How reliable are human phylogenetic hypotheses?

Cladistic analysis of cranial and dental evidence has been widely used to generate phylogenetic hypotheses about humans and their fossil relatives. However, the reliability of these hypotheses has never been subjected to external validation. To rectify this, we applied identical methods to equivalent evidence from two groups of extant higher primates for whom reliable molecular phylogenies are available, the hominoids and papionins. We found that the phylogenetic hypotheses based on the craniodental data were incompatible with the molecular phylogenies for the groups. Given the robustness of the molecular phylogenies, these results indicate that little confidence can be placed in phylogenies generated solely from higher primate craniodental evidence. The corollary of this is that existing phylogenetic hypotheses about human evolution are unlikely to be reliable. Accordingly, new approaches are required to address the problem of hominin phylogeny. T he upsurge in paleoanthropological field research over the past quarter century has resulted in the recognition of many new hominin species, including Australopithecus afarensis (1), Paranthropus aethiopicus (2), Ardipithecus ramidus (3, 4), Australopithecus anamensis (5), Australopithecus bahrelghazali (6), Homo antecessor (7), and Australopithecus garhi (8). This has led to commensurate interest in the generation of reliable hypotheses about human phylogeny To assess the likely reliability of standard hominin cranial and dental characters for interspecific and intergeneric phylogenetic reconstruction, we used hominin cladistic methods to analyze comparable characters from two groups of extant primates: the hominoids, the higher primate group most closely related to the fossil hominins, and the papionins, the Old World monkey tribe comprising the baboons, mangabeys, and macaques. We then judged the resulting cladograms against the groups' consensus molecular phylogenies In the present study, we concentrated on testing the phylogenetic utility of quantitative rather than qualitative craniodental data. Although more hominin cladistic analyses have been based on qualitative characters than on quantitative characters, there is in fact no intrinsic difference between qualitative and quantitative characters as far as the cladistic methodology is concerned (28). Moreover, qualitatively scored characters are necessarily more subjective than measurements, and the few studies that have examined the reproducibility of qualitative assessments of morphology have shown that many of them are impossible to replicate Materials and Methods Standard paleoanthropological measurements of the cranium, mandible, and dentition were used to compile quantitative data sets for the extant hominoids and papionins. The hominoid data set comprised values for 129 measurements recorded on males and females of Gorilla, Homo, Pan, Pongo, and an Old World monkey, Colobus, which was included as an outgroup. Seventyseven of the one hundred and twenty-nine measurements were recorded on 37 Gorilla gorilla