Archive for the 'Paleoanthropology' Category

Brains and eyes

After our first survey on the morphological relationships between eyes and brains, here a comprehensive second study on this same topic. We have analyzed data from computed tomography (orbits and endocranial space) and magnetic resonance (eyes and brain), investigating modern humans, apes, and fossils. Soft tissue variation mainly deals with the distance between eyes and temporal lobes. Cranial variation mainly concerns the orientation of the orbits, probably influenced by parietal morphology and variation of the head functional axis. Phylogenetic differences are generally associated with the distance between orbits and braincase, with fossil humans showing an intermediate position between modern humans and apes. Here a Skull Box post with more details.

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Language and fossils

This week I have published an opinion paper on language and paleoneurology, in a volume of Frontiers in Human Neuroscience dedicated to language, skulls, and brains. I review the fossil evidence on language, suggesting that most of such evidence concerns brain areas that are influenced by cranial structural constraints, or is based on speculations associated with individual bone remains. Thus, strictly speaking, there is no consistent evidence on language evolution when you deal with fossil anatomy. Ralph Holloway already stressed this point before, but it seems that most books and articles introducing this topic simply keep on stating the opposite, following a mantra (usually void of citations) according to which fossils must clearly reveal the cerebral (usually frontal) changes behind language evolution. The lack of scientific evidence in this context does not mean that there is no association between language and brain evolutionary changes in hominids, but just that fossils can provide only a very incomplete (and insufficient) view of this process. Firm statements, scientifically speaking, should be avoided, and relegated to storytelling and science marketing.

The dorsal and medial parietal areas, probably larger in Neanderthals and even more derived in modern humans, are not generally considered when discussing language processes, and most of the debate has been centred on frontal lobe functions. Nonetheless, the parietal areas are crucial for hand coordination and manipulative abilities, both factors that have always been regarded as influential in language evolution. Also, recent evidence suggests that language has an important embodied component: language coding passes through body experience and simulation, something which is profoundly associated with the functions of the deep parietal folds. Therefore, we should consider whether changes in the whole fronto-parietal system may have triggered or facilitated language in the human genus. The paper is open access.

Jebel Irhoud

New fossils and age for Jebel Irhoud. Jean-Jacques Hublin and colleagues have published new specimens, new analyses, and a new chronology pointing at 300 ka. All their results robustly confirm what we already knew on these populations: modern face, primitive braincase. Two major advances of these new findings are i) the morphology of Irhoud 10 (the new skull) is apparently so similar to Irhoud 1 (the old skull found back in the ’60s), suggesting that such phenoptype was common and representative, and not only the result of individual variation, and ii) the age around 300 ka, that suggests an earlier origin for our lineage. The braincase and endocast of the new skull were not analyzed in this study, probably because of some deformation, and there are no photographs of the fossils (in the paper we can only see the virtual reconstruction of the face), so an assessment of its paleoneurological traits is not available yet. But in this article they re-analyze the old specimens (Jebel Irhoud 1 and 2) through shape analysis, confirming a plesiomorph braincase, apparently (Extended Data Figure 4) because of a reduced parietal and frontal size and curvature. Here a 2013 study I coauthored with Osbjorn Pearson on Jebel Irhoud’s endocast, supporting the same conclusion: they were probably modern humans, but without modern brains. If they were our ancestors, something triggered a subsequent change in brain proportions and organization.

Frontal surfaces

beaudet-and-bruner-2017

More surfaces. This week we have published a surface comparison of the frontal endocranial morphology in OH9, Buia, and Bodo. The methods are the same applied previously by Amélie Beaudet and colleagues. Despite the importance generally assigned to the frontal cortex in our species, paleoneurology has not managed to reveal clear and patent changes in its gross form. Endocasts can only supply information on the general external appearance of the cortical anatomy, so we should expect they cannot be used to trace many aspects  associated with evolutionary variations. Also, the bad habits to defend firm statements based on single (and often reconstructed and fragmented) individuals unpleasantly crashes against the basic scientific principle of hypothesis testing, something that needs quantification, large samples and statistics. In this paper we compare these three specimens with the general scope of discussing some issues about frontal lobe evolution and paleoneurology. When compared with a modern human endocast, the younger fossils (Buia and Bodo) display flatter dorsal-lateral areas, while the older one (OH9) show a more extensive flattening of the whole dorsal surface. They all fit within a general trend observed in humans and hominoids: the more the eyes go below the frontal cortex, the more the frontal lobe bulges. So it seems reasonable to think that the curvature of the frontal lobes is but a structural consequence of the spatial relationships between face and braincase. In paleoneurology, we should exclude structural changes (cranial constraints and secondary consequences) if we want to localize functional ones, or if we want to reveal specific adaptations and primary evolutionary variations. Surface analysis is one more tool to go in that direction.

Brains and teeth

gomez-robles-et-al-pnas2017In anthropology it is commonly accepted that the evolution of larger brains was associated with the reduction of posterior teeth. Factors ranging from diet to cognitive ability have been used to explain this inverse correlation between cerebral complexity and masticatory structures. Aida Gómez-Robles and colleagues have analyzed brain and teeth changes using a multiple-variance Brownian motion approach, providing evidence against a brain-teeth phylogenetic association. Brain shape was analyzed by using eight linear variables as measured on endocasts. Teeth shape was analyzed through geometric morphometrics. The study found that endocranial proportions and dental geometry are largely characterized by similar rates of variation, which are indicative of a neutral and non-directional pattern of evolution. Brain size and tooth size show different rates of change throughout the phylogenetic tree, and the hypothesis of a reciprocal and inverse correlation is not supported. This seems to suggest independent factors at environmental and/or genetic level. Two characters show faster rates of change in specific lineages, and are probably associated with specific selective and adaptive processes: brain size in early Homo and brain globularity in Homo sapiens. The first result suggests that brain evolution in the genus Homo is strongly based on size increase rather than on changes of specific cortical proportions. However, caution is needed in this sense: the study is based on simple linear metrics such as arcs and chords, and reflects only the external appearance of endocranial anatomy. Despite these limitations, this result is consistent with other kinds of evidence. The second result reflects an exception to this size-only pattern of change: the globular brain shape in modern humans. Parietal lobe variations are again an issue.

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.

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.


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