Posts Tagged 'Homo sapiens'

Modern human brain shape

In a very comprehensive (and elegant!) article Simon Neubauer and colleagues have now analyzed brain shape variation along the modern human lineage. Since the description of the skull and endocast of Jebel Irhoud, it was clear that modern human brain form could have evolved after modern human origin. So, at that time (150,000-300,000 years ago) we had modern humans without modern brains. If Jebel Irhoud was Homo sapiens, then “early modern humans” lacked our characteristic globular brain shape, which is due to parietal lobe bulging and cerebellar form. Then, some later “archaic modern humans” seem to display a sort of intermediate morphology. Only recently (30,000-100,000 years ago) modern humans have evolved modern brains, at least in terms of general proportions and gross appearance. Of course, it’s difficult to say whether this transition was gradual or more abrupt. This article of the Max Planck team follows a previous one on the same specimens, and provides a very detailed analysis of many fossils that describe the evolution of our own species. Although the fossil record is not continuous because of the many chronological gaps, results suggest that a gradual change was likely. They also emphasize that a full-globularity can be found at the same time in which we find the archaeological evidence of behavioural modernity (arts, symbols, complex tools …). I remarked this same point many years ago, but the statement was not much appreciated because of the many uncertainties on the cultural “modern revolution” (more or less gradual, more or less discontinuous). Whatever the process behind, the appearance of a modern brain form (largely influenced by parietal districts associated with visuospatial functions, body cognition and visual imagery) matches the appearance of a modern behaviour (largely based on visual cognition and visuospatial managements, ranging from simulation and imaging to body-tool integration). Maybe it is but a coincidence, but nonetheless … they match.

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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.

Precuneus

Bruner et al JA2014Paleoneurological studies based on endocranial geometry suggested that a spatial dilation of the deep parietal areas was the major morphological difference between modern and non modern human brains. In our species, the morphogenetic change associated with this parietal bulging was then localized in a very early post-natal period, in a  stage which is absent in chimpanzees or in Neandertals. In the meanwhile the deep parietal areas were demonstrated to have also special cytoarchitectonic elements in modern humans, to be the main functional and structural node of the human brain organization, to be critically involved in major cognitive capacities through the fronto-parietal connections, to be central to the default mode network, and to be essential in human-specific cognitive processes involving imagination and simulation. Such specific parietal modifications have been also tentatively associated with species-specific vulnerability to neurodegeneration in our species. Actually, the early stages of Alzheimer’s disease are associated with metabolic, functional and structural impairments at the deep parietal areas, like the precuneus. These brain districts have been scarcely studied in term of morphology because of their difficult position, multifunctional roles, and blurred anatomical boundaries. Through a MRI shape analysis of adult human brains we have now identified the main character associated with individual brain variation in our species: the geometry of the precuneus. With a negligible effect of brain size or sex, the proportions of the precuneus are the main determinant of the midsagittal brain geometry. The brain morphological variation of the human genus and the brain morphological variation among adult modern humans share the same pattern: parietal bulging. And, at least for modern humans, this pattern is strictly determined by one single character: the longitudinal extension of the precuneal area. Evolutionary and functional evidence both converge toward the neural element which is at the same time the most variable at intra-specific level, strongly influencing our brain form. Many coincidences, which may be the result of the delicate spatial position of the deep parietal areas in the overall brain geometry. Or may there be more than this?


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