Archive for the 'Paleoanthropology' Category

The Pit

Eva Poza Rey and colleagues have now published a detailed paleoneurological survey of the endocasts from Sima de los Huesos, now dated at 430 ky. The study includes anatomical descriptions and a multivariate analysis of endocranial diameters. Fifteen years have passed since that early article on Sima 4 and Sima 5, showing that these two endocasts had an archaic phenotype, apparently missing any Neanderthal derived trait. This new article definitely increases the sample size, including a total of 16 endocasts. Results suggest that the endocasts from Sima de los Huesos display an intermediate morphology, between the human plesiomorphic brain form (like in Homo erectus or Homo heidelbergensis) and Neanderthals. The difference from an archaic condition would be in the posterior an inferior brain regions (posterior temporal, inferior parietal, and anterior occipital cortex), larger and wider in the Sima brains. Their endocranial size further supports the hypothesis that in Neanderthals encephalization was a gradual process. I think in the article there is probably too much space dedicated to asymmetries. Taking into considerations that all human species display a similar pattern of gross asymmetries, that differences between humans and apes could be a matter of allometry, that hemispheric differences can be very subtle and hence would require huge samples to be properly tested, and that in fossils we can only observe the superficial cortical dimensions with no information on the anatomical factors involved, I frankly can’t understand why this topic keeps on deserving so much attention in many paleoneurological papers.

The study is comprehensive and convincing, although I personally miss two points. First, there is no mention on the overall morphology of the frontal lobes, except some minor comments on the orbital gyri and frontal length in Neanderthals. Although there is no clear evidence of frontal expansion in the evolution of the human genus, Neanderthals and modern humans display relatively wider frontal cortex, probably because of a spatial constraint with the underlying orbits. In this aspect, the Sima endocasts show an archaic morphology, with narrow frontal lobes. Second, I would be really interested in a comparison with the endocast of Maba (China), which combination of traits is remarkably similar to Sima de los Huesos, showing Neanderthals features in the face but an archaic brain form. Convergence, same taxon, or shared ancestors?

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Ileret

I had missed this amazing study (2018) from Neubauer and colleagues on the skull and endocast KNM-ER 42700 from Ileret, Kenya, with a chronology of 1.5 million years. They performed a set of reconstructions for the skull and endocast, and compared these figures within the diversity of early hominids. Their results are quite convincing, and confirm that the specimen is definitely out of the range of variation of adult Homo erectus. Overall, the endocast is somehow similar to the endocast of KNM-ER1470 (H. rudolfensis). However, its morphology also lies midway along an ontogenetic trajectory going from Mojokerto to adult H. erectus. So, the taxonomy of KNM-ER 42700 can be uncertain, but the most likely explanation is a H. ergaster/erectus of a young age, much younger that predicted on the basis of other cranial features. Taking into consideration the approach based on multiple reconstructions, the multivariate shape toolkit, and the disclosure of the ontogenetic trajectory of H. erectus, this paper is definitely a great paragon in paleoneurology. A review on metric endocranial variation in H. erectus was published some years ago here.

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.

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.


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