BEYOND CLIMATE: FIVE FACETS OF THE EVOLUTION-ENVIRONMENT CONNECTION IN EASTERN AFRICA

This talk explores the intersection of five facets that help explain a fundamental adaptive shift related to the origin of Homo sapiens. The five facets are: climate dynamics, tectonics, herbivore feeding strategies, evolutionary processes, and vicariance (i.e., habitat fragmentation and loss of geographic range). The role of each is examined for the Acheulean-to-Middle Stone Age transition in the southern Kenya rift, which was associated with an 85% turnover in the mammalian fauna and a concurrent shift in vegetation and landscape dynamics recorded in the Olorgesailie and Koora basins. These developments occurred between 500,000 and 320,000 years ago, an interval that overlaps with estimates of African H. sapiens genomic divergence time. First, the effect of insolation on climate predicts that these major evolutionary and environmental transitions co-occurred beginning ~358 ka, i.e., near the the start of a long period of heightened climate variability. Second, tectonic faulting greatly increased the topographic relief and compartmentalization of the southern Kenya rift beginning ~500 ka. As a result, smaller basins and sub-basins were created that had varying sensitivity to moisture and thus distinct water/food resource histories. Third, the effect of large grazing herbivores on vegetation suggests that tall-vs-short grass dynamics did not necessarily follow moisture supply >500 ka but did so to a greater extent later in time. Ecological interactions can thus sometimes override expected climatic effects relevant to evolutionary transitions. Fourth, evolutionary processes that take into account large shifts in environmental variability are necessary to explain herbivore and hominin behavioral responses recorded in the southern Kenya rift between 500 and 320 ka. Fifth, although Acheulean technology continued for a while in other regions beyond 320 ka, reduction of its geographic range (e.g., elimination from southern Kenya) implies that local extinction of Acheulean behavior was an important step in its widespread, permanent replacement by Middle Stone Age behavior – and thus to the proliferation of adaptations unique to Homo sapiens.