Posts Tagged 'Neandertals'

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.

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Globularity genes

Today, Philipp Gunz and colleagues have published a real milestone for paleoneurology: a comprehensive analysis integrating brain anatomy and paleogenetics to identify the genes involved in brain form differences between modern humans and Neanderthals. They compute an individual globularization index for a very large modern human sample size, and then look for the effect of supposedly introgressed Neanderthal genes. They found correlations between our individual brain globularity and genes involved in neurogenesis and myelination, most of all in putamen and cerebellum. Interestingly, they don’t find morphometric signals for parietal changes, even if there is evidence of actual parietal cortical differences among humans, between modern and extinct humans, and most of all between humans and apes. Furthermore, putamen and cerebellum are seriously involved in motor circuitry (including tool use?), something which is crucially coordinated by the parietal cortex, at physical (body) and virtual (visual imaging) level. As usually, caution is required when such complex methods are employed (in this case, the many assumptions in shape analysis, the many assumptions in brain imaging, and the many assumptions in paleogenetics). These results should be probably intended more to support hypotheses than to supply conclusive answers. Although these results point to individual brain shape differences among modern humans associated with neurogenesis and myelination, the study does not provide specific comments about possible functional or cognitive aspects, naming only some very general behavioral issues. Some relevant cogntive effects are, indeed, expected. The issue is definitely thorny (Neanderthal introgressed genes into our own species associated with consequences in individual brain form and development!), but should have probably deserved a more courageous interpretation. After all, also in science one must take into account that old and wise adage: if you don’t like the answer, don’t ask the question. In the supplementary information there is an amazing comparison (S1) between CT endocasts and MRI brains. This supplementary analysis is, in my opinion, a real jewel for this field, and I really hope that more future papers will be dedicated to what is here a single figure. Here an article from the New York Times.

Neanderthal brains

After their chapter on the book Digital Endocasts, Kochiyama and colleagues have published this week a comprehensive reconstruction of a Neanderthal brain. An outstanding example of quantitative paleoneurology, indeed! They deformed our modern human brain into a Neanderthal endocranial cavity, as to allow an estimation of cortical volumes and proportions. They confirm that modern humans have larger parietal lobes and larger cerebellum, and that Neanderthals could have had larger occipital lobes. They also confirm that early modern humans did not display a modern human brain form. Of course, this simulation is based on the assumption that no specific and localized cortical changes have occurred along both modern and Neanderthal lineages since their separation. The assumption is a reasonable simplification, and is necessary to provide a shared comparative framework. Nonetheless, if specific and localized changes have occurred in one or both lineages, that one-to-one spatial fitting will lost local predictive power. In terms of brain anatomy, local cortical changes can actually occur as genetic adaptations to selective processes or else as induced plastic feedbacks in response to environment (including culture). Also, we must always consider that many brain regions (the cerebellum is one) have a gross morphology that is in part influenced by cranial constraints. It may be hence difficult, in some specific endocranial districts, to distinguish between brain cortical variations and cranial effects.

Ontogenetic dilemma

Ponce de Leon et al 2016

Marcia Ponce de León and colleagues have published a comprehensive shape analysis on modern human and Neandertal early ontogenetic endocranial changes, as Philipp Gunz and his team did back in 2010. Interestingly, results are different. The previous study from the Max Planck Institute concluded that only modern humans have a species-specific postnatal stage in which the braincase bulges (globularization stage). In contrast, this new analysis, coordinated by Christoph Zollikofer, suggests that after birth Neandertals and modern humans share a similar pattern of endocranial shape change. In this case, any endocranial difference between these two species must occur before birth. The discrepancy between the two studies may be due to differences in the samples (which, recognizing the good samples used in these analyses, would reveal a problematic instability of most paleoanthropological studies) or to differences in the reconstructions of the specimens (which, recognizing the good experience of both teams, would reveal a problematic instability of most paleoanthropological studies). Nonetheless, we must also take into account that both articles rely on very complex statistical and algebraic passages, and methodological biases should not be ruled out. After all, also paleontology deals with the same limits of any science: we do not work with skulls or brains, but with models made of variables and parameters. Models that work well in some cases, and do a worse job in some others, depending on the questions involved. In this new study, the fact that endocranial shape differences between Neandertals and modern humans are prenatal is used to state that there are no cognitive differences between the two species. Of course, cognition is more than shape, so the relationship between the timing of these changes (before or after birth) and the statement on cognition is not particularly straight. Inferences on cognition should be made on multiple evidence, dealing with something that goes well beyond a surface analysis.

Diploic veins

Rangel de Lazaro et al 2015Diploic channels run within the vault bones, and are therefore protected from external agents. This condition makes them an interesting topic in paleontology, archaeology, and forensics. At the same time, such secluded position has hampered detailed studies on their morphology, variations, and functions. In 1999 Hershkovitz and colleagues published a first pioneering survey on these “elusive” anatomical elements. This week we publish a segmentation procedure to visualize these channels after computed tomography, applying this method to modern humans and Neandertals. The diploic network displays a marked individual variability. It is frequently connected with the meningeal system at the pteric area, and with the emissary and venous systems at the occipital area. As for the meningeal arteries, also the diploic vasculature is apparently more complex in modern humans than in other hominids, mostly at the parietal area. Taking into account the large size of the parietal lobes and bones of our species, it is likely that such vascular development can be associated with metabolic and thermal functions. Beyond the large diploic channels, this vascular system counts with a widespread network of microvessels, which should be carefully investigated in the future.

Amud

Amano et al 2015 - AmudA new reconstruction of the Neandertal skull and endocast of Amud has been published by a team coordinated by Naomichi Ogihara, at the Keio University (Yokohama). They applied mathematical models to align the surfaces of the original fossil fragments (surface extrapolation). Thin-plate spline was then used to integrate the available anatomy from Amud with other Neandertals, namely La Chapelle-aux-Saints 1 and Forbes’ Quarry 1 (shape interpolation). Modern skulls were used to smooth all together. This new reconstruction shows a skull that is shorter and wider than the former one. The basicranial areas, largely missing in this specimen, were the most difficult parts to interpolate, because of their complex morphology influenced by different independent factors. Amud is dated to 50-70 ka, with a massive cranial capacity: around 1740 cc.

Visuospatial evolution

Descartes - Meditations metaphysiques 1641Visuospatial integration is essential in handling, tooling, simulation, and many specific tasks which are supposed to be crucial for human evolution. However, it may be even more important for theories on extended cognition, taking into account the relevance in coordinating the relationships among brain, body, and environment. This is something directly associated with concepts like embodiment, material engagement, and brain-artefact interface. And this is pretty intriguing when considering that the upper and medial parietal areas, which are major functional nodes of visuospatial integration, show a remarkable enlargement only in Homo sapiens. Together with Atsushi Iriki (Riken Brain Institute), we have now published a review trying to interlace all these issues: Extending mind, visuospatial integration, and the evolution of the parietal lobes in the human genus. We have tried to integrate topics in neurobiology, paleoneurology, cognitive archaeology, and comparative primatology, to understand why and how visuospatial integration may have been important, in our genus and in our species, for enhancing material engagement and embodying capacities. This article will be part of an issue of Quaternary International dedicated to the importance of “Material dimensions of cognition”. At the same time, the Journal of Anthropological Sciences is now publishing a second forum on the “three hands” of the Neandertals. The hypothesis of a mismatch between visuospatial functions and cultural complexity in this human species is further discussed with comments by Leee Overmann, Enza Spinapolice, Joseba Rios Garaizar, Ariane Burke, Carlos Lorenzo, and Duilio Garofoli. All the papers of the forum are free to download.


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