Molar proportions, endocranial volume, and insular nanism in fossil Homo

Background Recent fossil discoveries have emphasised the diversification of brain and body size in Plio-Pleistocene fossil hominids.1 Homo floresiensis has been described as having brain and body size smaller than any living humans, and the Homo luzonensis dentition suggests that this taxon was also diminutive.Aim To explore the hypothesis that third molar reduction in genus Homo may be linked to changes in brain size.Subjects and methods We assessed the phenotypic relationship between relative third molar length, endocranial volume, body mass, and maxillary and mandibular postcanine dental metrics for n = 15 hominid taxa.Results Maxillary and mandibular relative third molar lengths are significantly correlated with endocranial volume and body mass in Plio-Pleistocene hominids, but Homo floresiensis deviates from this scaling relationship. The molar lengths and proportions of Homo luzonensis are comparable to H. floresiensis, suggesting that these taxa were similar in size.Conclusion Geologically older hominid taxa tend to have relatively longer third molars and smaller endocranial volumes. We discuss the effects of growth rates on the dentition and explore the hypothesis that both H. floresiensis and H. luzonensis represent short-statured populations that evolved on islands via insular nanism and postnatal growth restriction.