Abstract Human migration north through Africa is contentious. This paper uses a novel palaeohydrological and hydraulic modelling approach to test the hypothesis that under wetter climates c.100,000 years ago major river systems ran north across the Sahara to the Mediterranean, creating viable migration routes. We confirm that three of these now buried palaeo river systems could have been active at the key time of human migration across the Sahara. Unexpectedly, it is the most western of these three rivers, the Irharhar river, that represents the most likely route for human migration. The Irharhar river flows directly south to north, uniquely linking the mountain areas experiencing monsoon climates at these times to temperate Mediterranean environments where food and resources would have been abundant. The findings have major implications for our understanding of how humans migrated north through Africa, for the first time providing a quantitative perspective on the probabilities that these routes were viable for human habitation at these times.

Citation: Coulthard TJ, Ramirez JA, Barton N, Rogerson M, Brücher T (2013) Were Rivers Flowing across the Sahara During the Last Interglacial? Implications for Human Migration through Africa. PLoS ONE 8(9): e74834. https://doi.org/10.1371/journal.pone.0074834 Editor: John P. Hart, New York State Museum, United States of America Received: May 20, 2013; Accepted: August 5, 2013; Published: September 11, 2013 Copyright: © 2013 Coulthard et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: The authors have no support or funding to report. Competing interests: The authors have declared that no competing interests exist.

Introduction The role of the Sahara as a geographical filter and launch zone for dispersals of Homo sapiens out of Africa is a controversial topic [1], [2], [3], [4]. At issue is the observation that 130-100,000 years ago there was a marked increase in humidity in the present desert and adjacent regions [5], [6], [7], [8], which coincided with some of the earliest appearances of H. sapiens in both the Sahara and the Levant [9], [10], [11], [12], [13], [14], [15], [16]. During MIS 5 (Marine Isotope Stage 5) [17], high insolation in the northern hemisphere caused the African monsoon to assume a position up to 1,000 km north of its location today [18], [19], [20], [21]. Isotopic and geomorphic evidence suggests that rain falling on the north of the Trans-Saharan mountains then flowed towards the Mediterranean [2], [22], [23], potentially creating migration pathways via a series of ‘green corridors’ [4] and ‘mega lake corridors’ [24] across the Sahara. Dating of human fossils from the Levantine sites of Skhul and Qafzeh imply that early dispersals occurred along the eastern margins of the Sahara prior to ∼100 ka [13], [14], [15], while other craniodental remains show that populations closely resembling those of the Near East were simultaneously present in north western Africa [9], [16], [18]. Given the combined dating uncertainties, many of these fossils and associated archaeological Middle Palaeolithic/Middle Stone Age finds are likely contemporary with the last interglacial period of peak humidity attested in marine cores [20], [23] and stalagmites [6], [25]. However, supporting evidence for the ‘green corridor’ hypothesis remains subjective. Aside from topographic analysis, there has been no quantitative hydrological assessment (i.e. calculation of fluxes and balances of water) that could test whether these freshwater pathways across the Sahara were physically possible during the Eemian (MIS 5e). Surface evidence of fossil river systems and dated lacustrine records show there has been water in the region, but this does not provide an effective quantitative view of when, where, or how much water was present in the wider landscape. Interpretation of the archaeological record is equally subjective, as although the general direction of dispersal (northward from sub-Saharan Africa) and its general timing (last interglacial) are clear, these data remain ambiguous between a single trans-Saharan migration with delayed subsequent expansion, multiple migrations via a single route or multiple migrations via multiple routes. The existing evidence is not sufficient to conclude whether contiguous ‘Green Corridors’ existed at the right time for migration. In this paper, for the first time, we simulate the balance and fluxes of water across this region. We use simulated precipitation from a state of the art Earth System Model (ESM) simulation of the Eemian (MIS 5e) climate to drive a combined hydrological and hydraulic model to reconstruct past rivers and flood events across 12,000,000 km2 of North Africa. For the first time, this reveals the seasonal and spatial patterns of Saharan surface palaeohydrology, predicting the presence of distinct river corridors and wetlands [24], [26]. Our simulations were carried out with the sole aim of testing the Green Corridor hypothesis; were contiguous corridors following surface water (i.e. rivers) really feasible during MIS 5e? Do all the buried rivers show the same history, or are there spatiotemporal differences? We achieve this by calculating a probability of surface water routes across the Sahara as a basis for further investigation.

Methods Rainfall data of a palaeoclimatic simulation (124–125 ka) are taken from an experiment with the coupled atmosphere-ocean-sea ice-biosphere general circulation model (ECHAM5/JSBACH/MPI-OM) [27].This simulation provides a series of realistic scenarios in which the African monsoon sits c.700 km further north than at present [20] (see materials and methods section ‘Generating precipitation data’). The good performance of the ECHAM5 model is shown by its predictions for pre-industrial precipitation that can be compared to gauged data (see materials and methods section ‘Validation and Calibration’). For the period 125–124 ka BP a 25 year snapshot is taken with a sequence of 12 hourly, gridded data (3.75o x 3.75o) across North Africa (Fig. 1). The northern edge of the MIS 5e monsoon spans the Ahaggar and Tibesti Mountains that are major topographic features covering a linear distance of ∼2500 km and including Southern Algeria, Southern Libya and Northern Chad with a maximum elevation of 2500 m. Previous spatial analysis of the regional topography [2] has shown there are major watersheds that are dry today but which would drain north from these mountains towards the Mediterranean. Satellite imagery reveals traces of major river channels linked to these watersheds, now partially buried under sand dune deposits [2], [22], [28]. Using a 1 km resolution digital elevation model (DEM) based on resampled GMTED 2010 topographic data (vertical accuracy RMSE 26–30 m) [29] the watersheds draining North from Ahaggar and Tibesti were delineated (see materials and methods section ‘Issues with the DEM’), excluding areas draining into the Nile basin to the East and the Senegal and Niger basins to the west. This area is shown as the shaded area in Figure 1 and is the focus of this study. The DEM is therefore based upon the present day topography though as sea level was up to 20 m higher during the earliest millennia of MIS 5e when our experiments are set [30], we adjusted the DEM by subtracting 20 m from all elevations, and redefined the coastline along newly submerged locations. Simulated rainfall data at 12 hour resolution were fed into a hydrological model [31] for each climatic grid cell which generated surface runoff across the 1 km DEM at 1 min resolution. Surface runoff was routed across this DEM using a 2d hydrodynamic flow model based on the Lisflood-acc model [32], (see material and methods section ‘Hydraulic Model’). As our aim was to test the viability of fluvial corridors, and for model parsimony, groundwater recharge was not simulated, though we acknowledge that there is evidence for palaeolakes in the region that were ground water fed [33] and these lakes could have formed part of a migratory route [2], [26]. A series of 25 year simulations were carried out with high, medium and low infiltration/evaporation rates (1.5, 3 and 6 m yr−1) [34], [35], [36] and for every model time step water was also removed from grid cells to account for evaporation and infiltration. Hydrological model outputs were plotted as probabilities of surface water being present at a location varying between 0 and 1. These were determined by recording daily outputs of water depths and summing over a 25 year simulation period the number of days where water depths greater than 0.1 m were present. The use of 1 km grid cells imposes limitations as all river channels must be a minimum of 1 km wide resulting in excessive channel width/depth ratios under lower flow conditions. This, along with no groundwater recharge, leads to an underestimation of cell flow depths and thus a poorer drainage network connectivity/delineation as well as an over estimate of water loss from infiltration/evaporation due to too large water surface/channel bed areas. This conservative model configuration gives us a far higher confidence in our results as it reduces the possibility of overestimating inundation timing and extents. Additional shorter runs were used to test model sensitivity to altering hydrology and rainfall rates and to demonstrate the model’s validity. Further validation of the palaeohydrology modelling method is provided in materials and methods section “Validation and Calibration”.

Results Figure 2 demonstrates the existence of a series of extensive ephemeral and perennial river systems draining North from the Ahaggar and Tibesti mountains across the Sahara to the Mediterranean during the period 125–124 ka BP. Some channels dissipate in the desert, but some converge forming three main systems; in the West the Irharhar river draining into the Chott Melrir basin, and to the East two larger systems named the Sahabi and the Kufrah (Fig. 2). As the rainfall is associated with the monsoon, flow is highly seasonal and the Irharhar river is ephemeral, flowing for ∼3 months. The Sahabi and Kufrah systems are close to perennial, due to larger contributing areas in the catchment headwaters located in the monsoon belt (Fig. 1). Figure 3 demonstrates the seasonality of flow in all three systems with precipitation in August taking over two months to reach the coast or near-coastal lake systems. The distinct climatic zoning (Fig. 1) means all three rivers are allogenic, losing water along their length with little or no hydrological contribution once they leave the mountains/uplands. In addition to rivers, the simulations predict massive lagoons and wetlands in NE Libya some of which are extensive (>70,000 km2). These are also fed from the Jebel Akhdar in Cyrenaica, which also received higher rainfall during this period. There are smaller lakes forming in Tunisia and Algeria due to water supplied via the Irharhar river system. PPT PowerPoint slide

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larger image TIFF original image Download: Figure 1. Palaeorainfall used to drive the combined numerical model. Yearly average rainfall from a 25 year snapshot of an ESM experiment and catchment area (hatched region) as well as the time series of zonally averaged precipitation for two stripes south of the catchment highlighting the (i) South-North gradient of rainfall during the wet seasons (June to September) and (ii) the modelled year to year variability of the monsoon system. The data is drawn from 12 hourly precipitation data produced during a time-slice experiment [27] of the last Interglacial (MIS 5e, ∼124 ka BP) performed by the fully coupled atmosphere-ocean- sea ice-biosphere general circulation model of the Max-Planck-Institute for Meteorology. The ESM consists of the spectral atmosphere model ECHAM5 [43] including the land surface model JSBACH [44] and a dynamic vegetation module [65] coupled to the general circulation ocean model MPIOM [45]. The model runs for the atmosphere at a truncation T31, which corresponds to a horizontal resolution of ∼300 km in the area under investigation. https://doi.org/10.1371/journal.pone.0074834.g001 PPT PowerPoint slide

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larger image TIFF original image Download: Figure 2. Simulated probability of surface water during the last interglacial. This figure details Archaeological sites, and an annual probability that a location has surface water. The archaeological data are derived from a number of sources (including [42], [66], [67], [68]. The findspots are characterised by Aterian and Middle Stone Age artefacts such as bifacial foliates and stemmed Aterian points and/or typical ‘Mousterian’ points, side scrapers and Levallois technology. Most are represented by surface scatters but where stratified examples exist these can be shown by dating (OSL and U-series techniques) and geomorphological setting to belong within MIS 5e [41], [42]. https://doi.org/10.1371/journal.pone.0074834.g002 Figure 2 and 3 present results from the conservative medium evaporation/infiltration scenarios, and are based on all 25 years of simulated hydrology - as rainfall within the ESM varies year to year. Simulations with higher evaporation/infiltration rates (6 m yr−1) gave the same drainage network but with lower probabilities. Simulations giving rise to wetter (1.5 m yr −1 evaporation/infiltration) conditions increased the longevity of a channel’s wetness but did not introduce any new major water courses, nor significantly increase the wetted area or size of any lake systems.

Discussion This study provides the first strong quantitative evidence for the presence of three major river systems flowing across the Sahara during MIS 5e. We simulated three river systems that are now largely buried by dune systems, but when flowing would have provided fertile habitats for flora and fauna in proximity to the channels [22]. Notably, the Sahabi and Kufrah would be major river systems with monsoon discharges significantly in excess of 2500 m3 s−1 and an extensive system of anabranches and wetlands. In the Libyan Kalanschiou region, the green corridor would have been 100 km wide, substantial and largely perennial. This reconstruction is highly compatible with evidence of widespread palaeosols deposited on the margins of this system during the less pronounced Holocene humid period [22]. Here we have simulated one wet phase, but this research strongly supports the occurrence of similar ‘Green Saharas’ recorded in the marine [37] and terrestrial [26] archive. Our simulation results quantitatively confirm previous hypotheses of these rivers shown in geomorphic and geochemical data [2], [4], [22], [23], [24]. For example, the radio-isotopic composition of the water identified in the Ionian Sea [4], [38] indicates that it was precipitated in the basaltic trans-Saharan range in Southern Libya as shown by our results. Runoff flowing rapidly to the coast, in a manner highly comparable to our simulated runoff waves, is shown by the light oxygen isotopic composition of the water flowing into the Mediterranean at this time [20]. Furthermore, the river systems that our research simulates are consistent with the well-preserved drainage network that has been identified in these regions by fieldwork and from satellite imagery [22], [24]. Overall, our confirmation that these hypotheses are physically realistic allows us to move on to questions of how and when the rivers operated, rather than their existence. Whilst we cannot state for certain that humans migrated alongside these rivers, the shape of the drainage systems indicate that anyone moving from south to north from a 2000 km wide region in the mountains would be funnelled into three clear routes. There is also a clear geographical split, with a 2000 km gap between the destinations of Irharhar and the combined Sahabi and Kufrah systems. Despite being ephemeral, the Irharhar river corridor could be the most suitable for dispersal of hominids beyond the Sahara. Uniquely, the Irharhar extends from humid to humid climes - ranging from the monsoonal Ahaggar and Tibesti region to the North Western Mediterranean climate zone that also received substantial winter rainfall (Fig. 1). High humidity in the destination region during the last interglacial is confirmed by the presence of significant water near the Chott Melrir basin [39]. Whilst the more extensive Sahabi and Kufrah also traverse the Sahara, their downstream limits remain within the arid/semi-arid regions [40]. Support for the significance of the Irharhar river corridor is provided by the high number of Middle Stone Age archaeological sites concentrated in the western region (Fig. 2). Many of these locations contain Levallois lithic artefacts with Aterian affinities that on comparative grounds can be plausibly dated to the last Interglacial [41], [42]. It is highly likely given the existing artefact distributions that humans migrated northwards from the relatively humid Trans-Saharan mountainous zones to the Maghrebian Mediterranean biome (Fig. 2). The loose clustering of sites along our simulated Irharhar river and associated channels implies this as a preferred route of dispersal. Furthermore, as the simulations are driven by present day topography, if the dune systems in this region were removed or reconfigured the Irharhar could flow further to the West. In contrast, the eastern region has a surprising lack of archaeological evidence despite the extensive simulated palaeo-river courses. It is likely that further surveys in this area will provide substantial evidence of Middle Stone Age activity, especially in the areas of buried palaeochannels. However, continued absence of this critical evidence of human migration would confirm our suggestion that a key factor in the western distribution of sites was the attractiveness of the richer Mediterranean-type environments of the Maghreb, which would have promoted permanent settlement in the region and further transit in both directions along the Irharhar river corridor.

Conclusions For the first time, our simulations demonstrate that Saharan “Humid Corridors” were highly likely during the last interglacial strongly re-affirming the viability of these routes as migratory corridors for early hominids. This research provides an unprecedented means of developing new hypotheses for past human, faunal and floral activity in this region and for validating the performance of palaeo climate simulations.

Author Contributions Conceived and designed the experiments: TC MR JR. Performed the experiments: TC JR. Analyzed the data: JR TC MR NB TB. Contributed reagents/materials/analysis tools: TC JR NB MR TB. Wrote the paper: TC JR NB MR TB.