Archive for the 'Brain evolution' Category

Evolution of Nervous Systems

evolution-of-the-nervous-systems-2ed

Second Edition of this outstanding reference in neuroscience and evolution edited by Jon H. Kaas, with four volumes dedicated to vertebrates, mammals, primates, and humans. Here a presentation of the contents, and a chapter on paleoneurology.

Brains and teeth

gomez-robles-et-al-pnas2017In anthropology it is commonly accepted that the evolution of larger brains was associated with the reduction of posterior teeth. Factors ranging from diet to cognitive ability have been used to explain this inverse correlation between cerebral complexity and masticatory structures. Aida Gómez-Robles and colleagues have analyzed brain and teeth changes using a multiple-variance Brownian motion approach, providing evidence against a brain-teeth phylogenetic association. Brain shape was analyzed by using eight linear variables as measured on endocasts. Teeth shape was analyzed through geometric morphometrics. The study found that endocranial proportions and dental geometry are largely characterized by similar rates of variation, which are indicative of a neutral and non-directional pattern of evolution. Brain size and tooth size show different rates of change throughout the phylogenetic tree, and the hypothesis of a reciprocal and inverse correlation is not supported. This seems to suggest independent factors at environmental and/or genetic level. Two characters show faster rates of change in specific lineages, and are probably associated with specific selective and adaptive processes: brain size in early Homo and brain globularity in Homo sapiens. The first result suggests that brain evolution in the genus Homo is strongly based on size increase rather than on changes of specific cortical proportions. However, caution is needed in this sense: the study is based on simple linear metrics such as arcs and chords, and reflects only the external appearance of endocranial anatomy. Despite these limitations, this result is consistent with other kinds of evidence. The second result reflects an exception to this size-only pattern of change: the globular brain shape in modern humans. Parietal lobe variations are again an issue.

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.

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.

What makes us humans

Hands (Leonardo)The Journal of Anthropological Sciences is now publishing the papers from the meeting “What Made Us Humans“, that took place in Erice on October 2014. The volume is edited by Telmo Pievani, Stefano Parmigiani and Ian Tattersall, and it includes contributions by Thomas Plummer, Dean Falk, Philip Lieberman, Jeffrey Schwartz, William Harcourt Smith, and many others. There is a section on brain and cognition, in which we publish a review on visuospatial functions and fossils. In this paper we discuss topics in extended cognition and embodiment, presenting the available sources of information from fossil anatomy: brain morphology, manipulative behaviors, and hand evolution. Modern humans displayed changes in all these traits, suggesting that differences in visuospatial integration processes may have been associated with changes of the embodying capacity, leading to derived and probably specialized relationships between brain, body, and environment. This article is a further reference on visuospatial integration and cognitive archaeology. All papers from this JASs volume are, as usually, free to download.

Parsimony

Mounier et al 2016Paleoneurology is rarely used to test taxonomic or phylogenetic hypotheses, at least for three reasons. First, the biology and variation of many endocranial traits are not even known for living humans. Second, plesiomorph traits, parallelisms, large intra-specific variability, and subtle inter-specific differences, make this issue very difficult to test through robust quantitative approaches. Third, the paucity and fragmentation of the fossil record often hamper meaningful statistical inferences. Of course these same problems concern many other anatomical districts, and that’s why probably morphology is not always recommended as a good and reliable source of taxonomic and phylogenetic information. Despite these limits, Aurelién Mounier and his coauthors have now tried to apply cladistics to paleoneurology, taking into consideration neurocranial and endocranial traits. According to their results, at least modern humans and Neandertals can be properly characterized in terms of braincase morphology, suggesting the existence of an actual phylogenetic signal behind the patterns of endocranial variation.

Precuneus and chimps

Bruner et al 2016 - Brain Structure and FunctionWe modern humans have larger parietal bones and large parietal lobes when compared with extinct human species and living apes. We also have an ontogenetic parietal bulging stage, a stage which is absent in apes and Neandertals. Interestingly, a main factor of variability in our brain morphology is the size of the precuneus, in terms of proportions and cortical surface area. Now we have compared human and chimp brains, and here it goes again: the main difference is a much larger precuneus in our species. It doesn’t look like an allometric issue, being possibly associated with that bulging stage specific of modern humans, and even absent in large-brained Neandertals. The precuneus is essential in visuospatial integration, coordinating brain, body, and environment, and bridging the somatosensorial experience with simulation and self-awareness. It is a key element to integrate space, time, and  social perception. It is also worth noting that parietal lobes are particularly vascularized in our species, and the precuneus is a high-metabolic and heat-accumulating element. This may be interesting when considering that it suffers early metabolic impairments in Alzheimer’s disease, a pathology particularly associated with our species. The precuneus is also a central hub of the default mode network. Interestingly, at least in adult modern humans the size of the parietal lobes is inversely correlated with the size of the frontal and temporal lobes, introducing some phylogenetic issues on the evolution of the fronto-parietal system.

For a long time we have been looking for subtle differences between human and ape brain. This one looks not that subtle. Any functional or histological change behind this expansion, at inter-specific and intra-specific level, is still to be investigated. But most of all, it remains to be established the nature of such morphological variations. Genetic factors and selective processes cannot be excluded, but these areas are also particularly sensitive to environmental influences, including training and cultural effects.


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