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: brain and genetics, cortical surface, genetic ancestry, sulcal patterns
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.
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: Cercopithecoids, cerebral complexity, endocranial volume, olfactory bulb
Macaques and chimps are still used in anthropology and neuroscience as “primitive models” for human evolution. This is of course a non-sense: all living species, after the divergence from a common ancestor with modern humans, have evolved and changed as humans did. The genus Macaca is as young as the genus Homo, and living macaques and living humans are recent species in evolutionary terms, approximately with a comparable age. The problem with chimps is that we miss fossils, so we ignore how and how much their lineages has changed. But we have more information on macaques, and in general on fossil cercopithecoids. A very detailed and informative study on the endocast of Victoriapithecus has been recently published, definitely a stimulating and comprehensive article for primate paleoneurology. This Old World monkey, dated to 15 Ma, had a small cranial capacity and large olfactory bulbs, but a sulcal pattern similar to modern cercopithecids. This suggests two major points. First, in Old World monkeys sulcal complexity evolved before brain size increase. Second, brain morphology evolved in cercopithecoids and hominoids through distinct processes, mixing primitive traits, different mechanisms, specific adaptations, and some convergences. These results stress further the necessity of caution and of a proper evolutionary perspective when dealing with comparative primatology and human brain evolution: macaques (and chimps) are derived species as we are, with their own independent evolutionary histories. They can provide information on biological factors which are shared among our respective lineages, but it would be an error to think that their anatomy, physiology, or genetics, represent an ancestral condition.