Integrated paleoneurology

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.

Imaging brains

In the last decade neuroscience has experienced an explosion of brain imaging studies, programs, and databases. The advance has been outstanding, indeed. Nonetheless, we have also been surprised by a large and unexpected number of strange and discordant results. There are so many examples in which similar studies reach different or even opposite conclusions. Differences in raw values, comparisons (from sexual variation to hemispheric asymmetries), or correlations and associations between variables, can be pretty frequent when dealing with specific aspects of brain anatomy. In some cases, these studies apparently deal with “simple” factors, like volumes or linear metrics. We have discovered new kinds of uncertainties on functions, software, and even on basic anatomy. A recent study has discussed the problem of reliability in functional MRI (here a comprehensive post). Of course this is not something strictly associated with neuroanatomy and brain imaging. Any analysis in molecular biology has many more methodological and technical passages which can hide some kind of processing problem and generate noise or even confounding outputs. In neuroimaging, there are at least four main steps which can be problematic. First, data sampling (machines, parameters, and so on). Second, the formatting procedures (databases, archives, platforms). Third, the processing of the data (programs, algorithms, etc.). Fourth, statistics (sample size, statistical power, adequacy of the statistical tools). Whatever it is, it is calling our attention. And this is the good part: we are more and more forced to be less superficial. All these unexpected uncertainties require a critical view, as discussed in this manuscript on future challenges for neuroimaging research. The most patent problem is confusion, and an epidemic spread of wrong information. But there is also another risk: too many discordant results can lead to a consistent loss of confidence in these methods (here another post – in Spanish). And this can have both scientific and economic consequences.

Ontogenetic dilemma

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

In 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

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.

Gran Dolina

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

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.