Posts Tagged 'hemispheric asymmetries'

Asymmetric brain boxes

A new paper by Melchionna and colleagues provides new insights on the never-ending issue of the evolution of human brain asymmetries. The study supplies a comprehensive shape analysis of endocasts from many hominoid taxa, including fossils. Differences that go beyond the allometric consequences of a larger brain can be detected in Homo sapiens, Homo heidelbergensis, and Homo neanderthalensis. The brain is asymmetric in its anatomy and functions and, after more than a century of debate, we still don’t know if our degree and scheme of asymmetry is due only to a larger cortex, or else if there are some specific evolutionary adaptations. Of course, a unique (all hominoids) trend is not necessarily expected, and each species can have had its own, independent and peculiar story. Indeed, we should not forget that there is a major limitation in paleoneurology: if things are not clear for living primates and living brains, little can be probably done with fossil species and gross endocranial moulds. The anatomical and statistical uncertainties are, at best, unquestionable. In this sense, there is still a major biological issue which is largely unresolved: we ignore the nature of the endocast asymmetries. White matter or grey matter? Are neurons, glia or blood involved? Skull- or brain-induced? It is a physiological or biomechanical issue? Unless we have clear evidence on these aspects, the information on “how much” and “where” a cast is asymmetric may be but a secondary information.

More asymmetries

Gomez-Robles et al 2013Cerebral asymmetries are a thorny issue in both paleoneurology and human neuroanatomy: despite their relevance, their origin and actual variations are elusive. Conceptual and technical problems tend to hamper conclusive statements, as evidenced by the many disagreements and uncertainties in the field. A recent paper on asymmetries in human and chimpanzee brains has added a geometric perspective to the topic. This shape analysis suggests that humans and chimpanzees have the same kind of asymmetries, with humans showing a larger degree of variation and expression of those patterns. Hence, again it seems it is more a matter of grade than of novel characters, at least in terms of geometry. Interestingly, allometry has a very minor role (if any) in intraspecific differences. In both species, shape variation and asymmetries are especially marked in the parietal areas.

Deep asymmetries

Stephanie Bogart and colleagues have published an interesting study on sulci asymmetries in chimps and macaques, on NeuroImage. Quantifying cortical depth and surface area, they found consistent  population-level brain asymmetries in chimpanzees but not in macaques. The paper is a good review on many issues related to brain asymmetries and evolution in primates. Asymmetries that, however, are the results of mechanisms and processes which are still poorly known.

Asymmetries

Brain asymmetries have always been a hot topic in neurosciences. Differences are often so subtle, and there are still many disagreements on this issue. If this topic is difficult to investigate in living species, when analyzing fossil taxa doubts are much more than certainties. In a recent paper Balzeau and colleagues quantify the hemispheric fronto-occipital projection in modern humans, African apes, and fossil hominids, analyzing fluctuating asymmetry, directional asymmetry, and antisymmetry. Differences between groups in distribution and variation are small, and generally related to the degree of asymmetry, not to the presence or combination of characters. It seems that differences between modern humans and apes (and fossil hominids) are a question of grade. Once more, it seems that on this issue the information available from the general endocranial morphology is necessary, but very limited.


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