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Ontogenetic dilemma

Ponce de Leon et al 2016

Marcia Ponce de León and colleagues have published a comprehensive shape analysis on modern human and Neandertal early ontogenetic endocranial changes, as Philipp Gunz and his team did back in 2010. Interestingly, results are different. The previous study from the Max Planck Institute concluded that only modern humans have a species-specific postnatal stage in which the braincase bulges (globularization stage). In contrast, this new analysis, coordinated by Christoph Zollikofer, suggests that after birth Neandertals and modern humans share a similar pattern of endocranial shape change. In this case, any endocranial difference between these two species must occur before birth. The discrepancy between the two studies may be due to differences in the samples (which, recognizing the good samples used in these analyses, would reveal a problematic instability of most paleoanthropological studies) or to differences in the reconstructions of the specimens (which, recognizing the good experience of both teams, would reveal a problematic instability of most paleoanthropological studies). Nonetheless, we must also take into account that both articles rely on very complex statistical and algebraic passages, and methodological biases should not be ruled out. After all, also paleontology deals with the same limits of any science: we do not work with skulls or brains, but with models made of variables and parameters. Models that work well in some cases, and do a worse job in some others, depending on the questions involved. In this new study, the fact that endocranial shape differences between Neandertals and modern humans are prenatal is used to state that there are no cognitive differences between the two species. Of course, cognition is more than shape, so the relationship between the timing of these changes (before or after birth) and the statement on cognition is not particularly straight. Inferences on cognition should be made on multiple evidence, dealing with something that goes well beyond a surface analysis.

Subparietal morphology

Pedro-Pereira and Bruner 2016In this last years we have been studying the morphology, surface and position of the precuneus in adult humans and chimps. This week we publish a survey on its coronal anatomy: lateral extension and sulcal pattern. The aim of this article is to provide a quantitative description of its parasagittal variation in terms of morphometrics and folding schemes. The subparietal sulcus is larger on the right side, and possibly larger in males. The size of the subparietal sulcus is not associated with the sulcal scheme, which is very variable even between hemispheres of the same individual. The height of the precuneus influences the outer cortical profile, but the morphology and width of the subparietal sulcus have no apparent effect on the external brain geometry. The precuneus in general influences the upper cortical shape, with scarce or no influence on the lateral outline of the upper parietal lobules. Therefore, shape changes in this lateral areas are more likely to be associated with changes of the intraparietal fold. Correlations between inner and outer morphology are useful to evaluate whether changes in deep anatomical elements can be indirectly evidenced in paleoneurology, through the analysis of the outer (endocranial) surface.

Bones and vessels

Eisova et al 2016The vascular traces left on the bones are remnants of physiological processes associated with blood flow and functions. Craniovascular traits can be used in archaeology, paleontology, and forensic science to deal with normal and pathological variations of the circulatory system, bridging interests between evolutionary and medical fields. Current information on these characters is, at best, scarce. After our recent work on diploic channels, this week we publish another morphometric study on the vascular traces, and specifically on their relationships with parietal bone size and thickness. We provide a quantitative description of the lumen size in adult modern humans for the middle meningeal and diploic vessels, as calculated from cranial anatomy after computed tomography, for different orders of branches. Vessel size and cranial thickness can be proportional if sharing growth factors, or inversely proportional if competing through structural constraints. However, we do not find any clear relationship between vascular size, cranial size, and cranial thickness. This result suggests that bone and vessel morphogenesis are probably influenced by independent factors, at least when dealing with differences among adult individuals.

Gran Dolina

ATD6_100_168(ESaiz)This week we publish a study on a parietal bone from Gran Dolina, Atapuerca, dated to more than 800.000 years and probably belonging to the species Homo antecessor. The general morphology  suggests small dimensions and an archaic appearance, with bossing lower parietal areas (supramarginal gyrus) and flattened upper parietal areas (upper parietal lobule). The vascular network is not particularly reticulated, and it is equally developed in its anterior and posterior branches. There is a well visible parietal foramen, an accessory parietal canal, and a lot of minor vascular passages, mostly around the lambda. The bone thickness and the distribution of the diploe suggest a young age. Therefore, the information available points to a juvenile archaic human. This fragment supplies at present the only evidence on the braincase of Homo antecessor. As far as we currently know, most archaic human species do not display consistent neuroanatomical differences, apart from variation in brain size. Nonetheless, this specimen can supply valuable information if, in the future, we will be able to improve sufficiently the fossil record as to support ontogenetic series.

Buia

Buia (Bruner et al 2016)This week we publish a morphometric analysis of the endocranial anatomy of Buia, a skull found in Eritrea and dated to 1 million years. The cranial capacity is 995 cc. The endocast is extremely dolichocephalic: very long and narrow. Nonetheless, it shows all endocranial traits that are commonly described in “archaic humans“. The bulging occipital lobes and the vascular system resemble the Chinese specimens from Zhoukoudian. Its pronounced parietal bosses are due to a narrow cranial base and temporal areas, and not to a real enlargement of the parietal lobes. Actually, the cranial base in Buia is very narrow and flexed, and it may have influenced both the neurocranial and splanchnocranial proportions (bulging parietal surface and tall facial block). At present, there is no reason to exclude this specimen from the Homo ergaster/erectus group. The skull from Daka show a similar chronology and a similar geographic origin, although it displays much more brachycephalic proportions. If all these Afro-Asiatic archaic specimens belong to the same species, the variability is notable. It remains to be established whether the evolutionary roots of more derived taxa (like Homo heidelbergensis) can be traced back to these archaic populations, or else if Buia and Daka are still part of an undifferentiated phylogenetic group.

Maba

Maba and Saccopastore (Wu and Bruner 2016)We have now published a study of the endocranial morphology of Maba, a Chinese fossil specimen dated approximately to the end of the Middle Pleistocene. The available portions of the upper face strongly resemble European Neandertals, like Saccopastore 1, found in Italy and supposed to have the same chronology of Maba, or Krapina 3, from Croatia. Also the spatial arrangement and the structural organization between face and braincase in Maba is reminescent of Neandertals. However, the frontal and parietal bones suggest an archaic endocranial morphology, more comparable with Homo heidelbergensis.  So we have here an archaic brain form assembled onto a derived facial block. A similar situation (Neandertal traits in the face and archaic features in the vault) was also described for the sample from Sima de los Huesos (Atapuerca, Spain). If such affinity is a matter of phylogeny, the range of the paleospecies H. heidelbergensisH. neanderthalensis should be revised, and extended to China. Otherwise, the facial Neandertal traits in this Chinese populations can be but a consequence of parallelism and analogy, and the specimen can therefore represent an archaic Asian taxon. Curiously, at the same time in Africa we have the opposite combination: Jebel Irhoud, a modern face with a Neandertal braincase! Definitely puzzling …

Folding brains

Tallinen et al 2016An amazing article has been published in Nature Physics. Brain cortical folding is influenced by genetic and physiological factors, but there are also many hypotheses concerning the possible role of mechanical forces associated with the cerebral tissues. These hypotheses are largely based on theoretical approaches and numerical simulations, integrating geometry and biomechanics. Because of the mechanical properties of cells and tissues, growth forces can be redistributed within and among the elements of the anatomical system, channeling morphogenesis and shaping the spatial organization of the anatomical components. This month Tuomas Tallinen and colleagues provide a further mathematical model of the growing cortex, introducing constraints associated with the sulcal pattern. But, more incredibly, they provide an extremely elegant and efficient experimental evidence. After MRI imaging, they prepare a physical model of the fetal brain with two gel components. The outer thin layer (simulating the cortex) swells when in contact with a solvent, undergoing a tangential expansion. When this happens, the growing outer surface and the stable inner volume must properly interact in terms of physical forces and distribution of the surface to volume adjustments. The result is amazing, because it really mimics the human cortical folding! There is an incredible correspondence between the real and simulated folding pattern, in terms of topology and degree of convolution. No programming here except the growing schedule, just physical properties, structural interaction, and forces redistribution.

“Morphology is not only a study of material things and of the forms of material things, but has its dynamical aspect, under which we deal with the interpretation, in terms of force, of the operations of energy.”
(D’Arcy Wentworth Thompson – On Growth and Form, 1942)


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