Climate, Heterochrony, and Human Evolution

The Late Neogene African records of climatic change at the start of the modern ice age, and of turnover in some mammalian groups including Hominidae, support the turnover pulse prediction. Many of the new species that appear 2.9-2.5 myr (millions of years) ago show similar suites of integrated character complexes, including larger bodies (consistent with Bergmann's Rule) and relative reduction in some body parts (Allen's Rule in the case of bodily extremities), together with enlargement of others including brains. I explore further the hypothesis (Vrba 1994) that the same evolutionary event of growth prolongation, or time hypermorphosis, as it acts on characters with different ancestral growth profiles in the same body plan, can result in a major reorganization--or "shuffling"--of body proportions such that some characters become larger and others smaller, some hyperadult and others more juvenilized. I suggest that this hypothesis applies to major features of hominid evolution including hominine encephalization.