Sea-level change, palaeotidal modelling and hominin dispersals: The case of the southern red sea

We examine the likelihood of early human sea crossings of the southern Red Sea during Pleistocene low sea-level stands, using palaeotopographic reconstruction of coastlines, modelling of palaeo-tidal flows and simulation of passive movements using Lagrangian particles. Existing isotopic and geological data demonstrate that the marine connection between the Red Sea and the Indian Ocean has remained open for at least the past half million years, ruling out the possibility of a land crossing. Many authors have argued for the plausibility of a successful sea crossing during the Pleistocene as a southern route for human dispersal from Africa, especially for the dispersal of Homo sapiens. However, decisive evidence is lacking. Other authors have preferred the default northern route of land-based dispersal via the Sinai Peninsula as the more likely option and viewed the southern sea crossing as a barrier rather than a gateway, especially if tidal flow was much stronger through the narrowed sea channel at low sea levels. We use Fluidity, a finite element modelling procedure, to model tidal flows and assess their validity by comparison with modern tide-gauge data. To model palaeotidal flows, we use reconstructions of palaeoshorelines and coastal palaeotopography extending for 150 km from the Bab al Mandab Strait to the Hanish Sill region, which take account of eustatic, GIA and tectonic effects. We then simulate passive movements using Lagrangian particles and a 4th-order guided search Runge-Katta algorithm. We ran simulations for six days from three different starting points on the African shore and 13 different times in the tidal cycle. We show that crossing distances are much shorter during the Pleistocene than today with clear inter-visibility of the opposing shorelines, but that tidal currents were much stronger. We also show that the highest chances of successful crossing, involving passive rafting or drifting, with a duration of 3–4 days, are in the vicinity of the islands in the Hanish sill region. With directed rafting or swimming, the crossing times would be much shorter. We conclude that sea crossings would be easily accomplished during long periods of the glacial cycle, regardless of hominin status, especially given attractive terrestrial landscapes and environments on both sides of the southern Red Sea.