Together with the recent article on modern vs Neandertal endocranial ontogeny, the team coordinated by Christoph Zollikofer has now published also a large and comprehensive study on endocranial ontogeny in humans and apes. The paper focuses on a specific question: to what extent endocranial differences are due to brain differences, and to what extent they are due to cranial constraints? Definitely, this is a key-paper in paleoneurology. They considered the integration between and within the main cranial districts to evaluate the influence on brain shape of two major cranial effects: spatial packing and facial orientation. Their analyses suggest that endocranial differences between humans and apes, as well as differences among apes, are the result of all those factors, the cerebral and the cranial ones. Therefore, the endocranial form is due to a complex admixture of specific brain differences (already present at birth) and cranial constraints. Comparisons among endocranial ontogenetic patterns of living hominoids, among adult fossil specimens, and among different neuroanatomical aspects of living species, can give different results, suggesting that the relationships between anatomical, morphological, and cytological elements is far from being understood. In my opinion, a limit of many shape analyses in general concerns the use of surface semi-landmarks to analyze brain geometry. Surface landmarks are necessary because of the lack of good anatomical references on the endocasts. Unfortunately, they can’t take into account the contribution of distinct cerebral areas, and as a consequence they consider brain morphology as a single homogeneous surface. The identification of boundaries or distinct and independent elements within this surface might seriously influence the multivariate output. I am particularly interested in the analysis of the parietal districts. When using surface landmarks the analysis of the parietal surface may give different (and sometimes contrasting) results. Hence, we may wonder whether the observed parietal variations are the result of brain differences (cortical expansion/reduction) or of geometry (bulging and flexion). Nonetheless, previous morphological studies based on cortical landmarks suggest that modern humans show an actual (absolute and relative) increase not only of the parietal “surface”, but also and specifically of the parietal “lobe”, when compared with extinct hominids or with living chimps. The localization of anatomical boundaries on endocasts may be difficult, although those results have been replicated on different samples. The identification of anatomical landmarks in living species is, in contrast, definitely more reliable. Therefore, whatever the result of a global surface analysis of the whole endocranium, we should not forget that comparisons of specific areas are suggesting a differential contribution of distinct brain components.
Archive for the 'Skull' Category
Tags: apes, braincase, cranial integration, facial orientation, parietal lobes, spatial packing
Tags: cranial thickness, diploic channels, middle meningeal artery, parietal bone
The 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.
Tags: computed tomography, diploic channels, Neandertals
Diploic channels run within the vault bones, and are therefore protected from external agents. This condition makes them an interesting topic in paleontology, archaeology, and forensics. At the same time, such secluded position has hampered detailed studies on their morphology, variations, and functions. In 1999 Hershkovitz and colleagues published a first pioneering survey on these “elusive” anatomical elements. This week we publish a segmentation procedure to visualize these channels after computed tomography, applying this method to modern humans and Neandertals. The diploic network displays a marked individual variability. It is frequently connected with the meningeal system at the pteric area, and with the emissary and venous systems at the occipital area. As for the meningeal arteries, also the diploic vasculature is apparently more complex in modern humans than in other hominids, mostly at the parietal area. Taking into account the large size of the parietal lobes and bones of our species, it is likely that such vascular development can be associated with metabolic and thermal functions. Beyond the large diploic channels, this vascular system counts with a widespread network of microvessels, which should be carefully investigated in the future.
Tags: frontal lobes, myopia, occipital lobes, orbits
This week, with a team coordinated by Michael Masters (Montana Tech), we have published a correlation analysis to evaluate the relationships between eye, orbit, and brain, in adult modern humans. As already evidenced in other studies in anthropology and primatology, the correlation between eye size and orbit size is very modest. Therefore, the orbit is really a poor predictor of the eye morphology, at both evolutionary and species-specific level. There is also a minor size correlation between the eye and the occipital cortical areas, probably because of their shared visual functions. However, there is also a similar (and even higher) correlation between eye and frontal lobe. In this case there is a structural issue: the frontal lobes lie just above the orbits, generating a spatial interaction between facial and neurocranial elements. Within hominids, this spatial proximity between prefrontal cortex and eyes is generally observed only in modern humans and Neandertals. These two taxa, possibly because of such vertical constraint, enlarged their frontal lobes mainly laterally. These correlations between soft and hard tissues, when dealing with inter-specific trends, can be useful to make inferences on brain proportions based on osteological evidence, providing an heuristic tool for indirect paleoneurology.
So, back to modern human evolution, the situation of the eye was pretty difficult: large eye (due to brain size increase), small orbit (due to facial reduction), upper constraints (the frontal lobes right on the orbital roof), posterior constraints (larger and closer temporal lobes). And, in industrial Countries we can also add more fat between eye and bone. Hard times for the eyeballs, forced to minor deformations blurring images on the retinal screen: myopia. Luckily for us crossing the 40s, the brain stops growing, but the face does not: it grows bigger, giving more space to the eye, which can enjoy a more comfortable environment year by year.
Tags: digital reconstruction, Neandertals
A new reconstruction of the Neandertal skull and endocast of Amud has been published by a team coordinated by Naomichi Ogihara, at the Keio University (Yokohama). They applied mathematical models to align the surfaces of the original fossil fragments (surface extrapolation). Thin-plate spline was then used to integrate the available anatomy from Amud with other Neandertals, namely La Chapelle-aux-Saints 1 and Forbes’ Quarry 1 (shape interpolation). Modern skulls were used to smooth all together. This new reconstruction shows a skull that is shorter and wider than the former one. The basicranial areas, largely missing in this specimen, were the most difficult parts to interpolate, because of their complex morphology influenced by different independent factors. Amud is dated to 50-70 ka, with a massive cranial capacity: around 1740 cc.
Tags: parietal bones, parietal lobes
Parietal lobes are a main source of morphological variation within humans and within hominids. This month we have published a study on the relationships between bones and lobes, to evaluate how and how much such variation in the cerebral areas can influence the variation of the corresponding cranial points. There is a size correlation between parietal bones and parietal lobes, but it is small. There is a lot of individual variation, most of all in the precuneus. Changes of the brain proportions do not seem to influence the spatial relationships of the bones. Therefore, when the boundaries of the parietal lobes change, the boundaries of the parietal bone do not. It is like the brain “slides” under the bones, without strict constraints due for example to the connective meningeal interface. So, the larger the parietal lobe, the more it approaches the frontal bone. Such lack of marked correspondence between bones and lobes suggest cautions when using cranial landmarks to estimate brain boundaries, like in neurosurgery or in paleoneurology. Now, two hypotheses can be put forward, taking into consideration that the growth of the parietal area in our species is characterized by a very early post-natal stage. First hypothesis: such lack of correspondence can be present since the beginning, and hence that early parietal bulging will separate the limits of the parietal lobe and bone. Second hypothesis: during that early stage parietal lobe and bone grow together (the latter in response to the former), but later stages of spatial changes in the anterior districts (frontal bone and lobe) alter their original correspondence. This study deals with modern humans, and it will be interesting to consider the same spatial relationships in other primates. Nonetheless, at least for Homo sapiens, we can say that between parietal lobes and parietal bones there is a good geometrical correspondence (overall curvature), a modest size correlation (length), and a variable spatial relationship (boundaries).
Tags: digital anatomy, Naomichi Ogihara, virtual anthropology
The 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.