Posts Tagged 'apes'

Humans, chimps, and brain size

Amélie Beaudet and colleagues have now published a new review on the evolution of the modern human brain. They introduce many traditional issues in paleoneurology, including frontal lobe evolution, asymmetries, lunate sulcus, brain growth, and brain shape. They also provide a detailed discussion of the information we have on the evolution of brain size. Studies on this topic are frequently biased by statistical or taxonomic problems, because of the intrinsic limitations of the fossil record. Actually, any model (gradual, random, punctuated, etc.) can be supported by the few and scattered data, generating disagreements and debates. Isaac Asimov said “Where any answer is possible, all answers are meaningless”. In this paper, they describe their own approach, trying to deal with such limitations. They support a gradualist perspective, although with some discontinuities within some clades. The review strictly deals with brain evolution, but I really appreciate the taxonomic considerations at the beginning of the article, defending their reasons to include humans and apes in one single family. I belong to the opposite faction, namely to the resisting supporters of two distinct families for this group, with the term hominids restricted to humans and (probably) australopiths. Firstly, because I think that taxonomy should not try to trace phylogeny too strictly, constrained and forced by cladistic schemes. The real phylogeny is unknown (we use genes as a proxy, which is but an estimation, with pros and cons), and the phylogenetic hypotheses are frequently changing. Instead, a taxonomy based on the whole biological model (that includes anatomy, physiology and so on) is more stable and, importantly, can add more information on the actual evolutionary, zoological and ecological organization and role of a group of species. Secondly, because I think that differences are the great value of evolution, and taxonomy should acknowledge such differences. In this case, we must admit that our lineage is particularly dissimilar from all the other apes. This does not mean that we are better, but surely much different, and taxonomy should take into account the importance of such outstanding changes. Many anthropologists give all these taxonomical issues for granted, using one label or another just by repeating or copy-pasting what they hear around, generally following a mainstream without a personal or competent opinion. But passively repeating statements is proper of dogmas and mantras, something that should be left out of science.  That’s why I really appreciate that, in this article, Amélie and her coauthors take a clear position, explaining their reasons.

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More on humans and chimps can be found in this other recent review on human paleoneurology. For Spanish readers, here a provocative dissemination article on humans and apes, and another one on the immense value of diversity.

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Shaping cortical evolution

Happy 2019 to everybody! To begin with this new year, here a new review on human paleoneurology, published in Journal of Comparative Neurology. Some conceptual and methodological issues in functional craniology, digital anatomy and computed morphometrics are introduced and discussed. The case-study on parietal evolution is also briefly summarized, with special attention to connectivity. Nonetheless, more specifically, the review points to theoretical and practical limitations of the field. Living species can provide information on the product of evolution, while fossils are necessary to provide information on the process. In the former case (extant species) we can rely on more comprehensive biological analyses, but results concern the final result of the process, not the process itself. In the latter case (extinct species) we can investigate directly the process, but samples are generally not representative neither at biological nor at statistical level. This dual framework is often not properly acknowledged, confounding taxonomy (the product) with phylogeny (the process). When samples and information are analyzed without these cautions in mind, conclusions can generate misleading hybrid perspectives. From the one hand, living species (monkeys and apes in anthropology and evolutionary neuroscience) are still frequenlty misinterpreted as primitive human ancestors. At the same time, scattered and descriptive information on individual and fragmented fossils are generalized to propose broad and inclusive theories. Both aspects are, scientifically speaking, crucial weaknesses, generating instability and unreliability within the field.

Another issue concerns the Homo-centric perspective that still contaminates evolutionary neuroanatomy and evolutionary anthropology. Apart from generating a deformed evolutionary scenario, anthropocentric views demote attention towards the other primates. Apes are generally used to “shed light on human evolution”. But living apes are not ancestral to humans. They could be bad models to understand our evolution, as we humans are probably bad models to understand their own one. They have their own specialized traits, which merit attention. In fact, apes are themselves an exceptional zoological case study. Anthropology is interesting, but apeology is interesting too. In cognitive terms, for example, apes could have capacities that we have never evolved. Finally, it can be also worth nothing that, charmed in searching for “what makes us humans”, we are neglecting “what makes us primates”. Because these latter features are associated with instincts, emotions, and cognitive constraints, they seriously deserve attention. Mostly when recognizing that they often deal with our social aspects, and with their consequences.

Integrated paleoneurology

Zollikofer et al 2016Together with the recent article on modern vs Neandertal endocranial ontogeny, the team coordinated by Christoph Zollikofer has now published also a large and comprehensive study on endocranial ontogeny in humans and apes. The paper focuses on a specific question: to what extent endocranial differences are due to brain differences, and to what extent they are due to cranial constraints? Definitely, this is a key-paper in paleoneurology. They considered the integration between and within the main cranial districts to evaluate the influence on brain shape of two major cranial effects: spatial packing and facial orientation. Their analyses suggest that endocranial differences between humans and apes, as well as differences among apes, are the result of all those factors, the cerebral and the cranial ones. Therefore, the endocranial form is due to a complex admixture of specific brain differences (already present at birth) and cranial constraints. Comparisons among endocranial ontogenetic patterns of living hominoids, among adult fossil specimens, and among different neuroanatomical aspects of living species, can give different results, suggesting that the relationships between anatomical, morphological, and cytological elements is far from being understood. In my opinion, a limit of many shape analyses in general concerns the use of surface semi-landmarks to analyze brain geometry. Surface landmarks are necessary because of the lack of good anatomical references on the endocasts. Unfortunately, they can’t take into account the contribution of distinct cerebral areas, and as a consequence they consider brain morphology as a single homogeneous surface. The identification of boundaries or distinct and independent elements within this surface might seriously influence the multivariate output. I am particularly interested in the analysis of the parietal districts. When using surface landmarks the analysis of the parietal surface may give different (and sometimes contrasting) results. Hence, we may wonder whether the observed parietal variations are the result of brain differences (cortical expansion/reduction) or of geometry (bulging and flexion). Nonetheless, previous morphological studies based on cortical landmarks suggest that modern humans show an actual (absolute and relative) increase not only of the parietal “surface”, but also and specifically of the parietal “lobe”, when compared with extinct hominids or with living chimps. The localization of anatomical boundaries on endocasts may be difficult, although those results have been replicated on different samples. The identification of anatomical landmarks in living species is, in contrast, definitely more reliable. Therefore, whatever the result of a global surface analysis of the whole endocranium, we should not forget that comparisons of specific areas are suggesting a differential contribution of distinct brain components.

Hominoids

Scott et al 2014After shape analysis of the endocranial growth and development in modern humans, chimps, and Neandertals, the team from the Max Planck Institute has published a study on apes endocranial ontogeny. In their former articles they evidenced a shared trajectory of form change in humans and chimps. The only exception is the “globularization stage” in modern humans, an early postnatal stage associated with parietal and cerebellar enlargement. This study now includes also gorillas, orangs, and gibbons, confirming that after eruption of the deciduous dentition all hominoids share a similar pattern of form variation. Differences among species are largely a matter of degree of change, but within a shared set of rules. This implies that most of the observed differences among their endocranial forms take place before, in prenatal stages.


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