HOMININ SURVIVAL DURING PRECESSIONALLY-PACED WET/DRY CYCLES AND CHANGING HABITATS: THERE IS NO SUBSTITUTE FOR FRESH WATER

Reliable water supplies are critical to human survival during times of water stress. In East Africa, where most hominin first occurrences are located, precession cycles with a ~ 20,000 year beat were pervasive, creating wet-dry cycles and triggering changes in vegetative and hydrological landscape. Precession cycles regulated water supply on the long term and seasonal monsoonal rains limited water on the short term. Aquifers are periodically recharged on the high topography (3,000-6,000 m) that traps the seasonal rains. Groundwater is shielded from evaporation and is discharged into topographic lows or fault conduits, effectively buffering the short term and long term variability in rainfall. There is a pervasive negative hydrological balance in eastern Africa where evapotranspiration exceeds annual rainfall. During dry periods, most rivers are seasonal and lakes are alkaline, groundwater is the only reliable water resource to maintain “hydro-refugia”. Hydro-refugia are sanctuaries for vertebrates (including hominins) centered on freshwater (springs, wetlands and groundwater-fed rivers) that persisted through the orbital-scale cycles and thus were the only option when potable surface water was scarce. The continually changing environment would have created difficulties for the “bare feet on the ground” (i.e. hominins) trying to find dependable potable water sources that were sufficiently closely spaced. The distance between water supplies is a key factor in survival of hominins particularly during droughts. Additionally, hominin food resources were likely tied to persistent sources of water. Termites, ants and edible plants such as papyrus, sedges, ferns, water lilies, portions of palms, figs, along with protein-rich (brain food) such as molluscs, turtles and fish are known to thrive in areas of groundwater discharge. Finally, several of the wetland sedges are C4 and edible which may explain the stable isotope signature found in the teeth of some hominins.