Early Pleistocene Millennial-Scale Hydroclimate Fluctuation in Kenya with Implications for Hominin Evolution

Our hominin ancestors in East Africa responded to dramatic oscillations in climate, according to the variability selection hypothesis (Potts, 1996, DOI: 10.1126/science.273.5277.922). Orbitally-resolved records confirm large changes in the amplitude of East African climate variability through time, yet it is unclear how such long-term variations, well beyond human generational time-scales, influenced hominin evolution. To date, there are very few records that evaluate East African climate variability at sub-orbital timescales that extend beyond the latest Pleistocene. We have produced a hydroclimate record based on the hydrogen isotopic composition of terrestrial leaf waxes (δD_wax) preserved in a paleolake sediment drill core from West Turkana, Kenya (WTK), a hominin fossil locale famous for Turkana Boy and other Homo erectus fossils. A previous orbital-scale hydroclimate record of δD_wax from WTK indicated an interval of extremely high climate variability at 1.7 Ma, a time that corresponds with multiple hominin evolutionary events (species turnover, new stone tool technology, Out of Africa I dispersal; Lupien et al., 2018, DOI: 10.1016/j.quascirev.2018.03.012). Here we compare millennial-scale variability in δD_wax during this high variability interval to a time interval with low orbital-scale variability to evaluate whether the amplitude of millennial-scale variability scales to the amplitude of orbital variations. We further this evaluation by comparing these early Pleistocene hydroclimate variations with high-frequency δD_wax variations during more recent intervals of higher global ice volume. We find that δD_wax fluctuations during the early Pleistocene in West Turkana were on the order of 10 ‰, about half amplitude of the Younger Dryas event in Lake Turkana ( 20 ‰). More importantly, the millennial-scale hydroclimate fluctuations between the high and low orbital-scale variability intervals are similar. This suggests that we cannot assume that changes in the amplitude of orbital-scale climate change directly impacts higher-frequency variations, with important implications for the mechanism of human evolution and variability selection.