Australopithecus afarensis endocasts suggest ape-like brain organization and prolonged brain growth

Human brains are three times larger, are organized differently, and mature for a longer period of time than those of our closest living relatives, the chimpanzees. Together, these characteristics are important for human cognition and social behavior, but their evolutionary origins remain unclear. To study brain growth and organization in the hominin species Australopithecus afarensis more than 3 million years ago, we scanned eight fossil crania using conventional and synchrotron computed tomography. We inferred key features of brain organization from endocranial imprints and explored the pattern of brain growth by combining new endocranial volume estimates with narrow age at death estimates for two infants. Contrary to previous claims, sulcal imprints reveal an ape-like brain organization and no features derived toward humans. A comparison of infant to adult endocranial volumes indicates protracted brain growth in A. afarensis, likely critical for the evolution of a long period of childhood learning in hominins

Accessory cusp expression at the enamel-dentine junction of hominin mandibular molars

Studies of hominin dental morphology frequently consider accessory cusps on the lower molars, in particular those on the distal margin of the tooth (C6 or distal accessory cusp) and the lingual margin of the tooth (C7 or lingual accessory cusp). They are often utilized in studies of hominin systematics, where their presence or absence is assessed at the outer enamel surface (OES). However, studies of the enamel-dentine junction (EDJ) suggest these traits may be more variable in development, morphology and position than previously thought. Building on these studies, we outline a scoring procedure for the EDJ expression of these accessory cusps that considers the relationship between these accessory cusps and the surrounding primary cusps. We apply this scoring system to a sample of Plio-Pleistocene hominin mandibular molars of Paranthropus robustus, Paranthropus boisei, Australopithecus afarensis, Australopithecus africanus, Homo sp., Homo habilis and Homo erectus from Africa and Asia (n = 132). We find that there are taxon-specific patterns in accessory cusp expression at the EDJ that are consistent with previous findings at the OES. For example, P. robustus M$_1$s and M$_1$s very often have a distal accessory cusp but no lingual accessory cusp, while H. habilis M$_1$s and M$_1$s show the opposite pattern. The EDJ also reveals a number of complicating factors; some apparent accessory cusps at the enamel surface are represented at the EDJ only by shouldering on the ridges associated with the main cusps, while other accessory cusps appear to have little or no EDJ expression at all. We also discuss the presence of double and triple accessory cusps, including the presence of a double lingual accessory cusp on the distal ridge of the metaconid in the type specimen of H. habilis (OH 7–M$_1$) that is not clear at the OES due to occlusal wear. Overall, our observations, as well as our understanding of the developmental underpinnings of cusp patterning, suggest that we should be cautious in our comparisons of accessory cusps for taxonomic interpretations

Dental morphology in homo habilis and its implications for the evolution of early homo.

The phylogenetic position of Homo habilis is central to debates over the origin and early evolution of the genus Homo. A large portion of the species hypodigm consists of dental remains, but they have only been studied at the often worn enamel surface. We investigate the morphology of the H. habilis enamel-dentine junction (EDJ), which is preserved in cases of moderate tooth wear and known to carry a strong taxonomic signal. Geometric morphometrics is used to characterise dentine crown shape and size across the entire mandibular and maxillary tooth rows, compared with a broad comparative sample (n = 712). We find that EDJ morphology in H. habilis is for the most part remarkably primitive, supporting the hypothesis that the H. habilis hypodigm has more in common with Australopithecus than later Homo. Additionally, the chronologically younger specimen OH 16 displays a suite of derived features; its inclusion in H. habilis leads to excessive levels of variation

Endostructural morphology in hominoid mandibular third premolars: discrete traits at the enamel-dentine junction

For access to specimens, we would like to thank Bernhard Zipfel, Lee Berger, Sifelani Jira (Evolutionary Studies Intitute, University of the Witwatersrand), Miriam Tawane (Ditsong Museum), Job Kibii (National Museums of Kenya), Metasebia Endalemaw, Yared Assefa (Ethiopian Authority for Research and Conservation of Cultural heritage), Yoel Rak, Alon Barash, Israel Hershkovitz (Sackler School of Medicine), Michel Toussaint (ASBL Archéologie Andennaise, Jean-Jacques Cleyet-Merle (Musée National de Préhistoire des Eyzies-de-Tayac), Ullrich Glasmacher (Institut für Geowissenschaften, Universität Heidelberg), Robert Asher, Hendrik Turni, Irene Mann (Museum für Naturkunde, Berlin), Jakov Radovčić (Croatian Natural History Museum), Christophe Boesch and Uta Schwarz (Max Planck Institute for Evolutionary Anthropology) and the Leipzig University Anatomical Collection (ULAC). For project support we thank Zeresenay Alemseged and Bill Kimbel. We would also like to thank the reviewers, the associate editor and the editor for their helpful comments and guidance, as well as Ottmar Kullmer for comments on an earlier version of this manuscript. This work was funded by the Max Planck Society, and financial support for L.K.D. was provided by a Connor Family Faculty Fellowship and the Office of Research and Development at the University of Arkansa

Two interpretations of human evolution: Essentialism and Darwinism

Despite intensive studies of a large number of fossils discovered during the 20th century there is no consensus as to the interpretation of the process of hominin evolution. Some authors see as many as six genera and some 17 species, while others argue for a single lineage from Plio/Pleistocene until today. Such diversity of interpretations of the same facts indicates lack of a uniform theoretical basis underlying studies of human evolution. Debates can be resolved using basic principles of scientific inquiry - parsimony and falsification of null hypotheses. Hypothesis testing is now possible with respect to the evolution of basic hominin characteristics such as brain size, body size and the size of the dentition that have sample sizes of a few hundred individual data points each. These characters display a continuous change with time. Analyses of variance do not falsify the null hypothesis of the existence of only one species at any time - variances around regression lines on time do not differ from the variance observed in the single species of Homo sapiens - distributions of residuals are normal. Thus, splitting of the hominin lineage into coeval species can only be based on descriptive characteristics that are liable to errors of subjective judgment.Maciej Henneber