Archive for the 'Paleoanthropology' 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.

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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.

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.

Naledi

Ralph Holloway and colleagues have just published a paleoneurological study of Homo naledi. They used seven cranial portions from at least five individuals to provide a general view of an endocast of this species. The study is comprehensive and very detailed, indeed. It turns out that, despite the very small endocranial volume (about 500 cc), the brain general organization is very similar to all the other human species. Beyond some particular features in Neanderthals and modern humans, all human (Homo) species display the same general sulcal pattern. If there were differences in their sulcal organization, these should have been pretty minor or hardly recognizable on an endocast, at least according to what we can test with the small samples generally available in paleoanthropology. So, it is not surprising that Homo naledi has a Homo brain form. But the interesting thing is the association between a human brain morphology and a small brain size, as suggested by this current study. If true, we have two main conclusions. First, our brain cortical complexity and our large brain size are two independent features. They have evolved together in many cases, but not in others. Second, our human cortical folding scheme is not simply an allometric (scaled) version of the apes’ one. Cortical folding is largely influenced by mechanical factors, most of all size-related effects, so one could think that our brain morphology, although distinct from apes, is a secondary consequence of having a big brain. The results presented in this study suggest that this is not the case. We humans have a specific cortical organization and, furthermore and additionally, a big brain too. Reasonably, both features have an influence on our cognitive capacities.

Of course, these results must be confirmed on a larger perspective. Remember that here we don’t have a “brain”, but some scattered endocranial surfaces of a few specimens. That’s not sufficient to reach detailed and reliable conclusions on the brain itself, not to say on cognition. Also, the species Homo naledi (and its chronology) is at present strictly associated with one specific site and needs further corroboration from a wider geographical scenario before supporting firm or generalized statements. Its striking feature is the very small brain size. In this sense, it is worth noting that we often use to mention “average” values, sometimes forgetting about their associated variation and variability. We modern humans have a normal cranial capacity spanning a range of more than 1000 cc. In this paper, Holloway mentions the case of Homo erectus, spanning from 550 cc to 1200 cc. Therefore, caution is still necessary when interpreting the small brain size of these individuals. Of course, the fact that this species (as the Flores hominid) could have undergone brain size reduction or small brain retention does not point against the importance of brain size and encephalization. According to the available fossil record, most human species bet on big brains. Exceptions are expected, but do not break the rule.

I want to focus on one more aspect of this article. Although the topic was definitely “sexy”, the authors avoided any speculation on cognition or phylogeny. Such attitude is so professional and definitely welcome, thank you!

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.


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