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A chronological framework for human origins.

Anatomically modern Homo sapiens is the sole living representative of an evolutionary radiation of species, collectively known as the hominids. The morphological and behavioural characteristics that define the hominids are believed to have originated in a single ancestral species, and the hominids therefore constitute a monophyletic group, or clade. Some authors have widened the formal taxon Hominidae to incorporate the African apes Pan and Gorilla, arguing that the close genetic relationship between these primates and modern humans warrants their inclusion in a single biological family. In this contribution I use the conventional cladistic definition of hominids as including all taxa more closely related to Homo sapiens than to any other living primate. By the late Miocene the common ancestor of the hominid clade had diverged from the lineages leading to Pan and Gorilla. A major task of hominid palaeontology is to document the events and processes that determined hominid diversity from about 8 million years (Ma) up to the present, and the purpose of this paper is to outline current views on the timing of the more important speciation events that have generated hominid diversity. Indirect evidence supports an African origin for the ancestral hominid. From their earliest appearance in the fossil record at about 6 Ma up to the middle of the Lower Pleistocene at about 1 Ma hominids were confined to Africa. This restricted biogeographic distribution is also characteristic of the hominids' sister taxa, the African apes, which have no close relatives (fossil or living) outside the continent. There is thus no evidence for the earliest hominids having dispersed from an Asian or European centre of origin. Corroboration comes from physiological studies which suggest that bipedal locomotion, one of the earliest hominid adaptations, served to reduce diurnal heat loads in equatorial environments (Wheeler 1984:93). If this explanation is accepted, hominid origins are restricted to a choice of just one of two Old World locations: sub-Saharan Africa or the peninsular and insular portions of South Asia.