiliani, 1955, 1957). Within this cycle, oxygen isotope stages demarcate the boundaries between long-term trends of global warming or cooling. Stages 2 (24–13 kyr) and 4 (71–59 kyr) were the most recent full-glacials, while Stage 3 (59–24 kyr) was a temperate, variable period, generally called interstadial. The period that is the focus of this research, OIS 4, has not been described in great detail by paleoclimatologists, but probably shared aspects that typiﬁed the glacial conditions of OIS 2. Field and Lahr

Generally, the onset of glaciation and cool temperatures in the higher latitudes led to global decreases in sea-levels, and increasing aridity in the tropics. Increased seasonality between summer and winter temperatures was also prominent. For much of the Old World, this would have resulted in the expansion of desert conditions throughout North Africa, Arabia, and part of South Asia, as well as an increase in savanna-type conditions and dry tropical forests in the tropical latitudes, in particular South Asia and Southeast Asia (Adams and Faure, 1997). A more detailed examination of the affects of OIS 4 conditions on the environments and coastlines of the Indian Ocean rim is provided in Sharma et al. (in preparation), but in general sea-levels lowered to 80–88 m below modern levels (Cutler, et al., 2003; Siddall, et al., 2003), exposing the Sunda Shelf and a large portions of western South Asia. The Red Sea would have remained connected to the Arabian Sea via the Bab al Mandab Straits; however, the Persian Gulf would have disappeared entirely, leaving a substantial expanse of sand dunes in its place (Glennie and Singvi, 2002, p. 867). Coastal environments, such as expanses of sand dunes, mangroves, alluvial plains, coral reefs, and lagoon systems would also have been affected by increasing/decreasing sedimentation, changing wind and rainfall patterns, and sea-level fall during the OIS 4 period. Glaciation also implies that the monsoon system that normally provides seasonal moisture would have been suppressed across the Indian Ocean basin, resulting in greater aridity along much of the Indian Ocean rim. However, it must be noted that in addition to these general trends for OIS 4, there is increasing evidence to suggest that there was regional variation in both temperature and humidity during and after glacial periods, which resulted in episodes of wetness in the desert environments of North Africa and South Asia (McClure, 1976; Overpeck et al., 1996). These events extended to both century and millennium time scales, and their timing appears to lag deglaciation by approximately 3000 years, suggesting that monsoon intensity would have increased at the end of OIS 4. These episodes would have led to an increase in vegetation and wildlife in what were for- merly desert environments, and perhaps also attracted populations into regions that had previously been avoided. Rapid transitions between periods of aridity and wetness would have had an impact on the subsistence regimes and mobility of populations that may have resided in these regions at this time. Although the chronology and frequency of these events have not been fully ascertained for OIS 4, their potential importance to human dispersals appears likely. However, our generation and assessment of the Southern Dispersal Route will focus on the more pervasive trends, in particular increased aridity and cooler conditions along the Indian Ocean coastline. The implications of a more varied climate on certain portions of the route will be touched upon again in the discussion section. Assessment of the Southern Dispersal

A MODEL FOR THE SOUTHERN DISPERSAL

The strengths of computer-based modeling of prehistoric dispersals has been demonstrated recently by demographic and spatial expansion simulations such as STEPPINGOUT (Mithen and Reed, 2002) and SPLATCHE (Currat et al., 2004), which trace the expansion of simulated populations within multivariate models of the Old World. These programs experiment with a number of scenarios and variables that represent climate, topography, rate of colonization, and open/closed land bridges between continents. The STEPPINGOUT simulations in particular illustrate how certain simulated outcomes (e.g., the rapid colonization of Europe) could be achieved after variables were calibrated and adjusted, but that other outcomes were impossible (Mithen and Reed, 2002, pp. 451–453). These findings are of interest to archaeologists, as they demonstrate how particular variables may have been more or less influential in the workings of the model, and thus in- form on processes that may have been active in prehistory. They also high- light the importance of assessing how closely models represent real-world conditions and processes. In contrast to a simulation that employs a number of variables at coarse resolution, we suggest that a high resolution investigation of only a few variables—in this case environmental and geographical conditions—can provide insights into the factors that affect dispersals and their evolutionary outcomes. The strengths of this method include a closer approximation of real-world conditions, and the generation of a hypothesis that can be tested against variables that are less easily quantified. This study uses GIS-based analyses to determine a potential route of dispersal from East Africa to Australasia. A simple model derived from anthropological literature directs this research: we assume that the first modern humans to leave Africa were mobile hunter-gatherers, and that these populations were familiar with, and perhaps subsisted on, coastal resources. Our focus is therefore on the coastlines, in particular the configuration of shorelines and fringing ecotones that would have hastened, or impeded, human dispersal from Africa eastwards. We also place emphasis on resources that may have been crucial to survival during the glacial conditions of OIS 4, such as the location of permanent water sources. This reflects our assumption that the initial expansion out of Africa required the traversal of North Africa and Southwest Asia, which was a predominantly arid environment during this period. However, we must stress that our goal is to explore the way that people would have moved had they dispersed along the coastline of the In- dian Ocean, and the route we discuss is a single outcome that best fits the constraints of our model of coastal foragers during an approximation of OIS 4 conditions. It is not intended as an independent confirmation of the Southern Dispersal hypothesis. Rather, the route presented here is itself a Field and Lahr hypothesis, and also a heuristic device for the discussion of the Indian Ocean rim in regards to human history and diversity.

SOUTHERN DISPERSAL ROUTE GENERATION: METHODOLOGY

The methodology we employ in the determination of the route is based on an evaluation of energetic cost and subsistence benefit. GIS allows for the construction of digital representations of geographic data, which can then be analyzed and reclassified according to specific criteria. As we assume that the Southern Dispersal was a largely coastal-based expansion of modern humans, our interest is focused on the finer details of the coastline of OIS 4; in particular physical features such as flat coastal surfaces that would have supported a wide intertidal zone, and also created a broad, easily traversed corridor. We are also interested in features that would have served as barriers, such as mountains or cliff-faces that would have forced dispersals inland. This interest stems from previous studies of the dynamics of human dispersals, which identify mountains, lakes, wide rivers, coastlines, and continental ice-sheets as features that can effectively block or redirect travel (Steele et al., 1998; Young and Bettinger, 1995;Rayet al., 1999, Surovell, 2003). These features may have had an impact on both the speed and direction of dispersals along the Indian Ocean rim. Our GIS-based analyses utilize a high-resolution representation of topographic slope as a fundamental measure of geographic cost and route configuration. Although slope is only one aspect of topography that affects human dispersals, its use in analyses allows for a relative assessment of energy expenditure, and it also allows for the detection of topographic constraints that are often present on coastlines, such as cliff faces or a broken and rocky coastal zone. Other topographic data, such as elevation or aspect, are less sensitive to the variations that are typical of coastlines, and thus of less value to our analysis. More specifically, we equate slope grade with a scaled approximation of energy expenditure, as outlined by Gorenflo and Gale (1990), Llobera (2000), van Leusen (2002, p. 5) and Bell and Lock (2000, pp. 88–89) (Table 1). This in turn was used to generate an isotropic cost surface, which could then be analyzed using algorithms embedded in the GIS to produce a route of least cost from one point to another. In the context of physical landscapes, this translates to a preference for routes along coastal plains or riparian areas that flank river courses, and an avoidance of more arduous routes over mountains and hills. This outcome also has some parallels in biological theory, which suggests that activities that are temporally shorter or require less exertion are more Assessment of the Southern Dispersal

Table 1 Variables and Weights Used in GIS Calculations of Least-Cost Routes Variable Weight Slope (0–25◦)a 0–25 Slope 26◦b 27.95 Slope 27◦ 29.19 Slope 28◦ 30.47 Slope 29◦ 31.76 Slope 30◦ 33.08 Slope 31◦ 34.43 Slope 32◦ 35.76 Slope 33◦ 37.21 Slope 34◦ 38.64 Slope 35◦ 40.12 Slope 36◦ 41.63 Slope 37◦ 43.18 Slope 38◦ 44.77 Slope 39◦ 46.40 Slope 40◦ 48.08 Slope 41◦ 49.81 Slope 42◦ 51.59 Slope 43◦ 53.43 Major riversc Impermeable Riparian areasd 0 Sand sease 15 Sea levelf Impermeable aSlope calculations were obtained from ancillary slope data sets of HYDRO1K and calculations based on the topographic coverage of ETOPO2. bSlope energy values were derived from the following calculation: tan slope x / tan (1◦)(Bell and Lock, 2000, pp. 88–89). cThe location of major rivers likely to have been present during OIS 4 was calculated from the HYDRO1K stream data set. Stream order (Strahler, 1964) was used to select streams of order 3 or higher, as these drainages are of significant size and volume, and more likely to have persisted in their present location during glacial periods. They are also more likely to have provided permanent water flow. dRiparian areas were calculated using the buffer function of ArcGIS 8.3. This produced riparian areas of 1 km widths along the edges of rivers of order 4 or higher. eThe extent of aeolian sand deposits is derived from the Global GIS Database (USGS 2000) and published data by Glennie and Singvi (2002) for the extent of desert sands during glacial episodes in Arabia and South Asia. The value of 15 prohibits crossing by least-cost pathways in nearly all circumstances, but does allow for crossing of short stretches (i.e., 90 km). fAs discussed previously, global sea levels for OIS 4 were given an average of − 80 m. The location of this level on the ETOPO2 data set was calculated for the study area, and this line was then reclassified as an impermeable barrier. economical in terms of foraging time and energy expenditure (Krebs and Davies, 1991). The use of slope-based algorithms in the reconstruction of least-cost routes across ancient landscapes has been demonstrated in archaeology- focused GIS studies (e.g., Anderson and Gillam, 2000; Bell and Lock, 2000; Gaffney et al., 1993; Madry and Rakos, 1996). However, there are distinct limitations to the method that should be stated at the outset. The analysis Field and Lahr involves the representation of a landscape in raster (grid) format, with increasing levels of energy expenditure assigned to cells as a factor of slope grade. In an isotropic analysis, cost is calculated from a specified point, and least-cost routes are generated by the determination of the lowest cumula- tive cost from the point to the destination. This produces a route that does not take into account travel direction, and treats the energetic cost of traveling either up or down slopes as equivalent (Wheatley and Gillings, 2002, p.152). Although several studies have attempted to more accurately calcu- late least-cost routes across anistropic surfaces (e.g., Eastman, 1993), our study, with its focus on relatively flat coastal topography, uses the simpler isotropic calculation. We also note that least-cost route generation in GIS applications is often hampered by the requirements of distance-based spatial analyses, in particular the use of starting and ending points. As GIS-based algorithms are designed to determine the least costly route between two points, both the distance and total cost of cells between the two locations plays a role in determining the most parsimonious route. This is particularly problem- atic for the analysis of human colonization, as having an endpoint forces directionality into the resulting route. Colonizing populations would not have known what lay before them, and thus their movements should not be modeled as a simple journey between two locations. It is more likely that knowledge of local environments (including spots that provided the best hunting and gathering resources, as well as the most easily traveled sec- tions) accrued after a period of residence in a particular region, and movement out of that region occurred in relation to minimum cost, rather than minimum distance (Anderson and Gillam, 2000, p. 47). In addition, colonizing populations may have simply followed natural trends in topography and environment, with little awareness of progressive migration. With these issues in mind, a “wandering” method of route determination was designed in which a 60 km search radius was placed around a starting point on the cost surface. The target radii distance was selected following a review of ethnographic literature that discussed hunting forays, average distances between residential moves, and total range of movement per annum (e.g., Kelly, 1995). Sixty kilometers was selected as a conservative average for combined mobility. Analysis of the cells surrounding the starting point determined the least costly path in any direction. Larger radii were not used as they often intersected with the coastal boundary, which then limited the wandering to fewer directions. Each route was thus a cul- mination of 60 km path segments; following each calculation the search radius was moved to the newest endpoint, and the next path segment of the route was generated from that point onwards. In instances where the route encountered obstacles that prohibited forward movement (i.e., the Assessment of the Southern Dispersal least costly path was back along the previously traveled direction), search targets in the form of three-quarter targets were used to enforce movement in a novel direction. For example, a route that persisted in returning to 180◦ would be forced forward using a three-quarter circular target that extended from 315◦ to 135◦. However, if forward movement was no longer possible (e.g., a cul de sac located on a peninsula bounded by the ocean), the analysis was started again from the previous point on the route, and three-quarter search targets were used to generate new route legs in novel directions. We also attempted to assess the potential for variability in route generation and diversions of equal cost. Occasionally routes bogged down in a depression (i.e., a lake bed or large basin), or encountered an extremely flat area that allowed for random wandering in all directions. If a route wan- dered in a circular manner for more than 600 km, a target of 0–180◦ was used to force the route eastwards. However, if routes seemed to wander randomly through an area, the analyses were run a second time from a previous point in order to assess the potential for secondary routes of equal cost. This produced a number of alternative routes, which are indicated in Figs. 1–13 by a dotted line. In addition to slope, the weighting of other variables within the model was critical to route generation. In this analysis, portions of rivers that are of order 4 or higher (e.g., the main channels of the Nile, Indus, and Ganges- Brahmaputra, but not the tributaries) were set as impermeable barriers (Table 1). This was also done for the extent of coastlines at OIS 4. Although the antiquity of watercraft and the ability of modern humans to cross bodies of water is subject to debate (Flemming, 2004; Oppenheimer, 2003;Walter et al., 2000; Stringer, 2000), rivers and coastlines were maintained as barriers in order to focus dispersals along terrestrial surfaces. However, if routes reached a cul de sac (either by rivers reaching the sea or steep slopes pro- hibiting upriver travel), new starting points were manually placed on the opposite sides of the river. We acknowledge that this is an arbitrary des- ignation, but as the extent to which rivers may have stopped flowing or been of low levels during OIS 4 is not well understood (especially the desert rivers such as the Nile and Indus), we maintain that this is an acceptable alternative to plotting routes around the high headwaters of each and every tributary. Similarly, a number of analyses have identified habitats such as deserts and jungles as semipermeable barriers that slow down traverse or colonization rates (Ray et al., 1999, Steele et al., 1998). In GIS applications, these features are equated with an energetic requirement similar to travel across steep terrains, and this restrains movement across them. Although the extent of deserts during OIS 4 has also not been completely determined, estimates based on OIS 2 levels from Adams and Faure (1997), and also Field and Lahr

Glennie and Singvi (2002) are available for the Indian Ocean rim. In this research, the extent of sand seas estimated for OIS 4 was given a value that was high in relation to surrounding slope costs, but not so high as to completely preclude the possibility of crossing narrow portions (Table 1). Other habitats that may have been attractive to humans were assigned a value based on theoretical conceptions of resource beneﬁt. As our model suggests that water sources and riparian areas would have been attractive to hunter-gatherers during OIS 4, these regions have been highlighted by assigning them a value of zero (no cost). In this study, 1 km buffers were placed around the edges of rivers that were order 4 or higher, and assigned a value of zero. This had the effect of attracting the route into the riparian corridor, mimicking a preference for these environments that was likely to have occurred in prehistory. The generation of routes for the Southern Dispersal was performed with ArcGIS 8.3 software, and utilized HYDRO1K slope data. The latter were derived from the 30-arcsecond digital elevation model GTOPO30 (LP DAAC, 2003). This data set contains cell values at 1 km resolution for the terrestrial surface of the planet, and is projected in the Lambert- Azimuthal Equal Area projection, which limits distance and area distortion in the data. Information on how the HYDRO1K and GTOPO30 data were generated is described in detail by the source literature. The slope data calculated from the global elevation data set ETOPO2 (approximately 3.7 km resolution) was used for secondary analyses of routes below modern sea levels (National Geophysical Data Center, 2004). These data were clipped into regional blocks, re-projected into the Lambert Azimuthal projection, and then merged with the HYDRO1K data at a cellular resolution of 1 km. This created a combined terrestrial and bathymetric dataset that would allow for the generation of routes above and below the modern coastlines. The loss of data resolution below − 80 m resulted in segments of the route being more generalized, but no border effect was observed. The source data for the rivers and streams, calculations for the riparian areas, and extent of sand seas were combined with the topographic layers in order to produce the ﬁnal cost surface. This process is described in the footnotes of Table 1.

LEAST-COST ROUTES FOR THE SOUTHERN DISPERSAL

Ethiopian Highlands to the Straits of Hormuz

The starting point for the ﬁrst route (Origin 1, or O1) is in east Ethiopia, which has been identiﬁed by the presence of fossils at Omo Kibish Assessment of the Southern Dispersal

Fig. 1. Routes along the Red Sea coast from Origins 1 and 2.

(Fleagle et al., 2003) and Herto (White et al., 2003) as a region of early modern human occupation (Fig. 1). Paleoenvironmental research also suggests that this area supported scattered savannas and brushy scrub during glacial periods, making it an island within the extensive deserts of North Africa (van Andel and Tzedakis, 1996). The first wandering route descends in a Field and Lahr northern direction towards the Red Sea, and quickly crosses onto coastal terrain that was exposed during OIS 4. This is the same area that has produced lithics and evidence for coastal exploitation dating to 125 kyr (Walter et al., 2000). In the absence of boat or raft technology for crossing the Bab al Mandab into Arabia, the route continues up the western coast of the Red Sea. At approximately the latitude of modern Aswan, a range of hills that extend down to the Red Sea coastline force the route inland, where it descends into the Nile Valley (Fig. 2). From there the route follows the alluvial terraces on the eastern bank of the river north until reaching the Mediterranean coast. The archaeological record for this region is both ancient and complex, with indications for the emergence of a number of discrete populations and riparian/desert based subsistence strategies during the late Middle and Upper Pleistocene (Van Peer, 1998). Further wandering to the east takes the route onto the northern coastline of the Sinai Peninsula. Additional analyses indicate that if travel along this route were to continue, the path wanders roughly northeast along the Mediterranean Coast (Fig. 3). The route then heads inland towards the ancient northern shores of the Dead Sea, passing close by the Carmel Caves complex (Bar-Yosef and Vandermeersch, 1993). From this point the route wanders north-eastwards into the vast, flat steppe north of Arabia. Upon reaching the south bank of the Euphrates River the route swings southwards, following the river corridor into the Persian Gulf basin. Although much of the basin was filled with active sand seas during OIS 4 (Glennie and Singvi, 2002, p. 867), a route along the northern edge of the Persian Gulf is possible, which perhaps follows the final drainage of the Tigris/Euphrates river system. The route follows this drainage eastwards towards the mouth of the Straits of Hormuz.

Bab al Mandab to the Straits of Hormuz

If populations were able to cross the Bab al Mandab Straits, a series of routes are possible along the coastline of the Arabian Peninsula, beginning from Origin 2 (O2). After crossing the Red Sea, the ﬁrst most parsimonious wandering route follows the eastern Red Sea coast northwards (Fig. 1). This route passes the location of several sites that contain Middle Paleolithic deposits (Petraglia, 2003). This route reaches a cul de sac on the eastern shore of the Gulf of Aqaba, and in secondary analysis could only be forced northwards through the Al Hijaz Mountains with the aid of directional search targets. The second most parsimonious route heads eastwards along the coastline of the Arabian Peninsula (Fig. 4). This route parallels the precipitous Assessment of the Southern Dispersal

Fig. 2. Routes along the Red Sea coast, the Nile Valley, and the coast of the Mediterranean Sea. Field and Lahr

Fig. 3. Route into the Euphrates Valley.

OIS 4 coastline until reaching the headlands of Ra’s Fartak, at which point it is forced inland. The route is able to return brieﬂy to the coastline, but again is pushed inland by the headland and at this point continues eastwards along a corridor provided by an intermittent river system (Wadi Shiban). Although it is unknown if Wadi Shiban was actually a running river during this period, the proximity of this drainage to the edge of the Arabian sand sea created a corridor that ran eastwards. Assessment of the Southern Dispersal

Fig. 4. Route from Origin 2; the Bab al Mandab Straits along the Arabian coastline.

This route then continues northwards, occasionally circumscribing several ancient basins that exist in the area (Fig. 5). Geological investigation by McClure (1976) has identiﬁed the remains of large terrestrial mammals in the shoreline deposits of these basins, which date to the early Holocene. It is likely that these areas would have also attracted similar fauna during wet episodes in earlier periods. Secondary analyses of routes from these basins suggest a return to the coastline of Oman, but only for a short distance. Upon encountering the inclines of the Al Hajar Mountains in eastern Arabia, the route swings west and northwards, skirting the sand seas that lie in the basin of the Persian Gulf. The route then encounters the coastline at Field and Lahr

Fig. 5. Routes through the Arabian Peninsula to the Straits of Hormuz. the Straits of Hormuz, which was considerably narrower during OIS 4, and only extended into the Persian Gulf for approximately 90 km.

The Makran Coast to Cape Comorin

The initiation point for this leg of the proposed Southern Dispersal route was placed on the eastern side of the Tigris/Euphrates delta and the Straits of Hormuz (Origin 3) (Fig. 6). From this point a single route Assessment of the Southern Dispersal

Fig. 6. Route along the Makran Coast from Origin 3. wanders eastwards along the Makran Coast, following the rugged coastline into the South Asian subcontinent. The route skirts the coastline that fringes the delta of the Indus River and continues southwards. It is important to note that the Indus Delta has been a dynamic environment throughout history; geological investigations suggest that the location of the river has repeatedly changed (e.g., Schuldenrein 2002), and alluvial deposits are substantial. It may also have posed a temporary barrier during OIS 4, as the mouth of the river is wide, and the surrounding environment and coastline was probably composed of salt ﬂats and marshes (Sharma et al., in preparation). Due to its low value in the cost-surface analysis, this allowed for the route to turn upriver, and head into the Indus Valley. Flanked by the Thar Field and Lahr

Desert and the Sulaiman¯ Range, this diversion offers a narrow corridor into the interior. This route was allowed to continue for several kilometers, and then halted to allow for additional route analyses down the coastline of South Asia. By placing a new starting point on the south bank of the Indus River, a least cost route was generated that ran southwards, fringing the sand seas associated with the Thar Desert, and following a very irregular coastline with numerous bays and inlets. At this point the route passes close to the location of the Hiran Valley complex, which contains lithics and other remains suggesting the presence of hominins utilizing Middle Palaeolithic technologies in this region between 69 and 56 kyr (Baskaran et al., 1986). Past this point the route continues southwards along the coastline, reaching the southernmost portion of South Asia near the location of modern day Cape Comorin (Fig. 7). Of note, in secondary analyses the least-cost route encounters Cape Rama, and is forced inland and across the Western Ghats. The hills around Cape Rama are not particularly steep, but they are considerably more costly than the ﬂat topography previously encountered on the coast. Once in the interior the wandering path ranges widely around the gently rolling landscape, ﬁrst traveling north along tributaries of the Kr- ishna River, and then swinging south to skirt the northern boundary of the Deccan plateau. After a second diversion northwards to the Krishna basin, the route turns southeast, and follows the Penner River to the Coromandel Coast. The route passes through several regions that have a high density of Upper Pleistocene-aged sites and a range of lithic assemblages (Pappu and Deo, 1994; Raju, 1988). During OIS 4 this region was predominantly grassland and savannah, and excavations in the Kurnool Caves has produced a variety of lithic artifacts in association with the remains of horse, rhino, boar, gazelle, deer, antelope, and wild cattle (Murty and Reddy, 1975; Prasad, 1996).

Cape Comorin to the Ganges-Brahmaputra Delta

From the southernmost tip of South Asia (Origin 4), the route heads northwards, following a series of lake shores across the OIS 4 surface of the Sri Lankan isthmus. The route continues to ﬂank the coastline as it travels north into the Penner Delta, and wanders throughout the Godavari Delta before proceeding northwards along the coast (Fig. 7). The route makes several diversions into coastal river valleys along the way, and then enters the Damodar Valley and heads north towards the Ganges River. As with the Indus, the value of the riparian areas surrounding the river pulled the route into the Ganges Valley (Fig. 8). This diversion into the interior would Assessment of the Southern Dispersal

Fig. 7. Routes along the western and eastern coasts of South Asia, and secondary route through the Western Ghats. probably have been attractive to hunter-gatherers, as the Ganges Valley was predominantly a grassland and savannah environment during OIS 4. Surveys in the nearby Son Valley have documented sites with lithic assemblages paralleling the Middle Paleolithic (Ahmed, 1984; Blumenschine et al., 1983; Sharma and Clark, 1983). Field and Lahr

Fig. 8. Routes from Origin 4 and Origin 5; the Ganges-Brahmaputra Delta.

The Ganges-Brahmaputra Delta to the Eastern Coastline of the Sunda Shelf

The Ganges-Brahmaputra River system may have also been a partial barrier to eastward migration. The size of this river, as well as the expanse of mangroves and swamps that constitute the delta (the Pleistocene version of the Sundarbans) suggests that populations would have had to travel far inland in order to cross (Agarwal and Mitra 1991). Therefore, the next leg of the Southern Dispersal Route was placed on the eastern side at a point that Assessment of the Southern Dispersal is approximately 250 km inland from the modern river mouth. From this point the route wanders northeastwards and encounters the Brahmaputra River. Again the route was restarted from the opposite side the river, from which it continues south-eastward to parallel the coast of modern Myanmar (Origin 5) (Fig. 8). At the termination of the Ragaing Yoma Mountains, the route turns eastwards and follows the western side of the Irawadi River. The route reaches a dead-end to the west of the river mouth, and in sight of the Pleistocene shoreline of North Andaman Island. Continuing eastwards, the Iriwadi River could only be crossed in from a point further north. The route then funnels along the coastal margin of western Myanmar and extends down the western coastline of the Malay Peninsula. After crossing what are now the Straits of Malacca, the route swings westward to run along the northern coastline of Sumatra. The route then reaches a dead-end at the north-western tip of the Sumatran coast as it stood at OIS 4. From here (Origin 6) migrants would have had a more difﬁcult journey along the rugged southern coastline of the Sunda Shelf (Fig. 9). The high frequency of volcanism in this region has resulted in a very precipitous topography, which was covered with dense dry forests and rainforests during OIS 4 (Adams and Faure, 1997). As indicated by the severe deviations from the coast into the interior of the Sunda Shelf, the southern Sumatran coastline was not passable in several locations, and migrants would have had to travel inland to get around these barriers (Fig. 10). The most severe of these barriers would have occurred in the vicinities of the modern city of Padang on Pleistocene Sumatra and the south-western edges of Pleistocene Java. Ultimately, these barriers force the route inland and eastwards until terminating at the south-eastern edge of the Sunda Shelf, at approximately the location of eastern Pleistocene Bali. Alternatively, migrants could have turned inland into the dry forests of the Sunda Shelf at approximately the location of the Straits of Malacca (Fig. 9). Although the lack of high-resolution bathymetric data for this region makes this portion of the route less precise, in general this secondary route runs south-eastwards across what is now northern Sumatra. The route then crosses what is now the bottom of the Java Sea, and then follows the southern coastline of Borneo. From this point the least-cost route runs eastwards and terminates at the eastern coastline of the Sunda Shelf as it stood at OIS 4.

Routes Through Wallacea

As has been noted by many scholars of Australasian prehistory, journeys past the edge of the Sunda Shelf into the islands of Wallacea would Field and Lahr

Fig. 9. Routes from Origin 6 through the Sunda Shelf, and secondary route through the Straits of Malacca. have required the use of ﬂoating or sailing craft. The most likely routes used by human migrants have been assessed by Birdsell (1977) and also by Irwin (1992) using studies of winds, currents, and intervisibility between islands. This research suggests that there were two major routes into Wal- lacea (Fig. 11). The ﬁrst route (Birdsell’s Route 1) crosses from the Sunda Shelf to the island of Sulawesi, and from there extends to the islands associated with Halmahera. Eastwards from here the voyagers would have been in sight of islands off the western coast of Papua New Guinea (Sahul). Assessment of the Southern Dispersal

Fig. 10. Routes through the Sunda Shelf, with terminations.

Alternatively, the southern Molucca Islands may also have been used as stepping-stones on the way to Sahul. Birdsell’s southernmost route (Route 2) extends along the Indone- sian island chain from Bali to Lombok, Sumbawa, Flores, and Timor. This route has been suggested as the most appropriate for voyages to Australia, although Irwin’s (1992, p. 22) analyses indicate that these islands were only intervisible from the highest points on westward journeys, rather than Field and Lahr

Fig. 11. Birdsell’s (1977) proposed routes through Wallacea, and route through southern Sahul from Origin 7.

eastward ones. This does not rule out the possibility for voyages between the islands, as the between-island distances are not greater than those along Route 1 to the north. A voyage from the end of this route to Australia has been proposed by many. Simulations by Wild (1985) and by Thiel (1987) both suggest that the crossing could have been made in a relatively short time, or perhaps even by accident. Assessment of the Southern Dispersal

Northern and Southern Sahul

Once landfall was made on Sahul, migrations along the coastlines could have continued on for thousands of miles. Least-cost analyses of a wandering route from Origin 7 on the northwest coastline of Australia (represent- ing a crossing from Birdsell’s Route 2, at Timor) run northwards briefly but then turn southwards, following an inland path through what was probably a dry brush and scrub environment. This route continues southwards and only parallels the coast southwards from the Tropic of Capricorn (Fig. 11). Origin 8, located further north, runs northwards and into the drainage of the Digul River (Fig. 12). These regions were largely composed of tropical forests, with steep mountain slopes and ravines. This route then runs eastwards, and enters the drainage of the Fly River, and continues on to the coastline of the Gulf of Papua. The routes runs eastwards along the coast, and terminates on the shores of Milne Bay (Fig. 13). Origin 9, the terminus for Birdsell’s Route 1, was placed on the western tip of the Vogelkop Peninsula (Fig. 12). This route immediately encounters the steep topography that typifies the Sahul highlands, and is forced southwards into the flat interior. This route intersects with the route originating from Origin 8, and also terminates at Milne Bay. Secondary analyses that forced travel along the northern coastline of Sahul similarly ran into topographic barriers, as the coastal shelf drops deeply into the ocean in this region. Directional search targets failed to produce a coastal route, and as a result the least-coast route follows the inland river courses of the Taritatu and Sepik Rivers. The route then turns southwards across the Huon Penin- sula and follows the coastal route along the Gulf of Papua to Milne Bay (Fig. 13).

SUMMARY: BARRIERS AND CORRIDORS ALONG THE SOUTHERN DISPERSAL ROUTE

The results of the least-cost route analyses suggest a network of barriers and corridors between Africa and Australasia. These are displayed schematically in Fig. 14. The ﬁrst corridor out of Africa runs northwards, along the western coast of the Red Sea, and into the Nile Valley. Although geological studies suggest that the morphology of the Nile Valley was well established around 300 kyr (Paulissen and Vermeersch, 1987), it is possible that the river was not a true barrier, and it could have been crossed during low-ﬂow episodes, allowing for additional dispersals to the west. How- ever, in our model the barrier posed by the Nile River, as well as the corridor of the Mediterranean Coast, funneled populations northwards into the Field and Lahr

Fig. 12. Routes through northern Sahul from Origins 8 and 9, with secondary route along Taritatu River.

Sinai, and eventually into the mountains of Lebanon. The headwaters of the Tigris/Euphrates Valley then provided the next corridor through the desert, ultimately ending at the Straits of Hormuz and the Makran coast. The continuation of the route to the east would have similarly relied on ecological corridors. If the Bab al Mandab Straits were not a complete barrier, then the crossing of the Red Sea at this point leads to two potential routes: the ﬁrst northwards, towards the Gulf of Aqaba and leading to a dead-end; the second south and eastwards along the rugged Arabian coast. The latter route runs only partially along the coastline, and travels a Assessment of the Southern Dispersal

Fig. 13. Routes from Origins 8 and 9 and secondary route along Sepik River, with termination at Milne Bay. considerable distance along the edge of the sand sea before ending at the Straits of Hormuz. Continuing eastwards from the Straits of Hormuz to the mouth of the Indus Valley, migrants would have followed a completely coastal corridor. However, once migrants reached the Indus River, the corridors turn back inland. Riparian environments, coupled with mangroves or swamps that are difﬁcult to cross, may have provided these corridors. The Ganges- Brahmaputra Delta also may have served as a barrier that deﬂected populations into the Ganges Valley. In sum, the combination of corridors and Field and Lahr

Fig. 14. Schematic diagram of potential routes of the Southern Dispersal. Parallel bars indicate the locations of barriers, while arrows indicate diversions into the interior. barriers in South Asia implies that this region would have absorbed populations, rather than pushed them further eastwards along the coastlines. Crossing the Ganges-Brahmaputra leads to additional corridors that run southwards, with perhaps a minor barrier at the Iriwadi River. Popula- tions that were blocked or slowed by this delta may have traveled southwards by boat or raft to the Andaman Islands from this point. Further south, the migrants faced a difﬁcult route southwards into Sunda, with al- ternating coastal and inland corridors, while crossing into Wallacea implies a familiarity with boats and coastal conditions. Two potential routes of dispersal have been identiﬁed: one from the south that eventually ends onto the western Australian coast; the other through the northern Mollu- cas and into northern Sahul. However, once Sahul was reached reliance on coastal travel may have again been reduced, as corridors again turned inland. This is also true of the northern coast of New Guinea, where analyses suggest that migrations would have had an easier route into the Assessment of the Southern Dispersal interior via river valleys, as opposed to strandlooping along the northern coastline.

DISCUSSION

The results of our analyses for the Southern Dispersal indicate that if migrants followed our proposed route, they would have encountered a variety of climates, environmental zones, and interior as well as coastal landscapes. These conditions, as well as the barriers and corridors described above, have marked implications for human subsistence, migration speed, and demographic expansion. The following discussion will focus on these issues, and also comment upon how the routes and their surrounding environments may have contributed to human evolutionary trajectories.

Subsistence within the Coastal Ecotone at OIS 4, and Transitions to Terrestrial Environments

The use of coastal resources is thought to date to 250–125 kyr, and perhaps occurred earliest along the coastal fringe of the Mediterranean (Stiner, 1994;Bartonet al., 1999; McBurney, 1967), the western and southern coasts of South Africa (Henshilwood et al., 2002; Klein, 2001; Klein, et al., 2004), and along the Red Sea coast (Walter et al., 2000). For this time period, the South African sites contain the richest assemblages of coastal food de- bris, including large vertebrates such as fur seals and sea birds, and also shell middens that contain the remains of mussels, limpets, turban shells, periwinkles, chitons, and abalone (Henshilwood et al., 2002; Klein et al., 2004). However, other faunal remains in these sites suggest that utilization of coastal resources occurred as part of a generalized subsistence strategy that included a variety of terrestrial vertebrates (e.g., eland, zebra, rodents, tortoises, ostriches etc.). The combination of terrestrial and intertidal marine resources suggests that the extension of foraging areas into the coastlines required minimal adaptation. However, in all deposits fish remains seem to have been rare, and their absence is thought to indicate a lack of fishing technology during the earliest periods. Similarly, the mix of coastal and terrestrial contexts for the South- ern Dispersal Route described above suggests the potential importance of generalized marine-terrestrial subsistence systems. Although the extent of coastal exploitation throughout Africa during OIS 4 is not known, the coastal/terrestrial ecotones within and outside Africa could have provided a diverse and reliable subsistence base that would have been attractive to Field and Lahr mobile foragers. It can also be expected that coasts would have been ecolog- ically pristine, as there is only limited evidence for collection or consump- tion of marine invertebrates prior to the arrival of the first modern humans (Erlandson, 2001). As has been suggested by the study of intertidal fauna from archaeological deposits at Blombos Cave and Ysterfontein, foragers would have encountered invertebrates in the intertidal regions that were of adult size, providing the maximum return for a minimal expenditure of foraging time and energy (Klein et al., 2004; Henshilwood et al., 2002). Moreover, these resources would have been available year round, and required minimal technology for collection and processing. These areas may also have been crucial in the most arid environments, such as Northeast Africa, Arabia, and part of South Asia, providing a steady supply of easily collected animals, and buffering against seasonal shortfalls (Anderson and Polis, 1998; Polis and Hurd, 1996). If, as hypothesized, the Southern Dispersal Route began east of the Ethiopian highlands, populations would have first encountered the Red Sea coastline. Archaeological investigations indicate that during the warm, interglacial conditions of OIS5e (ca. 125 kyr), subsistence in the region included oysters, giant clams, gastropods, and crustaceans (Walter et al., 2000, p. 67). However, the richness and abundances of species at OIS 4 was likely to have been somewhat less, due to decreased temperatures and loss of habitat associated with sea level fall. Lower sea levels would also have con- stricted the Bab al Mandab Straits, and increased the level of salinity in the Red Sea to some degree (Siddall et al., 2003). Sea level fall would have also resulted in the terrestrial exposure of marine continental shelves along the rest of the Indian Ocean rim. At OIS 5a (76 kyr), sea level stood at − 24 m, and by the onset of the OIS 4 it had fallen to − 81 m (Cutler et al., 2003). This rate of sea level fall (10.6 m/kyr) would have resulted in the terrestrial exposure of marine continental shelves, and high rates of mortality for coral reefs and lagoon-dwelling species. Therefore, reef-based subsistence along the precipitous coastlines of Arabia would have been significantly reduced during the early portion of OIS 4. However, the rocky coastline undoubtedly supported other marine animals, such as invertebrates, nesting sea birds, and larger marine taxa, such as sea turtles (Jennings, 1981; Arabian Wildlife, 2004). Brief monsoonal rains, as well as onshore breezes along the Arabian Sea coast also would have made the foothills that back the coasts considerably more attractive and habitable than the extreme xeric conditions of the Arabian interior during OIS 4. The collection of small desert animals in tandem with intertidal fauna may have been crucial to survival in this region. Similarly, the Makran Coast was extremely arid but would have provided a variety of marine food resources. Although reef and lagoon taxa Assessment of the Southern Dispersal may not have been as common, the predominance of clastic deposits and associated geomorphology would have supported populations of molluscs in the intertidal zone, in particular sand-dwelling shellfish, as well as crabs and mudskippers (Hopner et al., 2000). As this coastal regime stretched eastwards and crossed into regions that received significantly higher rainfall (i.e., the coastlines of South Asia), the near-shore environments would have gradually become richer and biologically more diverse. Deltaic features would have provided a rich habitat for migrating birds, and sandy beaches and dune complexes would have provided harvests of molluscs and nesting sea turtles. Mangroves would have also increased in frequency along the coastlines of South Asia. These environments provide a rich ecosystem for fish, crustaceans, birds, monkeys, bats, and other small animals. Dispersals that continued down the coastline of Southeast Asia towards the Sunda Shelf would have encountered coastal environments that were broadly similar to one another. Deltaic features, including tidal sand bars, beach dunes, alluvial plains, and mud flats with mangrove vegetation would have dominated the landscape, and these environments would have provided a variety of marine resources of a similar sort to those previously encountered in South Asia. This environment would have extended into the coastlines of Wallacea and Sahul, and in the case of Pleistocene Papua New Guinea incorporated remnants of coral reefs and lagoons. As described in the summary of corridors and barriers, the coastal corridor that stretched from east Africa to the Makran Coast would have certainly continued into South Asia, but past the Indus River subsistence strategies may have broadened substantially, incorporating savannahs and dry woodlands in the interior of South Asia (Adams and Faure, 1997). Man- grove forests may have similarly diverted populations into the interior, as the environments are difficult to traverse on foot (Tomlinson, 1995). More- over, transitions to terrestrial subsistence should be expected for South Asia and the Sunda Shelf, as the interiors of these regions would have been attractive to hunter-gatherers. Pleistocene-aged faunal remains imply an interior that was rich and habitable, with savannah, scrub, and brush environments, as well as expanses of moist tropical forests in the higher ele- vations (Earl of Cranbrook, 2000). Recent re-excavation of Niah Cave in Sarawak, which at 40 kyr was a considerable distance from the coast, indicates a fully terrestrial subsistence system focused on fruits, legumes, tubers, pigs, bats, birds, and rodents (Barker et al., 2002). However, the presence of freshwater molluscs and fish among the remains also points to the continued use of aquatic resources. Familiarity with both terrestrial and marine subsistence regimes is also suggested by the least-cost routes through the Sunda Shelf, which bounce between the interior and the southern coastline. Field and Lahr

As has been previously described, crossing from the Sunda Shelf to Sahul must have been performed by foragers with some familiarity with watercraft and marine environments. However, archaeological remains suggest landfall in Pleistocene Australia predates habitation of Pleistocene Papua New Guinea and Island Melanesia by approximately 10,000 years, and that nearly all of the early sites known for Australia (e.g., Malaku- nanja, Nauwalabila, Devil’s Lair, and Lake Mungo) are from woodland and savannah environments with abundant terrestrial resources (Simpson and Grun¨ 1998; Roberts et al., 1994; Roberts et al., 1998; Turney et al., 2001). Undoubtedly, sea-level rise since the LGM has partly destroyed the Pleis- tocene archaeological coastal record of Australia, but the lack of marine remains in these early sites implies that populations transitioned quickly to a full terrestrial subsistence (O’Connor and Chappell, 2003). However, the earliest archaeological sites in Papua New Guinea and Island Melane- sia (e.g., the sites of Lachitu, Matenkupkum, Buang Merabak, and Kilu) contain ample evidence for coastal subsistence, perhaps including special- ized ﬁshing technology (Gorecki, 1993; Gosden and Robertson, 1991; Allen, 1994; Balean, 1989; Leavesley, 2004; Wickler, 2001). Although the routes generated by the least-cost analysis indicate movements through the interior were more likely, the presence of watercraft likely made habitation of the coastlines possible. Therefore, differences between Papua New Guinea and Australia suggest the responses of a ﬂexible subsistence system to local environments—each foraging optimally with what was locally available, either terrestrial or marine.

The Speed of Migration

What is notable from the discussion of coastal barriers and corridors is the potential for particular regions along the Southern Dispersal Route to have rushed or slowed the speed of migration. In terms of distance and time frames, the speed of dispersal along the proposed coastal corridor was approximately 20,000 km in 10,000 years (beginning at 60 kyr in east Africa, and ending at 50 kyr in Australia). This produces a rate of approximately 2 km per year—an extremely modest distance, and in light of ethnographic studies of hunter-gatherers and their reliance on mobility (e.g., Kelly, 1995), one that is perhaps impossibly slow. The analysis of routes, environments, and potential subsistence-bases presented here leads us to suggest that dispersals along coastlines were generally much faster, but perhaps broken into phases of slow and swift migration-legs relating to the geomorphology of the coastlines and the char- acter of the flanking terrestrial landscape. Increased speed along coastlines Assessment of the Southern Dispersal is due to the “funneling” effect of the coastal corridor, which forces travel in a single direction. It is also associated with population growth and migration being limited to a very narrow spatial front, which can expand only in a single direction (Surovell, 2003, p. 583; Gruhn,¨ 1994, p. 254). Thus, the dispersal along the desert coastlines of Africa, Arabia, and South Asia during OIS 4 may have been the most rapid, as the aridity of the interior would have forced populations to the coastal fringe, while the intensive collect- ing would have kept them moving forward. Variation in the geomorphology of the coastline may have also encouraged populations to move quickly through areas that were less productive, resulting in rapid “jumps” down the coastline (Sharma et al., in preparation). This is consistent with the hypothesis for “jump dispersals” defined by Tchernov (1992b), and suggested by Lahr and Foley (1994). More specifically, we hypothesize that if dispersals of modern Homo sapiens occurred during OIS 4, the swiftest migrations would have been those that took people immediately out of Africa, either via the Bab al Mandab Straits and the Arabian Coast, or via the Nile Valley and the Tigris/Euphrates Valley. These corridors would have provided habitats for hunting and gathering, and the severity of the surrounding deserts would have served to funnel populations through the regions quickly. Potentially, this expeditious migration could have occurred within a few hundred years, and involve an extremely small number of individuals. It also may not have left any living descendents in the regions of northeast Africa, Arabia, or the Makran Coast, as well as no archaeological signature, as populations would have moved through very quickly (i.e., Luis et al., 2004). The hypothesis of a rapid migration through Arabia also implies that the first modern Homo sapiens may have reached South Asia at a very early date (Field et al., in review). However, migrations may have slowed upon reaching South Asia and the Sunda Shelf. The attractiveness of the interior implies a sce- nario in which populations would have expanded into the open savannahs and tropical forests of the interior, and eastward travel may have slowed down dramatically. Populations could have reached the edge of Sahul ca. 50 kyr, which fits the more conservative 45–50 kyr colonization timetable (O’Connell and Allen, 2004). This “slowdown in the savannah” hypothesis does not preclude the possibility that some portion of the population also passed through South Asia and Sunda very quickly, and then went on, ostensibly with navigable watercraft, to Wallacea and Sahul ca. 60 kyr. In addition, it is important to note that the assessment of the South- ern Dispersal present here has been calibrated in relation to the earliest dates for human occupation in Australia. If our hypothesis for a slowing Field and Lahr of dispersals out of South Asia is correct, then the age of the first leg of the process—the initial expansions out of Africa and into South Asia—may have occurred significantly earlier.

The Potential for Demographic Expansion

We suggest that the Southen Arabian coast is the least likely region to have contributed to demographic expansion along the Southern Dispersal Route, and it probably played only a minimal role in the dispersal of populations out of Africa during OIS 4, or even later periods (cf. Oppenheimer 2003; Lahr and Foley 1994). Although there is potential for periods of wetness associated with the supressed monsoon system (Overpeck et al., 1996), this route is considerably more hazardous than the route through the Nile and Tigris/Euphrates valley, as it passes through an extremely dry and rocky environment, with little access to fresh water and almost no association with riparian areas. The route that proceeds from Origin 1 and utilizes the Nile Valley corridor would have offered a richer habitat, and also avoided the most extreme desert areas of the Arabian Peninsula. We also suggest that the open savannah and woodland environment of South Asia would have encouraged populations to expand demographically and geographically from the outset. This period of population growth and expansion implies that genetic vestiges of the earliest modern Homo sapiens out of Africa should be found in South Asia. Although archaeological investigations have yet to detect a signature that can be exclusively linked to modern humans at this time period, the Y-chromosome genetic lineages YAP/M145/M203/M174, RPS4Y/M216, the mtDNA M haplogroup, and the 9-bp deletion, have been put forward as evidence for the early dispersal of populations from Africa into the interior and eastern coastal areas of South Asia (Macaulay et al., 2005; Thangaraj et al., 2005; Underhill et al., 2001, pp. 51–52; Quintana-Murci et al., 1999b; Watkins et al., 1999). Although an African origin for these markers, as opposed to genetic drift, is currently under debate (i.e., Vishwanathan et al., 2004, Cordaux et al., 2003), the degree of heterozygosity among autosomal DNA markers in hill-tribe Indian populations at a minimum indicates high levels of genetic diversity, and a potentially lengthy history within South Asia (Vishwanathan et al., 2004, p. 135). Demographic expansion, as well as early episodes of colonization, are also suggested for Sahul. Analyses of mtDNA variation in modern highland populations in New Guinea by Redd and Stoneking (1999) indicate coalescent times of 122,000—88,000 years, which suggests that the populations in the region have been isolated for a lengthy period. Moreover, Australians Assessment of the Southern Dispersal appear to be largely unrelated to the populations of the New Guinea highlands, but can be linked to populations in South Asia (Harpending et al., 1996; Redd and Stoneking, 1999; Roychoudhury, 1984). These distinctions suggest multiple dispersals into Sahul between 30,000 and 60,000 years ago, but cannot speciﬁcally pinpoint a single migration event at either the earlier or later dates.

CONCLUSION

The results of the analyses presented here offer a fresh look into issues surrounding human dispersals using both modern and ancient palaeoen- vironmental data. Primarily, they suggest that routes through the coastal corridor of the Indian Ocean were viable during OIS 4, and that the local environments would have played a crucial role in overall subsistence and mobility. These environments also would have affected the speed by which populations advanced eastwards from Africa along the coastal corridor, and may have at times directed demographic expansions into the interior, or encouraged additional migrations along eastward-trending coastlines. Specif- ically, our analyses suggest a predominance of coastal routes through the arid environments of Arabia, the Persian Gulf, and the Thar Desert, and expansions into the interiors of South Asia and the Sunda Shelf. Dispersals through the more arid regions would have been the most rapid, and been the least likely to leave traces in the form of archaeological remains and/or genetic lineages among local modern populations. In contrast, dispersals along the coastlines of South Asia and the Sunda Shelf are likely to have included an inland-terrestrial component, and been of slower pace as populations expanded demographically into broadly habitable environments. Traces of this first expansion of modern Homo sapiens are more likely to be found in these regions, either in the form of genetic heritage and/or archaeological signatures. The least-cost routes themselves serve as hypotheses that may be tested archaeologically. Although it has been generally assumed that sea- level rise since the LGM would have erased all traces of earlier dispersals, surveys of the remote and undeveloped coastlines that the routes passed through (e.g., the coastlines of Yemen, Oman, Iran, and Myanmar) could discover caves or shelters that contain significant deposits. Off-shore islands that were connected to the coast during OIS 4 may also have preserved archaeological sites from later disturbance or destruction. The aridity of OIS 4 also suggests that springs and rivers that flow into the coasts would have been crucial to survival, and these areas are likely to be associated with archaeological deposits. Field and Lahr

Related studies, such as the analysis of genetic lineages surviving in populations along the proposed routes, may also contribute substantially to current understanding of the genetic ancestry and diversity of modern humans. As was discussed above, the timing of dispersals, the arrival of modern humans in Sahul, the diversity and ancestry of these populations, and the potential for genetic drift, are all crucial elements in this debate. The use of the routes as hypotheses for testing genetic lineages may yield some insights into which demographic processes were more at work in prehistory, and also “ﬁll in the gaps” between what is known for populations of Aus- tralia, New Guinea, and South Asia. In addition, the examination of populations within geographic “bottlenecks” (e.g., the Ganges-Brahmaputra Delta) or side paths (e.g., the populations of Sulawesi and Mollucca Islands, or the highlands of Southeast Asia) may provide critical links between the populations of Sahul and the rest of the world. In conclusion, the evidence for understanding the ﬁner details of human history will be gained from palaeoanthropological and human genetic data. However, this research demonstrates how the analysis of ancient environments and landscapes provides a rich venue for the contextual assessment of those data, and also allows for commentary on the evolutionary factors that may have led to particular biological or demographic trends. Additional research of the potential avenues for dispersal in other parts of the world, or even smaller-scale dispersals within regions, may generate additional insights.

ACKNOWLEDGEMENTS

This paper is the ﬁrst in a series of studies under the UK NERC- EFCHED project (Searching for Traces of the Southern Dispersal) that aim at estimating the likelihood of a given route of dispersal—the South- ern Dispersal—given speciﬁc temporal, geographical, palaeoclimatic, and anthropometric parameters. Additional funding was provided by the Lev- erhulme Trust. We wish to acknowledge and thank two anonymous review- ers, and also the editor of this journal, Angela Close, for a series of discern- ing critiques. Collaborative input and insight was also provided by Charu Sharma, Mike Petraglia, Robert Foley, Stephen Stokes, Geoff Duller, Mark Siddall, and Chris Clarkson

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