The most critical need for survival of hominins on both the short term (days) and long term (102
years) is a persistent source of freshwater. The two possible sources of potable water were surface water (lakes, streams) and areas of groundwater discharge (springs, seeps). Lakes in the East African Rift System (EARS) were few and often alkaline and/or saline due to high evapotranspiration (ET). Most rivers are seasonal (ephemeral), especially during the driest parts of precessionally-forced climate cycles in the Plio-Pleistocene. However, the locations of groundwater discharge in the EARS are not controlled by climate, but by geology (topography, permeability of rocks, and rift-related faults). This is counter-intuitive, but supported by basic hydrogeologic principles. Recharge occurs on topographic highs (fault blocks and volcanoes); the groundwater is shielded from evaporation, moves downhill under hydraulic head and discharges in topographic lows. Precession cycles (~23 kyr) dominate climate in the EARS. Computer modelling for multi-millennial periods indicates that spring flows lag groundwater recharge by 100 s to 1000 years because of the geology. Thus, rainfall during wetter periods ends up producing persistent freshwater during dry periods. The lag creates long buffer periods allowing hominins to adapt to new habitats as potable surface water from rivers or lakes became increasingly scarce.
On the short term, the volume of water required for an individual ranges from 1 to 5 liters/day depending on age, gender, ambient temperature and activity level. Intense competition is expected at water holes between hominin and other animal species and hence selective pressure favors those with adaptations for sufficient mobility to discover a new and more persistent groundwater source, or for those already settled within home range of such a resource. On the long term, localized groundwater systems are likely to have been widespread within the EARS providing refugia and sites of intense competition during dry periods, thus being an important factor in natural selection and evolution. During wetter periods, springs may have formed ways of ‘bridging’ longitudinal dispersal of hominins between larger freshwater bodies or rivers providing a critical resource during hominin migration within and out of Africa.