Franck Guy

During the Neogene, major tectonics events occurred: uplift of mountain ranges (including the Tibetan Plateau and surrounding regions), opening and closing of seaways, and large variations of atmospheric CO2 and sea level. What were the consequences of such changes on the dispersal of hominoid populations? We focus on the Miocene to Pliocene time interval (23 Ma-2.6 Ma). First, we analyze the spread of hominoids from their original geographic range, the tropical forests of Africa during the Early Miocene to the first expansion to Eurasia during the mid-Miocene Climate Optimum. Niche modelling combined with paleoclimatic simulations, provides means to circumvent the fragmentary nature of this record. We identify how the large climatic changes during Miocene Transition impact the potential habitats of hominoids and compare our findings to both the related fossil records and paleoenvironmental proxies.Second, we analyze human distribution during the Mid to Late Pliocene (4-3 Ma), i.e. before the triggering of the large perennial Greenland ice sheet, and of huge amplitude northern hemisphere glacial interglacial cycles, while CO2 evolved between 400 and 300 ppm. In Africa, tropical areas experienced drastic hydrological changes, mainly driven by precession cycles, which deeply modulated monsoon intensity and precipitation patterns, as illustrated by the paleoshore level record of Mega Lake Chad. To explore how orbital parameters strongly modify hydrological cycles over tropical Africa and, the associated dispersion of the genus Australopithecus, we simulated the response of climate, vegetation, and hydrological cycles of the mid to Late Pliocene conditions.For both geological contexts, we provide and analyze a series of Earth System models (IPSL-CM5A2 LR-low resolution-) coupled simulations. Moreover, we associate these fully-coupled simulations with high resolution atmosphere-only model simulations, and a dynamic vegetation model (ORCHIDEE). Both models were used to estimate ecological niches with a spatial resolution of 50 km.We describe the imprint of slow climate changes during the Miocene Climate Transition (MCT) as well as more rapid climate evolution during mid to Late Pliocene, associated with higher frequency oscillation of orbital parameters.This study demonstrates how, for these periods, climate and especially hydrological variations were pivotal to the understanding of hominoid migrations. We compare our findings to fossil records and paleoenvironmental proxy reconstructions.To conclude, we discuss the strengths and limitations of such approaches, which were made possible through a large trans-disciplinary effort between paleontologists, paleoanthropologists, paleoenvironmentalists, paleoclimatologists, and niche modelers.

Discoveries in recent decades indicate that the large papionin monkeys Paradolipopithecus and Procynocephalus are key members of the Late Pliocene – Early Pleistocene mammalian faunas of Eurasia. However, their taxonomical status, phylogenetic relationships, and ecological profile remain unclear. Here we investigate the two latter aspects through the study of the inner ear anatomy, as revealed by applying micro-CT scan imaging techniques on the cranium LGPUT DFN3-150 of Paradolichopithecus from the lower Pleistocene (2.3 Ma) fossil site Dafnero-3 in Northwestern Greece. Using geometric morphometric methods, we quantified shape variation and the allometric and phylogenetic signals in extant cercopithecines (n = 80), and explored the morphological affinities of the fossil specimen with extant taxa. LGPUT DFN3-150 has a large centroid size similar to that of baboons and their relatives. It shares several shape features with Macacina and Cercopithecini, which we interpret as probable retention of a primitive morphology. Overall, its inner ear morphology is more consistent with a stem Papionini more closely related to Papionina than Macacina, or to a basal crown Papionina. Our results, along with morphometrical and ecological features from previous studies, call into question the traditional hypothesis of a Paradolichopithecus-Macacina clade, and provide alternative perspectives in the study of Eurasian primate evolution during the late Neogene-Quaternary.

Bipedal locomotion is one of the key adaptations that define the hominin clade. Evidence of bipedalism is known from postcranial remains of late Miocene hominins as early as 6 million years ago (Ma) in eastern Africa^1,2,3,4. Bipedality of Sahelanthropus tchadensis was hitherto inferred about 7 Ma in central Africa (Chad) based on cranial evidence^5,6,7. Here we present postcranial evidence of the locomotor behaviour of S. tchadensis, with new insights into bipedalism at the early stage of hominin evolutionary history. The original material was discovered at locality TM 266 of the Toros-Ménalla fossiliferous area and consists of one left femur and two, right and left, ulnae. The morphology of the femur is most parsimonious with habitual bipedality, and the ulnae preserve evidence of substantial arboreal behaviour. Taken together, these findings suggest that hominins were already bipeds at around 7 Ma but also suggest that arboreal clambering was probably a significant part of their locomotor repertoire.

Les molaires des mammifères jouent un rôle central lors de la mastication, ou la fragmentation des aliments par l’occlusion dentaire cyclique. Les mammifères fragmentent les aliments avec un degré d’efficacité variable en fonction de leur morphologie dentaire, cela suggère qu’il existe un lien adaptatif à estimer. Dans cette étude, nous testons l’effet de la morphologie des molaires lors de l’intercuspidation en occlusion centrée, c’est-à-dire entre les phases I et II, sur la fragmentation de cinq aliments qui correspondent à différents défis mécaniques rencontrés par les catarrhiniens. En utilisant un simulateur de mastication, nous mesurons expérimentalement le nombre de particules alimentaires produits après cinq intercuspidations successives pour deux forces différentes, afin d’évaluer leur influence. L’échantillon comprend des spécimens de cercopithèques et d’hominidés englobant les principaux modèles d’occlusion dentaire et les habitudes alimentaires des catarrhiniens actuels et fossiles. Nous montrons que l’augmentation de la force de morsure entraîne globalement une meilleure dégradation des aliments, mais des exceptions sont signalées notamment pour les aliments ductiles et coriaces (pommes, gingembres et feuilles). L’intercuspidation fragmente les aliments fragiles et complexes (noisettes et grillons). Cette expérience a permis de montrer que seules une occlusion et une force de morsure appropriées sont suffisantes pour fragmenter les aliments fragiles et que des mouvements supplémentaires sont nécessaires pour fragmenter les aliments coriaces (feuilles et herbes). Elle met également en évidence les capacités masticatoires de différents modèles d’occlusion chez les primates et notre capacité à les tester avec la robotique. Cette nouvelle avancée permettra d’étudier des questions anthropologiques majeures portant sur l’évolution de la morphologie dentaire.

Terrestrial bipedal locomotion is one of the key adaptations defining the hominin clade. Evidences of undisputed bipedalism are known from postcranial remains of late Miocene hominins as soon as 6 Ma in eastern Africa. Bipedality of Sahelanthropus tchadensis was hitherto documented at 7 Ma in central Africa (Chad) by cranial evidence. Here, we present the first postcranial evidence of the locomotor behavior of the Chadian hominin with new insights on bipedalism at the early stage of our evolutionary history. The original material was discovered at locality TM 266 (Toros-Menalla fossiliferous area), and consists in one left femur and two antimeric ulnae. These new findings confirm that hominins were already terrestrial biped relatively soon after the human-chimpanzee divergence but also suggest that careful climbing arboreal behaviors was still a significant part of their locomotor repertoire.

The form of two hard tissues of the mammalian tooth, dentine and enamel, is the result of a combination of the phylogenetic inheritance of dental traits and the adaptive selection of these traits during evolution. Recent decades have been significant in unveiling developmental processes controlling tooth morphogenesis, dental variation and the origination of dental novelties. The enamel-dentine junction constitutes a precursor for the morphology of the outer enamel surface through growth of the enamel cap which may go along with the addition of original features. The relative contribution of these two tooth components to morphological variation and their respective response to natural selection is a major issue in paleoanthropology. This study will determine how much enamel morphology relies on the form of the enamel-dentine junction. The outer occlusal enamel surface and the enamel-dentine junction surface of 76 primate second upper molars are represented by polygonal meshes and investigated using tridimensional topometrical analysis. Quantitative criteria (elevation, inclination, orientation, curvature and occlusal patch count) are introduced to show that the enamel-dentine junction significantly constrains the topographical properties of the outer enamel surface. Our results show a significant correlation for elevation, orientation, inclination, curvature and occlusal complexity between the outer enamel surface and the enamel dentine junction for all studied primate taxa with the exception of four modern humans for curvature (p<0.05). Moreover, we show that, for all selected topometrical parameters apart from occlusal patch count, the recorded correlations significantly decrease along with enamel thickening in our sample. While preserving tooth integrity by providing resistance to wear and fractures, the variation of enamel thickness may modify the curvature present at the occlusal enamel surface in relation to enamel-dentine junction, potentially modifying dental functionalities such as blunt versus sharp dental tools. In terms of natural selection, there is a balance between increasing tooth resistance and maintaining efficient dental tools. In this sense the enamel cap acts as a functional buffer for the molar occlusal pattern. In primates, results suggest a primary emergence of dental novelties on the enamel-dentine junction and a secondary transposition of these novelties with no or minor modifications of dental functionalities by the enamel cap. Whereas enamel crenations have been reported by previous studies, our analysis do not support the presence of enamel tubercles without dentine relief nuclei. As is, the enamel cap is, at most, a secondary source of morphological novelty.