Brain evolution involves changes in size and morphology, but also changes in the capacity to be changed. Plasticity refers to the range of phenotypic variation allowed within a given genetic structure. Environment is a major factor influencing the phenotypic expression, and we humans have a special additional environmental component called “culture”. The ecological, cultural, and social niches, shape each others, with dynamics which are far from being understood. Aida Gómez-Robles and colleagues have now published a morphological analysis of human and chimp brain, taking into consideration heritability. In both species cranial capacity is highly heritable, more for humans than for chimps. Also the general dimensions of the main brain areas show in both species an apparent genetic component. The situation is different when dealing with sulcal morphology, which are still heritable for chimps but not that much for humans. This means that brain morphology in chimps has a stricter genetic program, while humans are more sensitive to non-genetic factors and individual responses. Environmental influences are supposed to be the key, mostly when considering the altricial condition and heterochronic changes associated with the human brain growth and development. In a recent review on the evolution of visuospatial integration with Atsushi Iriki we focused on the necessity to understand to what extent brain changes associated with human evolution are due to genetic, epigenetic or environmental factors. We pointed to the sensitivity of the brain to be “trained” through feedbacks between biology and culture as a crucial variable targeted by selection. This new study stresses further the possibility that selection can act on the capacity to change, more than on the change itself.
Tags: brain size, heritability, phenotypic plasticity, sulcal patterns
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: brain and genetics, cortical surface, genetic ancestry, sulcal patterns
The skull has represented, since ever, a crystal ball to investigate history and geography of past and present human populations. Considering the reciprocal influences between brain and braincase, we can wonder whether the brain can also provide traces of that long run. It looks like it does, according to a recent study which evidences a correlation between cortical patterns and genetic ancestry. It seems not a matter of size or surface area, but of cortical organization and sulcal geometry. If confirmed, these results are extremely interesting. Taking into account these differences among human groups, the authors of the study cast some doubts on the possibility to obtain robust information from fossil species, questioning the relationships between brain shape changes and specific volumetric variations of the brain districts. I must confess I can’t really see an antagonistic relationship between these results and the paleoneurological data. Intra-specific and inter-specific variations do not necessarily undergo the same rules and patterns. Most importantly, paleoneurological evidence is also aimed at considering specific changes of surface and volume proportions, beyond sulcal appearance. And, as recently described for the precuneus, larger size of a brain element may generally mean larger cortex of that element. Brain and braincase share a lot of morphogenetic mechanisms, but of course they are also influenced by independent factors. Their boundaries may vary according to different rules, but the intimate relationships between their respective surfaces allow at least a gross quantification of spatial organization, volumetric changes, and relative proportions among brain areas. We know that morphological changes as shown on endocasts are only a part of the story, and we know they are not always associated with neural (or even cognitive) changes. But reductionists approaches should be avoided in any fields, including genetics or neuroimaging, not only in paleoneurology. Conversely, a genetic signal on the sulcal pattern may promote further interest in brain shape variation. Last but not least, the study seems to support, rather than contradict, the information available from bones: it turns out that the conclusions of the analysis match the results of the Howells‘ craniometric studies. Good.
Tags: apes, endocranial ontogeny
After shape analysis of the endocranial growth and development in modern humans, chimps, and Neandertals, the team from the Max Planck Institute has published a study on apes endocranial ontogeny. In their former articles they evidenced a shared trajectory of form change in humans and chimps. The only exception is the “globularization stage” in modern humans, an early postnatal stage associated with parietal and cerebellar enlargement. This study now includes also gorillas, orangs, and gibbons, confirming that after eruption of the deciduous dentition all hominoids share a similar pattern of form variation. Differences among species are largely a matter of degree of change, but within a shared set of rules. This implies that most of the observed differences among their endocranial forms take place before, in prenatal stages.
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: frontoparietal system, parietal lobes
Roberto Caminiti and his colleagues have just published a very large and detailed review on the fronto-parietal network, comparing functional anatomy, histology, and connectivity in humans and macaques. The organization of the fronto-parietal system is similar in these two taxa, suggesting a shared conservative structure rooted in a long evolutionary history. However, there are also discrete differences, most of all at the intraparietal sulcus and in the precuneus. Because such differences were apparently put forward on a shared background, they support the hypothesis of Fred Coolidge about exaptation of the parietal lobes, as reuse of primitive characters to achieve new functions. Visuo-spatial integration and the eye-hand system are of course central in this perspective, but those parietal elements are also involved in different kind of processes ranging from consciousness to numerosity. As usually, we are supposing that macaques show a primitive organization, and humans a derived one. As recently discussed, such assumption is very general and it has no logic or experimental support, and caution is recommended in this sense. In fact, both macaques and humans could display derived characters, evolved independently. The review carefully considers also the human paleoneurological evidence, supplying a very complete image which effectively synthesizes at once more than ten years of works published by myself and by Simon Neubauer and Philipp Gunz.