THE IMPORTANCE OF SPRINGS AND WETLANDS IN REDUCING “WATER STRESS” IN HOMINIDS DURING ARID PERIODS
Climatic forcing operates on time scales of 106 yrs (tectonic) to 105 yrs (orbital) to 103-100 yrs (climatic) to 101-100 yrs (solar). Environmental changes in Africa were long-term (cooling, aridification, and expansion of savanna) with superimposed short-term excursions (wet/dry cycles, 19 to 23 kyr) that had the greatest impact in low latitudes. In arid climates, a switch from humid to dry conditions altered surface water and groundwater systems and increased "water stress" for animals as potable water sources become limited. Those plant and animal species able to adapt to episodic reduction in water resources occurring on a variety of time scales are more likely to succeed (i.e., "variability selection", Potts 1996). Two paleoenvironmental records of water resources that were sustained by groundwater reserves during prolonged dry periods are (1) Olduvai Gorge (Beds I & II), Tanzania and (2) Loboi Swamp, Lake Baringo, Kenya. Sedimentary records at Olduvai between ~1.79-1.74 Ma document a spring-fed wetland complex that flourished during the arid portion of precession-driven (~ 21 kyr) lake cycles. Olduvai wetlands developed on lake-margin flats. The source was meteoric water from the adjacent Ngorongoro Highlands that exited along faults in the valley. Concentrations of artifacts and cut-marked bone are higher in the wetlands than in adjacent environments suggesting the wetlands were frequented by hominids during arid periods. Loboi Swamp is also fed by springs discharging along rift valley fractures. The wetland developed and expanded during "The Little Ice Age" which was a millennial-scale wet period (~700-130 BP) and associated with migrations of indigenous peoples into the normally dry rift valley. These examples representing wet/dry cycles of different times scales document aquifer-fed water sources that could have relieved "water stress" on hominids during arid periods and may have been an important ecological resource during human evolution.