Early hominid biogeography ( phylogeny y human evolution )

We examined the biogeographic patterns implied by early hominid phylogenies and compared them to the known dispersal patterns of Plio-Pleistocene African mammals. All recent published phylogenies require between four and seven hominid dispersal events between southern Africa, eastern Africa, and the Malawi Rift, a greater number of dispersals than has previously been supposed. Most hominid species dispersed at the same time and in the same direction as other African mammals. However, depending on the ages of critical hominid specimens, many phylogenies identify at least one hominid species that dispersed in the direction opposite that of contemporaneous mammals. This suggests that those hominids may have possessed adaptations that allowed them to depart from continental patterns of mammalian dispersal. A critical biogeographic question in early human evolution concerns the relationships between the eastern and southern African hominids. Given that the mammalian fossil record preserves evidence of faunal interchange between these two regions (1, 2), it is very unlikely that hominids in one region evolved entirely independently from those in the other. Yet, despite the strong probability that early hominids dispersed between eastern and southern Africa, few studies (1–4) have examined the paleobiogeography of early hominid species. These studies agree that either two, or three, dispersal events must have taken place and that, in general, the early hominids followed the continental dispersal patterns of other largebodied mammals (Fig. 1a). Some of these studies are predicated by the hypothesis that hominid speciation and dispersal events are driven by environmental change, and in particular by the hypothesis that mammals shift their ranges to match changes in the distributions of vegetational zones (2, 5, 6). Prior studies have addressed the impact of ecology on hominid biogeography, but the biogeographic implications of phylogeny have yet to be fully explored. Obtaining a reliable phylogeny is a crucial, initial step in the formulation of any biogeographic hypothesis because, if a species in one region is descended from an ancestor in another region, then a vicariance (splitting of ranges) or dispersal event must have taken place (7, 8). The present study examines the biogeographic implications of various early hominid phylogenies (Fig. 2) and compares the inferred dispersal patterns of the early hominids to those of other large mammals. Note that dispersal is used here in a generic sense to refer to the movement of taxa across the landscape and thus comprises range expansion, range shift, and dispersal across a faunal barrier (7). The present study does not test whether hominid species arose as a result of vicariance (8). Although vicariance may have played a role in early hominid evolution, only one early hominid species (Homo habilis sensu stricto, hereafter referred to as H. habilis) is known from both eastern and southern Africa. Thus, because vicariance implies that an ancestral species will have a larger range than its descendants, it is likely that most hominid vicariance events were preceded by range expansion (a type of dispersal). Biogeographic patterns were obtained by plotting information about the geographic distribution of species onto early hominid cladograms and phyletic trees. This procedure assumes that the chronological and geographic ranges of hominid species are closely approximated by the times (6, 9–19) and places at which their fossils are found. The validity of this assumption undoubtedly varies on a locality-by-locality basis, and the discovery of hominid fossils in Chad (20) has reminded researchers that the absence of evidence is not evidence of absence, but it is necessary to make the assumption to generate working biogeographic hypotheses. Obviously, the possibility that the fossil record has been affected by taphonomic bias, such that fossil preservation and the stratigraphic record are better in eastern than in southern Africa, means that biogeographic conclusions must be drawn cautiously. Nonetheless, the hominid fossil record is not so poor as to make biogeographical inferences untenable. Obviously, new fossil discoveries have the potential to overturn certain of the results of the present study (a truism of paleontological research). However, unless a new hominid species is discovered that largely invalidates the basic patterns that are common to current phylogenetic hypotheses, new hominid fossils will not substantially alter the biogeographic patterns reported here unless hominid species endemic to eastern Africa are found in southern Africa (and vice versa), or if any of six hominid species (Paranthropus robustus, Paranthropus boisei, Paranthropus aethiopicus, H. habilis, Homo rudolfensis, Homo ergaster) are found to be older than 2.8 million years (Myr). These conditions provide reasonable ‘‘confidence limits’’ for the present study because the geographic distributions of Plio-Pleistocene hominids have been established over the course of decades of research and because, of the six species listed above, only one (P. aethiopicus) is currently known to be older than 2.4 Myr (6, 10, 12–19). Regardless, even if new fossil discoveries necessitate a dramatic revision of early hominid phylogeny, the methods described here will allow researchers to determine the biogeographic patterns implied by any new tree. MATERIALS AND METHODS Patterns implied by cladograms and phyletic trees (Fig. 2) were documented by treating geography as an ordered cladistic character in which regions correspond to character states. Two main regions were recognized: eastern and southern Africa. A third region, ‘‘the Corridor,’’ represented by the Malawi section of the Rift Valley, also was defined because at least one hominid specimen has been reported from there (16) and because the Corridor is likely to have served as a faunal conduit between the other two regions (2). Although mammal The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked ‘‘advertisement’’ in accordance with 18 U.S.C. §1734 solely to indicate this fact. PNAS is available online at www.pnas.org. Abbreviation: Myr, million years. *To whom reprint requests should be addressed. E-mail: dstrait@ gwu.edu.