The craniodental morphology of hybridising macaques, and implications for detection of hybrids in the human fossil record

Where the possibility of hybridisation in the human lineage was once largely speculative and ultimately untestable, recent advancements in genome sequencing techniques have brought it to the forefront of palaeoanthropological research. A genetic mode of investigation has some clear advantages over a morphologically based one, and undoubtedly has a great deal left to contribute to our understanding of human evolution. Nevertheless, insurmountable limitations on the preservation of DNA mean that anatomically based research remains perpetually indispensable. The ability to identify hybrids based upon skeletal morphology has enormous potential for palaeontological research, but has proven to be notoriously difficult. In the human fossil record particularly, possible hybrids have typically been identified based upon unusual mosaics of modern and archaic features. Though suggestive, these claims remain contentious; ultimately, we have very little idea what a hybrid in the human lineage actually ought to look like. This research applies 3D geometric morphometric methods and analysis of select non-metric traits to investigate the craniodental manifestation of hybridisation in Asian macaques. This research complements, critically examines, and elaborates upon a slowly growing body of literature addressing various aspects of primate hybrid morphology. In particular this research addresses the existing theory that an increased frequency of certain developmental abnormalities in the primate skull may be indicative of hybrid origin, and that this may be useful for the identification of hybrids in the human fossil record. Data have been gathered from the skeletal remains of 453 adult macaques (genus Macaca), spanning 12 species and originating from 10 countries across Asia. The particular focus is on the Japanese and Taiwanese macaques (M. fuscata and M. cyclopis), and a considerable sample of hybrids between them. Craniofacial shape results are largely consistent with prior research, and raised frequencies of certain developmental abnormalities are identified in the hybrid group. However, other results directly contradict prior findings and cast doubt on existing assumptions about hybrids morphology. Prospects for the identification of hybrids in the human fossil record are reduced, though much remains to be discovered