DID THE PLIO-PLEISTOCENE ICE AGE TRIGGER THE EVOLUTION OF ADVANCED HOMINIDS?

The late Neogene expansion of continental glaciers in the Northern Hemisphere heralded the beginning of the modern Ice Age and resulted in vast biotic change in Africa. Cooler and drier climates replaced forests with grasslands, prompting turnover of various mammalian faunas such as antelopes and rodents. Of particular interest is the evolution of advanced hominids (early Homo) from a population of gracile australopithecines close to the onset of this ecological crisis some 2.4 million years ago (Ma). While capable of rudimentary bipedal locomotion, australopithecines possessed a suite of arboreal adaptations. Indeed, their reliance on trees for food and as refugia against predators precluded the larger brain sizes and fully terrestrial mode of life which characterize early Homo. By forcing australopithecines to abandon trees, climatic disruption of woodland habitats provides a plausible mechanism for the rapid emergence of Homo through delayed maturation in ontogeny and natural selection for greater intelligence. Because the hominid fossil record is relatively poor, however, a mathematical model of taxonomic evolution and fossil preservation was used to estimate the true time of origin for the genus Homo. If model parameters are calculated from all known extinct hominid species (n=25), the stratigraphic range extension (or interval of complete non-preservation) for Homo is 5.54 × 10⁵ years at the 50% confidence level. If only extinct species within Homo (n=11) are used, the range extension increases to 6.81 × 10⁵ years at the 50% confidence level. Both estimates place the origin of Homo at approximately 3.0 Ma, somewhat prior to the major pulse of cooling about 2.5 Ma but generally coincident with the large-scale climatic change associated with the Ice Age. Evidence of temporal overlap between both groups of hominids suggests that Homo arose by divergent speciation rather than anagenesis of a bottlenecked ancestral australopithecine species.