Networks

Network analysis is nowadays employed in so many fields, ranging from economics to engineering, to investigate any kind of system in terms of its elements and their relationships. Sociology is probably the discipline that has mainly exploited and improved this approach, and all the complex methods and statistics behind it. In neuroscience, it is mainly used to investigate neuronal connections. The perspective supplied by network analysis is incredibly powerful because it allows localizing roles and constraints within very complicated systems. Some elements can be crucial as hubs of local influences, some others can be important as bridges between distant components. Some elements can be very sensitive to local changes, some others can be more independent from neighboring variations. Systems can be complicated because of the many elements and relationships (hundreds or thousands or millions of nodes) or because of the nature of their relationships (many variables influencing the relationships). Or both. In any case, network analysis is an amazing tool to step into the organization of functional or structural systems. When applied to anatomical elements, we can talk of Anatomical Network Analysis. In its basic form, nodes are the anatomical elements, and relationships can be just their physical contact. The network, therefore, represents a topological model, in which the position and neighboring properties of each component are the evolutionary result of a selection balancing the architecture of the system, generating at the same time limitations and constraints. In evolutionary neuroanatomy and paleoneurology, too often each macromorphological change in a specific cortical region is directly (and speculatively) interpreted as a functional (cognitive) adaptation. However, brain geometry is also influenced by skull constraints, and each cortical region is also sensitive to spatial variations of its neighboring cortical elements. Therefore, some anatomical changes may be due to intrinsic changes (for example, cell proliferation or growth), while others may be due to secondary extrinsic factors (for example, pressures and strains of the neighboring environment). In a first attempt to apply network analysis to brain macroanatomy, one year ago we analyzed the cortical organization as described by the traditional Brodmann map, finding an interesting correspondence with the general subdivision in “lobes” and cranial fossae, and further integration of the parieto-occipital block. We have now published a more detailed study, on the regions generally considered in evolutionary neuroanatomy and paleoneurology, as to characterize their architectural roles and relevance in terms of overall topology. The posterior cortex is more integrated than the frontal regions, with the precentral gyrus acting as a spatial hinge between these two districts. Topological complexity matches, in this case, sulcal complexity. The temporal cortex is particularly sensitive to distinct cortical influences. We also considered a preliminary model including the main endocranial (bone) elements, evidencing the role of the anterior fossa and frontal bone in influencing the prefrontal cortical morphology. This first targeted analysis represents an invitation to go beyond simplistic approaches to brain morphological changes in paleoneurology and evolutionary neuroanatomy. Yet, it is just a kickoff. More steps are on the way.

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Domesticated bodies

Humans are hypothesized to have undergone a process of self-domestication, associated with reduction of aggressive behavior and enhanced sociability. We have also undergone anatomical changes in our parietal cortex, which is crucial for body cognition and visuospatial integration. In a recent Opinion Paper published in Frontiers in Psychology, Ben Gleeson and I wonder whether these two factors, self-domestication and body cognition, may have reciprocal influences, or even share some evolutionary mechanisms. A first likely bridge is our heightened use of technology, which is strictly associated with our body-tool prosthetic capacity and with our special life-cycle (adolescence and creativity, longevity, post-reproductive stages, etc.). The concept of “tool” is considered, in this article, through functional and cognitive parameters. A second connection is the social system, because body cognition and the association cortex affect group size and social skills based on egocentric perspectives. Neural plasticity could represent an important organic link between these anatomical and behavioral aspects. The article is part of a volume dedicated to Self-Domestication and Human Evolution.

Humans, chimps, and brain size

Amélie Beaudet and colleagues have now published a new review on the evolution of the modern human brain. They introduce many traditional issues in paleoneurology, including frontal lobe evolution, asymmetries, lunate sulcus, brain growth, and brain shape. They also provide a detailed discussion of the information we have on the evolution of brain size. Studies on this topic are frequently biased by statistical or taxonomic problems, because of the intrinsic limitations of the fossil record. Actually, any model (gradual, random, punctuated, etc.) can be supported by the few and scattered data, generating disagreements and debates. Isaac Asimov said “Where any answer is possible, all answers are meaningless”. In this paper, they describe their own approach, trying to deal with such limitations. They support a gradualist perspective, although with some discontinuities within some clades. The review strictly deals with brain evolution, but I really appreciate the taxonomic considerations at the beginning of the article, defending their reasons to include humans and apes in one single family. I belong to the opposite faction, namely to the resisting supporters of two distinct families for this group, with the term hominids restricted to humans and (probably) australopiths. Firstly, because I think that taxonomy should not try to trace phylogeny too strictly, constrained and forced by cladistic schemes. The real phylogeny is unknown (we use genes as a proxy, which is but an estimation, with pros and cons), and the phylogenetic hypotheses are frequently changing. Instead, a taxonomy based on the whole biological model (that includes anatomy, physiology and so on) is more stable and, importantly, can add more information on the actual evolutionary, zoological and ecological organization and role of a group of species. Secondly, because I think that differences are the great value of evolution, and taxonomy should acknowledge such differences. In this case, we must admit that our lineage is particularly dissimilar from all the other apes. This does not mean that we are better, but surely much different, and taxonomy should take into account the importance of such outstanding changes. Many anthropologists give all these taxonomical issues for granted, using one label or another just by repeating or copy-pasting what they hear around, generally following a mainstream without a personal or competent opinion. But passively repeating statements is proper of dogmas and mantras, something that should be left out of science.  That’s why I really appreciate that, in this article, Amélie and her coauthors take a clear position, explaining their reasons.

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More on humans and chimps can be found in this other recent review on human paleoneurology. For Spanish readers, here a provocative dissemination article on humans and apes, and another one on the immense value of diversity.

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?

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.


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