Archive for the 'Brain morphology' Category

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.

Advertisements

Little Foot

The endocast of the australopith StW 573 is pretty complete, and now Amélie Beaudet and colleagues have published a very detailed and comprehensive anatomical analysis of its features. For many paleoneurological traits we still miss a reliable knowledge on intra- and inter-specific variation but, according to what we can currently see in Australopithecus, Paranthropus and chimpanzees, StW 573 does not display derived sulcal patterns in the frontal and parietal regions. Its overall endocranial form resembles the morphology of some Paranthropus specimens, although in this case there are still some issues on deformation and possible taphonomic effects (specially at the frontal bone). The study supplies a careful description of the vascular patterns, in particular for the middle meningeal artery. In humans, only our species has generally a complex vascular network, while vessels are more scarce and less connected in extinct human taxa. Nonetheless, these same vessels (or, at least, their analogous networks) are more developed in apes. Therefore, australopiths are a key group to understand what happened with these traits, and to assess the polarity of these features in the evolution of distinct hominoid branches.

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.

Human brain variation

One year ago Croxson and colleagues published a survey on human and macaques brain variation, a paper which has been issued this month in Cerebral Cortex. They considered variation in white and grey matter, comparing inter and intra-specific patterns, and discussing similarities between the evolutionary and individual degree of variability. This study evidences the importance of variation as a source of evolutionary possibilities and constraints. The survey was based on only 10-20 individuals and, despite any statistical reassurance, we have to recognize that this is an unusual sample size for a study targeted to describe and quantify intra-specific diversity. Furthermore, in these kinds of analyses one has constantly the sensation that phylogenetic differences (macaque-human) are still interpreted as evolutionary differences (ancestral-descendant), which is definitely an inappropriate perspective when dealing with extant species. Also, the fact that we keep on using the term “monkey” when referring to one single species of hundreds of living, independent and diverse ones, denotes a still-alive linear approach to the evolutionary schemes (the old fashion progression monkey -> ape -> human). This paper was then commented by Aida Gómez-Robles, who discussed the pros and cons of this study. Some months later, Reardon and colleagues published a similar analysis, but on a huge sample. In her review, Aida Gómez-Robles pointed to endocasts as a potential source of additional information on intra-specific brain variation. Definitely a good point, and a valiant position to be presented in a mainstream journal on cognition. Endocasts and macroanatomy are issues which are often neglected in neuroscience. Nonetheless, two aspects must be taken into account. First, macroanatomy and morphology still hide many issues which suffer a dramatic lack of information, and that can reveal unexpected suprises. This is also true taking into account traditional neuroanatomy and, for example, in our last survey on human brain variation (on 265 individuals) the precuneus still stands as a major source of gross morphological human diversity. Second, although endocasts can’t provide a comprehensive information on brain biology, they can remarkably help to increase the sample size when dealing with primates and especially hominoids, because of the many collections available as dry or digital skulls. A recent study on the degree of endocranial metric variation in apes, humans, and hominids can be found here.

Newborn folding

Amazing study this one on neonate cortical folding! They analyzed almost 600 newborn brains with an automated method based on surface and curvature and, according to the results, the adult folding scheme is already expressed after birth. This means that most folding mechanisms act before birth. Taking into account that the brain then undergoes a dramatic increase in size, we can probably say that size-related effects may be important but indeed not determinant, for the final sulcal scheme. There could be allometric influences in brain folding but, if a neonate already displays the adult cortical pattern, this means that other early non-allometric factors are involved. Here a post on this study. And, a recent review by Van Essen and colleagues on brain development and evolution, addressing the issue of cortical folding.

Parietal cortex

In November 2017 Ashley Morhardt organized a Karger Workshop at Hyattsville (USA), entitled “From fossils to function: integrative and diverse approaches to vertebrate evolutionary neuroscience“. The workshop was included in the activities of the J. B. Johnston Club, and papers are  now published in Brain Behavior and Evolution. My contribution is a review on the evolution of the parietal cortex in the human genus. Articles will be freely accessible for the next six months. Have a look!

Neanderthal’s visual cortex

A recent study published by Antonio García Tabernero addresses issues about the morphology of the occipital lobes in Neanderthals. It is an anatomical description of a new occipital fragment from El Sidrón, and a morphological analysis of the Neanderthal occipital variation. Neanderthals display more occipital asymmetries than other hominids, including in the vascular pattern of the venous sinuses. They have been also hypothesized to have larger occipital cortex when compared with modern humans. It remains to be evaluated whether differences in the occipital lobe morphology can suggest distinct functional capacity, and here is a stimulating article right on this topic. However, it should be considered the possibility that occipital proportions in Neanderthals are not a derived feature, but a plesiomorph human (Homo) condition, being modern humans the ones who depart from this scheme.


Enter your email address to follow this blog and receive notifications of new posts by email.

RSS Brain News

RSS Cognitive archaeology

RSS The Skull Box

  • Primate brain folding
    A recent work, analyzing the development and evolution of the primate cortical folding, evidences two separate folding processes of the neocortex. Namba and colleagues examine two subtypes of neocortex, the dorsal isocortex, defined as the portion of neocortex limited laterally by the lateral fissure (LF) and medially by the cingulate sulcus (CiS), and the p […]

RSS Anthropology

  • Seven Million Years of Human Evolution
    This fascinating visual presentation from the American Museum of Natural History outlines what we know about human evolution by combining …Continue reading →

RSS Human Evolution

  • An error has occurred; the feed is probably down. Try again later.

RSS Neurophilosophy

  • Researchers develop non-invasive deep brain stimulation method
    Researchers at MIT have developed a new method of electrically stimulating deep brain tissues without opening the skullSince 1997, more than 100,000 Parkinson’s Disease patients have been treated with deep brain stimulation (DBS), a surgical technique that involves the implantation of ultra-thin wire electrodes. The implanted device, sometimes referred to as […]

Disclaimer

This blog publishes texts and comments of the author, which can not be referred to institutions or contexts outside of the blog itself. The published material may be partly derived or reported from the Web, and therefore evaluated in the public domain. If some content violates copyright or if it is considered inappropriate, please contact me, to promptly remove it. On the other hand, please cite this source whenever using images or texts from this website.
Advertisements