Morphological variation of the maxilla in modern humans and African apes

Morphological variation of the Australopithecus afarensis maxilla

Central to discussions about hominin diversity in the mid-Pliocene of eastern Africa is whether or not certain fossils should be attributed to Australopithecus afarensis, instead of representing separate species. Key to answering this question is a good understanding of the magnitude and pattern of intraspecific variation shown by A. afarensis. Given the importance of maxillary characteristics in species diagnoses of early hominins, we explored morphological variation among all nine sufficiently preserved A. afarensis maxillae from Hadar. After CT-based virtual reconstruction, these were analyzed in the comparative context of 448 extant hominine (modern human and African ape) maxillae, representing all currently recognized subspecies, large geographic areas, and both sexes. Maxillary morphology was captured by three-dimensional landmarks, and size and shape were examined using geometric morphometric methods. The main findings are that 1) A. afarensis has high degrees of size and shape variation compared with extant hominines, potentially linked with sexual dimorphism, 2) no allometry was found, despite the large size variation, 3) a temporal trend in maxillary size is suggested but not in shape, and 4) the inferred patterns of sexual dimorphism in form and shape are different from those observed in Homo sapiens, Pan, and Gorilla. These results provide a greater understanding of A. afarensis, enable quantitative comparisons with contemporary maxillae attributed to Kenyanthropus platyops, Australopithecus deyiremeda, and Australopithecus bahrelghazali, and can help evaluate variation in other Plio-Pleistocene hominins, such as those assigned to species of early Homo.

Maxillary morphology of chimpanzees: Captive versus wild environments

Morphological studies typically avoid using osteological samples that derive from captive animals because it is assumed that their morphology is not representative of wild populations. Rearing environments indeed differ between wild and captive individuals. For example, mechanical properties of the diets provided to captive animals can be drastically different from the food present in their natural habitats, which could impact cranial morphology and dental health. Here, we examine morphological differences in the maxillae of wild versus captive chimpanzees (Pan troglodytes) given the prominence of this species in comparative samples used in human evolution research and the key role of the maxilla in such studies. Size and shape were analysed using three-dimensional geometric morphometric methods based on computed tomography scans of 94 wild and 30 captive specimens. Captive individuals have on average larger and more asymmetrical maxillae than wild chimpanzees, and significant differences are present in their maxillary shapes. A large proportion of these shape differences are attributable to static allometry, but wild and captive specimens still differ significantly from each other after allometric size adjustment of the shape data. Levels of shape variation are higher in the captive group, while the degree of size variation is likely similar in our two samples. Results are discussed in the context of ontogenetic growth trajectories, changes in dietary texture, an altered social environment, and generational differences. Additionally, sample simulations show that size and shape differences between chimpanzees and bonobos (Pan paniscus) are exaggerated when part of the wild sample is replaced with captive chimpanzees. Overall, this study confirms that maxillae of captive chimpanzees should not be included in morphological or taxonomic analyses when the objective is to characterise the species.

Making meaning from fragmentary fossils: Early Homo in the Early to early Middle Pleistocene

In celebration of the 50th anniversary of the Journal of Human Evolution, we re-evaluate the fossil record for early Homo (principally Homo erectus, Homo habilis, and Homo rudolfensis) from early diversification and dispersal in the Early Pleistocene to the ultimate demise of H. erectus in the early Middle Pleistocene. The mid-1990s marked an important historical turning point in our understanding of early Homo with the redating of key H. erectus localities, the discovery of small H. erectus in Asia, and the recovery of an even earlier presence of early Homo in Africa. As such, we compare our understanding of early Homo before and after this time and discuss how the order of fossil discovery and a focus on anchor specimens has shaped, and in many ways biased, our interpretations of early Homo species and the fossils allocated to them. Fragmentary specimens may counter conventional wisdom but are often overlooked in broad narratives. We recognize at least three different cranial and two or three pelvic morphotypes of early Homo. Just one postcranial morph aligns with any certainty to a cranial species, highlighting the importance of explicitly identifying how we link specimens together and to species; we offer two ways of visualizing these connections. Chronologically and morphologically H. erectus is a member of early Homo, not a temporally more recent species necessarily evolved from either H. habilis or H. rudolfensis. Nonetheless, an ancestral-descendant notion of their evolution influences expectations around the anatomy of missing elements, especially the foot. Weak support for long-held notions of postcranial modernity in H. erectus raises the possibility of alternative drivers of dispersal. New observations suggest that the dearth of faces in later H. erectus may mask taxonomic diversity in Asia and suggest various later mid-Pleistocene populations could derive from either Asia or Africa. Future advances will rest on the development of nuanced ways to affiliate fossils, greater transparency of implicit assumptions, and attention to detailed life history information for comparative collections; all critical pursuits for future research given the great potential they have to enrich our evolutionary reconstructions for the next fifty years and beyond.