Why the human brain size evolved has been a major evolutionary puzzle since Darwin, but addressing it has been challenging. A key reason is the lack of research tools to infer the causes of a unique event for which experiments are not possible. We suggest that analogous problems have been successfully addressed in other disciplines using what has been recently termed simulation-based inference. Following that approach, we outline a strategy to address why the human brain size evolved: hypotheses are expressed in mechanistic models that yield quantitative predictions for evolutionary and developmental trajectories of brain and body sizes, the predicted trajectories are compared with data, and models are chosen by their ability to explain the data. We discuss a recently published model that makes quantitative predictions for evolutionary and developmental trajectories of brain and body sizes for six hominin species, and compare the model predictions with data, finding that the model recovers many aspects of hominin evolution and development. Counter-intuitively, the human brain size evolves in this model as a spandrel or by-product of selection for something else, namely, fertility-determinant traits. Our analysis indicates that simulation-based inference offers a way forward to infer why the human brain size evolved.This article is part of the Theo Murphy meeting issue 'Selection shapes diverse animal minds'.

Humans are considered to be altricial (strongly underdeveloped at birth) with respect to other primates, but this observation is driven by the strong postnatal enlargement of human brains. We inferred that the developmental stage of human brains at birth does not differ substantially from that of other fossil hominins.

Palaeoneurology is a complex field as the object of study, the brain, does not fossilize. Studies rely therefore on the (brain) endocranial cast (often named endocast), the only available and reliable proxy for brain shape, size and details of surface. However, researchers debate whether or not specific marks found on endocasts correspond reliably to particular sulci and/or gyri of the brain that were imprinted in the braincase. The aim of this study is to measure the accuracy of sulcal identification through an experiment that reproduces the conditions that palaeoneurologists face when working with hominin endocasts. We asked 14 experts to manually identify well-known foldings in a proxy endocast that was obtained from an MRI of an actual in vivo Homo sapiens head. We observe clear differences in the results when comparing the non-corrected labels (the original labels proposed by each expert) with the corrected labels. This result illustrates that trying to reconstruct a sulcus following the very general known shape/position in the literature or from a mean specimen may induce a bias when looking at an endocast and trying to follow the marks observed there. We also observe that the identification of sulci appears to be better in the lower part of the endocast compared to the upper part. The results concerning specific anatomical traits have implications for highly debated topics in palaeoanthropology. Endocranial description of fossil specimens should in the future consider the variation in position and shape of sulci in addition to using models of mean brain shape. Moreover, it is clear from this study that researchers can perceive sulcal imprints with reasonably high accuracy, but their correct identification and labelling remains a challenge, particularly when dealing with extinct species for which we lack direct knowledge of the brain.

American Journal of Biological AnthropologyVolume 180, Issue 2 p. 245-251 LETTER TO THE EDITOR The human remains found in 1967 in Axlor: Still not convincingly Neandertals: A reply to González-Urquijo et al Asier Gómez-Olivencia, Corresponding Author Asier Gómez-Olivencia asier.gomezo@ehu.eus orcid.org/0000-0001-7831-3902 Department of Geología, Facultad de Ciencia y Tecnología, Universidad del País Vasco/Euskal Herriko Unibertsitatea (UPV/EHU). Barrio Sarriena s/n, Bilbao, Spain Sociedad de Ciencias Aranzadi, Donostia-San Sebastian, Spain Centro UCM-ISCIII de Investigación sobre Evolución y Comportamiento Humanos, Madrid, Spain Correspondence Asier Gómez-Olivencia, Department of Geología, Facultad de Ciencia y Tecnología, Universidad del País Vasco/Euskal Herriko Unibertsitatea (UPV/EHU). Barrio Sarriena s/n, 48940 Bilbao, Spain. Email: asier.gomezo@ehu.eus Contribution: Conceptualization (equal), ​Investigation (equal), Writing - original draft (equal), Writing - review & editing (equal)Search for more papers by this authorDiego López-Onaindia, Diego López-Onaindia orcid.org/0000-0002-5266-6416 UMR 5199, PACEA, Université de Bordeaux, Allée Geoffroy Saint-Hilaire, Pessac Cedex, France Contribution: Conceptualization (equal), ​Investigation (equal), Writing - original draft (equal), Writing - review & editing (equal)Search for more papers by this authorNohemi Sala, Nohemi Sala orcid.org/0000-0002-0896-1493 Centro Nacional de Investigación sobre la Evolución Humana (CENIEH), Burgos, Spain Centro UCM-ISCIII de Investigación sobre Evolución y Comportamiento Humanos, Madrid, Spain Contribution: ​Investigation (equal), Writing - review & editing (equal)Search for more papers by this authorAntoine Balzeau, Antoine Balzeau orcid.org/0000-0002-4226-611X Département Homme et Environnement, Équipe de Paléontologie Humaine, UMR 7194, CNRS, Muséum national d'Histoire naturelle, Paris, France Department of African Zoology, Royal Museum for Central Africa, Tervuren, Belgium Contribution: ​Investigation (equal), Writing - review & editing (equal)Search for more papers by this authorAna Pantoja-Pérez, Ana Pantoja-Pérez orcid.org/0000-0001-9302-1756 Centro Nacional de Investigación sobre la Evolución Humana (CENIEH), Burgos, Spain Centro UCM-ISCIII de Investigación sobre Evolución y Comportamiento Humanos, Madrid, Spain Contribution: ​Investigation (equal), Writing - review & editing (equal)Search for more papers by this authorIgnacio Arganda-Carreras, Ignacio Arganda-Carreras orcid.org/0000-0003-0229-5722 Department of Computer Science and Artificial Intelligence, University of the Basque Country (UPV/EHU), Donostia, Gipuzkoa, Spain Ikerbasque, Basque Foundation for Science, Bilbao, Spain Donostia International Physics Center (DIPC), Donostia, Gipuzkoa, Spain Contribution: ​Investigation (equal), Writing - review & editing (equal)Search for more papers by this authorMikel Arlegi, Mikel Arlegi orcid.org/0000-0001-5665-9275 Institut Català de Paleoecologia Humana i Evolució Social (IPHES-CERCA), Tarragona, Spain Departament d'Història i Història de l'Art, Universitat Rovira i Virgili, Tarragona, Spain Contribution: ​Investigation (equal), Writing - review & editing (equal)Search for more papers by this authorJoseba Rios-Garaizar, Joseba Rios-Garaizar orcid.org/0000-0001-8474-2156 Independent researcher, Aretxabaleta, Spain Contribution: Conceptualization (equal), ​Investigation (equal), Writing - original draft (equal), Writing - review & editing (equal)Search for more papers by this authorAida Gómez-Robles, Aida Gómez-Robles orcid.org/0000-0002-8719-2660 Department of Anthropology, University College London, London, UK Department of Genetics, Evolution and Environment, University College London, London, UK Department of Life Sciences, Natural History Museum, London, UK Contribution: Conceptualization (equal), ​Investigation (equal), Writing - original draft (equal), Writing - review & editing (equal)Search for more papers by this author Asier Gómez-Olivencia, Corresponding Author Asier Gómez-Olivencia asier.gomezo@ehu.eus orcid.org/0000-0001-7831-3902 Department of Geología, Facultad de Ciencia y Tecnología, Universidad del País Vasco/Euskal Herriko Unibertsitatea (UPV/EHU). Barrio Sarriena s/n, Bilbao, Spain Sociedad de Ciencias Aranzadi, Donostia-San Sebastian, Spain Centro UCM-ISCIII de Investigación sobre Evolución y Comportamiento Humanos, Madrid, Spain Correspondence Asier Gómez-Olivencia, Department of Geología, Facultad de Ciencia y Tecnología, Universidad del País Vasco/Euskal Herriko Unibertsitatea (UPV/EHU). Barrio Sarriena s/n, 48940 Bilbao, Spain. Email: asier.gomezo@ehu.eus Contribution: Conceptualization (equal), ​Investigation (equal), Writing - original draft (equal), Writing - review & editing (equal)Search for more papers by this authorDiego López-Onaindia, Diego López-Onaindia orcid.org/0000-0002-5266-6416 UMR 5199, PACEA, Université de Bordeaux, Allée Geoffroy Saint-Hilaire, Pessac Cedex, France Contribution: Conceptualization (equal), ​Investigation (equal), Writing - original draft (equal), Writing - review & editing (equal)Search for more papers by this authorNohemi Sala, Nohemi Sala orcid.org/0000-0002-0896-1493 Centro Nacional de Investigación sobre la Evolución Humana (CENIEH), Burgos, Spain Centro UCM-ISCIII de Investigación sobre Evolución y Comportamiento Humanos, Madrid, Spain Contribution: ​Investigation (equal), Writing - review & editing (equal)Search for more papers by this authorAntoine Balzeau, Antoine Balzeau orcid.org/0000-0002-4226-611X Département Homme et Environnement, Équipe de Paléontologie Humaine, UMR 7194, CNRS, Muséum national d'Histoire naturelle, Paris, France Department of African Zoology, Royal Museum for Central Africa, Tervuren, Belgium Contribution: ​Investigation (equal), Writing - review & editing (equal)Search for more papers by this authorAna Pantoja-Pérez, Ana Pantoja-Pérez orcid.org/0000-0001-9302-1756 Centro Nacional de Investigación sobre la Evolución Humana (CENIEH), Burgos, Spain Centro UCM-ISCIII de Investigación sobre Evolución y Comportamiento Humanos, Madrid, Spain Contribution: ​Investigation (equal), Writing - review & editing (equal)Search for more papers by this authorIgnacio Arganda-Carreras, Ignacio Arganda-Carreras orcid.org/0000-0003-0229-5722 Department of Computer Science and Artificial Intelligence, University of the Basque Country (UPV/EHU), Donostia, Gipuzkoa, Spain Ikerbasque, Basque Foundation for Science, Bilbao, Spain Donostia International Physics Center (DIPC), Donostia, Gipuzkoa, Spain Contribution: ​Investigation (equal), Writing - review & editing (equal)Search for more papers by this authorMikel Arlegi, Mikel Arlegi orcid.org/0000-0001-5665-9275 Institut Català de Paleoecologia Humana i Evolució Social (IPHES-CERCA), Tarragona, Spain Departament d'Història i Història de l'Art, Universitat Rovira i Virgili, Tarragona, Spain Contribution: ​Investigation (equal), Writing - review & editing (equal)Search for more papers by this authorJoseba Rios-Garaizar, Joseba Rios-Garaizar orcid.org/0000-0001-8474-2156 Independent researcher, Aretxabaleta, Spain Contribution: Conceptualization (equal), ​Investigation (equal), Writing - original draft (equal), Writing - review & editing (equal)Search for more papers by this authorAida Gómez-Robles, Aida Gómez-Robles orcid.org/0000-0002-8719-2660 Department of Anthropology, University College London, London, UK Department of Genetics, Evolution and Environment, University College London, London, UK Department of Life Sciences, Natural History Museum, London, UK Contribution: Conceptualization (equal), ​Investigation (equal), Writing - original draft (equal), Writing - review & editing (equal)Search for more papers by this author First published: 28 October 2022 https://doi.org/10.1002/ajpa.24633 Funding information: Euskal Herriko Unibertsitatea, Grant/Award Number: GIU19/027; Eusko Jaurlaritza, Grant/Award Number: Research Group IT1418-19; H2020 European Research Council, Grant/Award Numbers: 949330 (DEATHREVOL), MSC Actions Individual Fellowship (Project N°8957; Ministerio de Ciencia e Innovación, Grant/Award Numbers: Proyecto PID2021-122355NB-C31 financiado por MCIN/, RYC2020-029656-I, Ramón y Cajal fellowship (RYC-2017-22558) Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. 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Human newborns are considered altricial compared with other primates because they are relatively underdeveloped at birth. However, in a broader comparative context, other mammals are more altricial than humans. It has been proposed that altricial development evolved secondarily in humans due to obstetrical or metabolic constraints, and in association with increased brain plasticity. To explore this association, we used comparative data from 140 placental mammals to measure how altriciality evolved in humans and other species. We also estimated how changes in brain size and gestation length influenced the timing of neurodevelopment during hominin evolution. Based on our data, humans show the highest evolutionary rate to become more altricial (measured as the proportion of adult brain size at birth) across all placental mammals, but this results primarily from the pronounced postnatal enlargement of brain size rather than neonatal changes. In addition, we show that only a small number of neurodevelopmental events were shifted to the postnatal period during hominin evolution, and that they were primarily related to the myelination of certain brain pathways. These results indicate that the perception of human altriciality is mostly driven by postnatal changes, and they point to a possible association between the timing of myelination and human neuroplasticity.

Objectives We reassess the taxonomic assignment and stratigraphic context of a permanent upper first molar and a permanent lower third premolar recovered from the archeological site of Lezetxiki in the North of the Iberian Peninsula. Materials and Methods We assessed the external and internal morphology of the teeth using qualitative descriptions, crown diameters, dental tissue proportions, and geometric morphometrics. The teeth from Lezetxiki were compared with Middle Pleistocene specimens, Neanderthals, Upper Paleolithic modern humans, and recent modern humans. Results Both teeth were consistent with a Neanderthal classification. The upper first molar shows taurodontism, and its cusp proportions and overall morphology match those of Neanderthals. Geometric morphometric analyses of occlusal anatomy classify this molar as a Neanderthal with a posterior probability of 76%. The lower third premolar, which was originally classified as a lower fourth premolar, also shows a Neanderthal morphology. This premolar is classified as a Neanderthal with a posterior probability of 60%. Discussion These teeth represent the only adult Neanderthal teeth from the Western Pyrenees region found to date. The teeth were found at a stratigraphic level (designated Level III) that marks the transition level from Mousterian to Aurignacian, and are among the most recent Neanderthal remains from the north of the Iberian Peninsula.

Fossil endocasts record features of brains from the past: size, shape, vasculature, and gyrification. These data, alongside experimental and comparative evidence, are needed to resolve questions about brain energetics, cognitive specializations, and developmental plasticity. Through the application of interdisciplinary techniques to the fossil record, paleoneurology has been leading major innovations. Neuroimaging is shedding light on fossil brain organization and behaviors. Inferences about the development and physiology of the brains of extinct species can be experimentally investigated through brain organoids and transgenic models based on ancient DNA. Phylogenetic comparative methods integrate data across species and associate genotypes to phenotypes, and brains to behaviors. Meanwhile, fossil and archeological discoveries continuously contribute new knowledge. Through cooperation, the scientific community can accelerate knowledge acquisition. Sharing digitized museum collections improves the availability of rare fossils and artifacts. Comparative neuroanatomical data are available through online databases, along with tools for their measurement and analysis. In the context of these advances, the paleoneurological record provides ample opportunity for future research. Biomedical and ecological sciences can benefit from paleoneurology’s approach to understanding the mind as well as its novel research pipelines that establish connections between neuroanatomy, genes and behavior.

Mountain gorillas are particularly inbred compared to other gorillas and even the most inbred human populations. As mountain gorilla skeletal material accumulated during the 1970s, researchers noted their pronounced facial asymmetry and hypothesized that it reflects a population-wide chewing side preference. However, asymmetry has also been linked to environmental and genetic stress in experimental models. Here, we examine facial asymmetry in 114 crania from three Gorilla subspecies using 3D geometric morphometrics. We measure fluctuating asymmetry (FA), defined as random deviations from perfect symmetry, and population-specific patterns of directional asymmetry (DA). Mountain gorillas, with a current population size of about 1000 individuals, have the highest degree of facial FA (explaining 17% of total facial shape variation), followed by Grauer gorillas (9%) and western lowland gorillas (6%), despite the latter experiencing the greatest ecological and dietary variability. DA, while significant in all three taxa, explains relatively less shape variation than FA does. Facial asymmetry correlates neither with tooth wear asymmetry nor increases with age in a mountain gorilla subsample, undermining the hypothesis that facial asymmetry is driven by chewing side preference. An examination of temporal trends shows that stress-induced developmental instability has increased over the last 100 years in these endangered apes.

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OBJECTIVES:Molar crenulation is defined as the accessory pattern of grooves that appears on the occlusal surface of many mammalian molars. Although frequently used in the characterization of species, this trait is often assessed qualitatively, which poses unavoidable subjective biases. The objective of this study is to quantitatively test the variability in the expression of molar crenulation in primates and its association with molar size and diet.METHODS:The variability in the expression of molar crenulation in hominids (human, chimpanzee, gorilla, and orangutan) was assessed with fractal analysis using photographs of first, second and third upper and lower molars. After this, representative values for 29 primate species were used to evaluate the correlation between molar complexity, molar size, and diet using a phylogenetic generalized least squares regression.RESULTS:Results show that there are statistically significant differences in fractal dimensions across hominid species in all molars, with orangutan molars presenting higher values of occlusal complexity. Our results indicate that there is no significant association between molar complexity and molar size or diet.DISCUSSION:Our results show higher levels of occlusal complexity in orangutans, thus supporting previously published observations. Our analyses, however, do not indicate a clear association between molar complexity and molar size or diet, pointing to other factors as the major drivers of complexity. To our knowledge, our study is the first one to use fractal analysis to measure occlusal complexity in primates. Our results show that this approach is a rapid and cost-effective way to measure molar complexity.

The Sima de los Huesos (SH) endocranial sample includes 16 complete or partial endocasts corresponding to European Middle Pleistocene hominins. Different anatomical and molecular studies have demonstrated that these hominins are phylogenetically related to Neanderthals, thus making them the earliest unquestionable representatives of the Neanderthal lineage. The description of endocranial variation in this population is fundamental to shedding light on the evolution of the Neanderthal brain. In this contribution, we analyze and describe endocranial variation in this sample, including aspects related to brain size (endocranial volume and encephalization) and brain organization (through qualitative descriptions and quantitative analyses). Our results indicate that the SH hominins show a transitional state between a primitive hominin endocranial configuration (which is found in Homo erectus and non-SH Middle Pleistocene Homo) and the derived configurations found in Neanderthals and modern humans, without a clear anticipation of classic Neanderthal endocranial traits. In comparison with other cranial and postcranial traits that show a fully Neanderthal or clear pre-Neanderthal condition in the SH collection, endocranial variation in these hominins is surprisingly primitive and shows no Neanderthal affinity. These results and the comparison with other cranial traits confirm that Neanderthals evolved in a mosaic fashion. Traits related to mastication (dental, facial and mandibular anatomy) led the Neanderthalization process, whereas neurocranial anatomy must have acquired a fully Neanderthal condition considerably later.

The origin of Neanderthal and modern human lineages is a matter of intense debate. DNA analyses have generally indicated that both lineages diverged during the middle period of the Middle Pleistocene, an inferred time that has strongly influenced interpretations of the hominin fossil record. This divergence time, however, is not compatible with the anatomical and genetic Neanderthal affinities observed in Middle Pleistocene hominins from Sima de los Huesos (Spain), which are dated to 430 thousand years (ka) ago. Drawing on quantitative analyses of dental evolutionary rates and Bayesian analyses of hominin phylogenetic relationships, I show that any divergence time between Neanderthals and modern humans younger than 800 ka ago would have entailed unexpectedly rapid dental evolution in early Neanderthals from Sima de los Huesos. These results support a pre-800 ka last common ancestor for Neanderthals and modern humans unless hitherto unexplained mechanisms sped up dental evolution in early Neanderthals.

Objectives We provide the description and comparative analysis of six new teeth from the site of La Ferrassie. Our goal is to discuss their taxonomic attribution, and to provide an updated inventory of Neandertal and modern human remains from La Ferrassie in their associated archeological context. Materials and methods We use external and internal anatomy, classic morphometrics, and geometric morphometrics. The teeth from La Ferrassie are compared to several samples of contemporary Neandertals and upper Paleolithic modern humans and to recent modern humans. Results Three specimens are classified as Neandertals, two as modern humans, and one remains unclassified. Discussion Based on the previously known fossil samples and the new teeth reported here, there are currently a minimum of four adult and five immature Neandertal individuals coming from the "Grand Abri" and a minimum of two modern human adult individuals: one from "Grand Abri" and one from "Grotte." It is noteworthy that the spatial distribution of the recovered Neandertal remains is not restricted to the area where the LF1-LF 8 were found but now covers the full extension of the excavated area. Moreover, while both Neandertal and modern human occupations have yielded isolated human remains, the partial-to-complete skeletons only belong to Neandertals. These considerations open new perspectives for the understanding of the occupation and use of the La Ferrassie site.

We provide the description and comparative analysis of all the human fossil remains found at Axlor during the excavations carried out by J. M. de Barandiarán from 1967 to 1974: a cranial vault fragment and seven teeth, five of which likely belonged to the same individual, although two are currently lost. Our goal is to describe in detail all these human remains and discuss both their taxonomic attribution and their stratigraphic context.We describe external and internal anatomy, and use classic and geometric morphometrics. The teeth from Axlor are compared to Neandertals, Upper Paleolithic, and recent modern humans.Two teeth (a left dm2 , a left di1 ) and the parietal fragment show morphological features consistent with a Neandertal classification, and were found in an undisturbed Mousterian context. The remaining three teeth (plus the two lost ones), initially classified as Neandertals, show morphological features and a general size that are more compatible with their classification as modern humans.A left parietal fragment (Level VIII) from a single probably adult Neandertal individual was recovered during the old excavations performed by Barandiarán. Additionally, two different Neandertal children lost deciduous teeth during the formations of levels V (left di1 ) and IV (right dm2 ). In addition, a modern human individual is represented by five remains (two currently lost) from a complex stratigraphic setting. Some of the morphological features of these remains suggest that they may represent one of the scarce examples of Upper Paleolithic modern human remains in the northern Iberian Peninsula, which should be confirmed by direct dating.

Studies of brain evolution tend to focus on differences across species rather than on variation within species. A new study measures and compares intraspecific variation in macaque and human brain anatomy to explore the effect that short-term diversity has on long-term evolution.