Figure 5: Modelled dispersal of hominins between hydro-refugia through a wet–dry climate cycle.

As the climate changes from the driest (a) to wettest (c) state envisaged during a precessional cycle, the modelled dispersal of hominins between hydro-refugia increases until dispersal is almost ubiquitous. Agent-based modelling results based on three hydrological scenarios, using a maximum three-day travel distance of 150 km and surface roughness as the cost layer scaled according to Supplementary Fig. 11. The black lines shown represent the tracks of agents in the model. It is important to note that just because the model shows a potential pathway between two water sources it does not necessarily mean that it was actually followed. (a) Driest scenario using persistent springs (Fig. 1), geothermal springs, fresh water lakes, major rivers with a flow >5 km³ y⁻¹ (Run-4). Note the networks of springs acting as hydro-refugia, which persist irrespective of the topographic cost layer and scaling used in the model (Supplementary Figs 9–12). (b) Dry scenario using modern springs (perennial+geothermal), lakes (fresh+saline), perennial wetlands and major rivers with a flow >0 km³ y⁻¹ (Run-3). Note the potential for dispersal transverse to the rift axis (white dashed line) and the absence of along-axis dispersal routes. Springs act to connect rivers on the rift flank with those of the rift floor. The cross-rift movement in the vicinity of Lake Abaya (Ethiopia), where the rift cuts the Ethiopian Highlands, is sensitive to the topographic cost layer and scaling used in the model (Supplementary Fig. 9); however, the principle holds in southern Kenya. The next stage in the increased water availability continuum is shown in Supplementary Fig. 10, which includes perennial streams and reveals a progressive increase in cross-rift movement and the start of dispersal along the rift (Run-2). (c) Wettest scenario uses modern springs (seasonal, perennial+geothermal), wetlands (perennial+seasonal), lakes (fresh+saline), major rivers with a flow >0 km³ y⁻¹ (Run-1). Note the potential for widespread dispersal of hominins and genes. Background relief map is based on a 30 × 30 m SRTM model: http://earthexplorer.usgs.gov/. Projection: WGS 1984.