Environmental hypotheses of Pliocene human evolution

Abstract

Substantial evolutionary change in Pliocene hominins affected a suite of behaviors and anatomical features related to mobility, foraging, and diet – all related to the ways in which hominins interacted with their biotic and physical surroundings. The influence of environment on evolutionary change can be stated as a series of hypotheses. Adaptation hypotheses include the following: novel adaptations emerged in hominins and contemporaneous mammals (1) within relatively stable habitats; (2) during progressive shifts from one habitat type to another; and (3) due to significant rises in environmental variability. These ideas further suggest the “adaptability hypothesis”: (4) since adaptations potentially evolved in environmentally stable, progressively changing, or highly variable periods, lineages have differed in their ability to endure environmental fluctuation. Thus, extinction of certain adaptations (and lineages) should have corresponded with heightened environmental variability, while new adaptations evolved during those periods should have enabled a lineage to persist (and spread) through a novel range of habitats. Turnover hypotheses, on the other hand, concern the timing and processes of species origination and extinction in multiple clades. These hypotheses state that species turnover; (5) was concentrated in a narrow interval of time related to a major climate shift; (6) spanned several hundred thousand years of climate change, and occurred in a predictable manner dependent on the nature of species adaptations; and (7) took place gradually over a long period as lineages originated, persisted, or went extinct within a changing mosaic of habitats. A separate, biogeographic hypothesis regarding faunal change posits that; (8) substantial climatic and tectonic disruptions resulted in multiple episodes of faunal community formation (assembly) and breakup (disassembly). This assembly-disassembly process may have had profound effects on Pliocene and Pleistocene faunas of Africa and on researchers’ ability to infer significant events of faunal evolution from fossil sequences at the basin or sub-basin scale. Since all ideas about environmental effects on evolution depend on temporal correlation, an important challenge is to match faunal sampling to the precessional (~20 Kyr) and obliquity (41 Kyr) scale of Pliocene climate dynamics.