More asymmetries

Cerebral 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.

Advertisements

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.