Paleobiogeography

The available evidence indicates that the hominins evolved in northern and eastern Africa before dispersing south into southern Africa (Strait & Wood, 1999). The early proto-australopithecines are all found in East Africa and Chad, ranging from between 4.2 and 2.5 Ma. The southern dispersal of A. africanus occurred between 3.5 and 3.0 Ma. It first turns up in Sterkfontein in Member 2 deposits, about 3.5 Ma, where there is an apparently complete skeleton, still in the process of being carefully excavated. The expansion of the australopithecine range into southern Africa appears to have been accompanied by the emergence and dispersal of Kenyanthropus platyops in eastern Africa around 3.5 Ma. The emergence of Homo habilis first occurred in east Africa by 2.3 Ma, but by 1.8 Ma H. habilis was possibly also present in southern Africa. By 2.4 Ma K. rudolfensis had moved south into central Africa, just north of the Malawi Rift. Following a similar temporal and spatial dispersal with Homo habilis, the species of Paranthropus first appear in east Africa around 2.5 Ma with P walkeri, which then gave rise to the later species P. boisei, which survived in east Africa for almost another million years, and another species of Paranthropus (P robustus) dispersed to southern Africa, becoming extinct around 1.0 Ma.

The dispersal of the hominins from north to south during a number of "migratory" phases strongly correlates with a number of large-scale mammal faunal migrations during the same periods. The evolutionary branching of the hominins—their emergence, proliferation of species, and extinction times—closely resembles the pattern observed in the rise and fall of other large mammal groups as well as of rodents (Vrba, 1985, 1999). In other words, the early evolution and dispersal patterns of the earliest hominins and other large mammals in Africa were the result of a similar response to environmental perturbations (Strait & Wood, 1999; Vrba, 1999). Between the major radiations of early hominin species, between 3.0 and 2.5 Ma, the paleohabitats of the hominins in East Africa were undergoing significant and prolonged periods of climate change. Most proto-australopithecine species appear to have occupied forest and forest edge or gallery forest, though there is also some evidence, especially at Hadar and Koobi Fora, for a more diverse and mosaic paleohabitat over time. However, at the time of the emergence of Kenyanthropus, Paranthropus, and Homo around 2.5 Ma, the climate had changed to a more arid state and palaeohabitats were now largely dry savanna, open savanna woodland, and/or arid, semiarid steppe scrub (Vrba, 1999). A similar trend was also occurring in southern Africa (McKee, 1999). So the emergence of the new genera correlates with increasing aridity within the African continent. Paranthropus appears to have focused more on the hard fruits and nuts (as suggested by its unique robust "nutcracker" masticatory apparatus), thus adapting to a more specific habitat and dietary niche, while later K. rudolfensis and early Homo were more generalized in their dietary requirements, probably more opportunistic, taking advantage of a broad range of food types, including meat in the case of Homo at least. Clearly the early species of Homo were able to outcompete the later species of Kenyanthropus, for it soon disappears from the hominin fossil record. It is only with the later Homo species, H. ergaster, however, that finally the hominins break out of Africa into Eurasia proper.

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