Continuity

Humanchimp_2015Since the earliest hypotheses on human evolution there is major issue on continuity vs discontinuity. Charles Darwin suggested that only a matter of degree separates human and non-human species, also in the cognitive sense. From the opposite side, many biologists (mostly those involved in molecular sciences) are constantly looking for unique features, single changes that can switch the light on. The fact that there is still no agreement or evidence giving definitive support to any of the two perspectives may suggest that the debate is simply oriented toward an unfruitful direction. If there is still no good answer, maybe it is because there is a bad question. Is a faster car faster just because it supports higher speed or because it is differently designed? Both. What about evolutionary “shifts” based on the same processes? Are they a continuous or a discontinuous phenomenon? Both. Brain evolution is particularly sensitive to the continuity vs discontinuity debate. Is there a real biological frontier between continuity and discontinuity? It looks like a fractal loop and any change is, after all, a discontinuity in something. In paleontology, continuity is often a matter of appearance concerning the homogeneity of the fossil record, which gives a partial and largely incomplete view of the variation. Paleontology is furthermore based on a specific biological component – morphology – which may not necessarily have a linear correspondence with the underlying evolutionary processes. What if continuity and discontinuity are just in our head, in our eyes, in the form we perceive reality, in the form we analyze reality? We need fixed categories to decompose the scene and then recompose it by searching for relationships. Maybe this necessity is a limit, or maybe it is an advantage. But we think through categories. Evolution does not.

Warping brains

Ogihara et al 2015The team coordinated by Naomichi Ogihara has published an analytical review on computed reconstruction of fossil crania and interpolation of their brain morphology. The article presents and discusses the applications of biomedical imaging in paleontology, including technical and algebraic details. Automated assembling of fossil fragments is approached following geometric similarity, fracture surfaces, pattern matching, smoothness, and anatomical correspondence. Skulls, endocasts, and brains are integrated mixing information from computed tomography and magnetic resonance, and spatial deformation functions are used to interpolate brain morphology in fossil species. This is a very useful paper both for the technical issues and for general perspectives in digital anatomy and computed morphometrics in paleoneurology. Additional information on this topic can be found in the paper by Gunz and colleagues on virtual reconstruction and in the review by our team on functional craniology.

Brain ontogeny

Hublin et al 2015After that recent article on endocasts, the team from the Max Planck Institute has now published one more review, this time on brain ontogeny and life history. This paper introduces issues concerning encephalization, energy budget, birth, maturation, ecology, and culture. It provides also many general perspectives on hominid paleoneurology, resuming much paleontological evidence published in the last decade. It is a good and effective recompilation of literature and concepts, integrating morphometrics, development, and evolution. It is part of a special issue dedicated to brain, birthweight and the immune system.

Precuneus and surface

Bruner et al 2015One year ago we showed that a main source of variation among adult human brains is due to the proportions of the precuneus. This seems to be a stand-alone feature, not integrated with other patent morphological changes of the brain form. The spatial pattern associated with the dilation/contraction of the precuneus is particularly similar to the parietal bulging characterizing the brain of our species in evolutionary terms. Now we have published a study of the anatomical factors associated with this shape change, namely an analysis of the whole precuneal volume (cortical surface and cortical thickness) in a sample of adult humans. The results suggest that the observed changes of precuneal shape and proportions are associated with actual changes in precuneal surface area. Therefore, it is a matter of absolute cortex volume, and not just of relative size. There are no differences in cortical thickness. The precuneal volume increases with positive allometry as brain size increases (that is, it increases more than the rest of the brain, as brains get bigger), but the individual differences – as well as the differences between hemispheres – are important. What is the cellular reason of such morphological variation? Number of neurons, connections, or other components?

Interestingly, such marked anatomical variation seems not influencing any standard psychometric variable. It can be hypothesized that traditional psychometric performances are not adequate to quantify the functions of the precuneus. This is likely most of all when taking into consideration its importance in the default mode network, which functions are not easy to capture with task-based metrics.

Endocasts

Neubauer 2014Simon Neubauer has recently published a review on endocasts. There is a general  and effective introduction to functional craniology and paleoneurology. Then, some debated case-studies in paleoanthropology are presented and discussed, following a welcome and sensate objective approach. Computed tools and methods are also briefly introduced. Finally, evo-devo concepts and principles are applied to current paleoneurological data, integrating life-cycles and morphogenesis. This is a very useful article for teaching, and for those who are looking for a quick updating on applications and perspectives in paleoneurology. You can find another recent review of Simon Neubauer in the book Human Paleoneurology, a chapter entitled “Human brain evolution: ontogeny and phylogeny”.

Homo erectus

Bruner et al (Quaternary International 2014)Despite most than one century of studies, the taxonomic and phylogenetic status of Homo erectus is still largely debated. There is no agreement whether or not the African and Asian specimens belong to the same species, or on the meaning of the variation within the Asian group. The relevant influence of a shared allometric component, the large geographic and chronological span, the marked individual and idiosyncratic variability and (most important) the small sample size, hamper any definitive conclusion. Because of all these actual limits, we should seriously consider if our insistence in searching fixed and stable taxonomic certainties represents a necessary and useful effort. Because of these limits, probably paleoanthropology should rely on a different approach to taxonomy, more centred on the actual information available than on hypothetical and conceptual schemes. In the meanwhile, this week we  publish a general review on Homo erectus paleoneurology. We describe the general morphology of the African and Asian Homo erectus endocasts, providing a quantitative perspective of their variation and variability by means of traditional endocranial metrics. As expected, no patent differences are evidenced among different geographic groups, being size and allometry the main source of variation. Of course, the absence of morphological differences in the endocasts does not necessarily means the absence of differences in brain organization, and it does not give information on the underlying taxonomical structure. The limits of the sample size are evident: a power analysis suggests that, beyond the issue of biological representativeness, for a simple variable like cranial capacity groups of at least 40 specimens would be necessary to deal with the statistical uncertainties! Nonetheless we can now state that, at least according to the current metric information, all the possible taxa included in the Homo erectus hypodigm share similar endocranial proportions.

Temporal sulcal pattern

Rosas et al 2014Neuroanatomical evidence suggests that we have relatively larger temporal lobes when compared with the apes’ allometric brain variation. Actually, there are also some form differences in our middle cranial fossa, housing the temporal lobes. However, the morphology of the middle endocranial fossa is influenced by many factors involved in the cranial base phylogeny and ontogeny, and we can wonder whether it strictly represents, in terms or direct linear variations, corresponding changes of the temporal lobes. The structural relationship with the underlying mandible is just one of the many non-neural influences of the middle endocranial area. Nonetheless, the middle endocranial surface can also provide information on the sulcal pattern of the temporal cortex, now further investigated by Antonio Rosas and Markus Bastir. In this case, the resulting morphology is more likely to be the direct consequence of brain morphogenesis and cortical organization, being less influenced by structural cranial constraints. That is, possible species-specific differences in the sulcal pattern can be more easily interpreted in terms of intrinsic brain factors (independently upon their functional meaning), more than in terms of extrinsic  secondary consequences of the complex spatial dynamics of the endocranial base.


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