The fate of Middle Pleistocene

My welcome for the new year is a perspective review entitled “Evolving human brains: paleoneurology and the fate of Middle Pleistocene“. The article introduces human paleoneurology and functional craniology, and a topological model of the skull-brain network system. Then, I discuss the paleoneurological variations associated with Middle Pleistocene, and the following fate: modern humans and Neandertals. Finally, in this article I provide a further development of my hypotheses on cognitive extension and prosthetic capacity, according to the evidence associated with the evolution of the parietal cortex. The article is published as part of a special issue of Journal of Archaeological Method and Theory, edited by Marlize Lombard and Anders Högberg and entitled “Theoretical pathways: thinking about human endeavour during Middle Stone Age and Middle Palaeolithic”. Here a full-text view-only version of the article.

Extending the body into digital technology

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Here above the full video of the Round Table “Extending the body into digital technology: an evolutionary perspective“, organized by Claudio Tuniz and myself at the recent EuroScience Open Forum 2020 (Section: I compute therefore I am). Here the Program of the meeting, and the complete Programme Book. The conference includes a talk of mine on paleoneurology and visuospatial evolution, a talk by Atsushi Iriki on extended body and environment, a talk by Luke Miller on extending tools into the self, and one by Barbara Pernici on chess and technology.

Florisbad

The skull of Florisbad was found in 1932 in South Africa, and it is dated to about 260 ky. It could belong to a very early Homo sapiens population, to a derived stage of H. heildelbergensis, or to a side branch of some unknown human taxon. We have now published a report on its braincase, as to present a general view on some metric and non-metric features. The curvature of the frontal squama is definitely modern, suggesting a modern spatial relationship between the face and the neurocranium. Nonetheless, the frontal lobes are very broad, as often observed in Neandertals. In contrast, the parietal regions display a more archaic morphology (in their general form but also in their vascular anatomy), more affine with H. heidelbergensis. There are some individual epigenetic features, (like the interparietal groove) which are generally associated with local changes of the diploic thickness. The mosaic combination of derived and archaic traits hampers a reliable taxonomic assessment of this specimen. Furthermore, all these features are not diagnostic in terms of taxonomy, because they are characterized by noticeable individual variation and idiosyncratic differences. Fossils with distinct admixtures of antero-posterior cranial features (generally, the posterior regions use to be less derived than the anterior ones) have been described in Africa (Jebel Irhoud), Asia (Maba) and Europe (Sima de los Huesos). It can be useful to remind that taxonomic labels are but useful conventional elements aimed at supplying conceptual and lexical tools, necessary to organize our thinking capacity. Our human brains do need those conventional units, but nature, of course, does not.

Krapina

Zachary Cofran and colleagues have just published an outstanding methodological paper presenting new digital reconstructions of the Krapina endocasts, with estimated endocranial volumes. They revise the Krapina specimens for two main reasons. First, they suggest that some previous endocranial estimates for their cranial capacity are questionable, because of the uncertainty associated with the remains, which are fragmentary and incomplete. Second, they recommend to substitute specific values for fossils’ cranial capacity (that convey a – false – sense of precision) with ranges, according to different reconstructions and error. In my opinion this approach, remarkably discussed back in 2012 by Neubauer and colleagues in their study on Australopithecus africanus, seriously represents a main advance in paleoneurology. Estimates for Krapina 3 and Krapina 6, which are the most complete skulls, does not change particularly, when compared with previous values obtained from physical reconstructions of the endocasts. New figures for Krapina 1 and Krapina 5 are provided. A first reconstruction for Krapina 2 is also presented. Estimations are based on endocast morphological interpolation based on geometric morphometrics and shape analysis. Interestingly, using a Neandertal as template (Spy II) seems to work better than using a modern human template. This latter also works fine, but the final virtual endocasts are a bit “modernized”, because the modern template introduces (as expected) a slight parietal and cerebellar bulging. This study also confirms that the Krapina sample (dated to 130 ka) shows a range of endocranial volume (and, possibly, overall brain size) that is similar to the range of Middle Pleistocene Europeans (Sima de los Huesos) and of other pre-Wurmian Neandertals, namely smaller than later classic Neandertals. Here, an old report we published on these same specimens back in 2006, comparing Krapina and Saccopastore.

More precuneus

One more paper on precuneus morphology. The precuneus displays a remarkable (and definitely striking) variation in its size and shape in adult humans. One can think that such geometric diversity is due to cranial deformation and constraints, and not to real (primary) brain differences. And, if real cortical differences are involved, one can think that its longitudinal and vertical extensions are the result of the same process: overall larger (or smaller) precuneus. In this new study, we have analyzed its longitudinal, vertical and lateral proportions, as to see whether there is a unique morphological pattern, or else if those directions of growth are independent. Results support the latter conclusion: its relative development in length is not patently associated with its relative development in height or width. This may suggest that distinct areas (and functions) are involved. And, of course, that such variations are due to real (primary) brain features, and not to (secondary) deformation due to cranial factors. It remains to be evaluated to what extent the grey or the white matter do generate these spatial variations, and which specific areas are involved. The precuneus is much more developed and expanded in humans when compared with other primates, including apes, and its corresponding regions appear also larger in modern humans when compared with extinct human species, including Neandertals. The homology of its areas, when comparing humans, apes and monkeys, remain to be fully understood, although new stimulating information is now available, in this sense. Previous articles on the same topic dealt with precuneus general shape, sulcal patterns, population variability, brain-braincase relationships, primate variations and cortical surface area. Here a review on the evolution of the parietal lobes. The precuneus is mainly involved in visuospatial integration, and it represents a central node for the integration between brain, body and environment, as well as a crucial center for body cognition.

[Full-text View-only PDF]

Hands on the Lower Paleolithic

After our articles on hand-tool morphometrics, on tool handling and electrodermal activity, and on hand morphology and haptic perception in Lower Paleolithic tools, here a new study on ergonomics and finger flexion. By using a digital glove for coordinates, we analyzed the comfortable grasping position in choppers and handaxes. “Comfortable” doesn’t deal with tool functions, but instead with haptic exploration and ergonomics. This is not about using, but about sensing. According to the results, choppers and handaxes trigger distinct patterns of finger flexion. Interestingly, the last three fingers are particularly involved, and not the thumb or the index finger. When we handle a tool, it becomes included into the body scheme at cerebral level, extending our physical body to peripheral elements and adding the tool’s properties to our cognitive system. Therefore, different haptic patterns during grasping can also suggest distinct levels, types or grades of tool embodiment.

On brains and endocasts

A recent article by Dumoncel and colleagues provides an interesting comparison between brains and endocasts from the same subjects. Results suggest that correspondence is pretty good and able to reveal many cortical regions, at least when taking into account the sulcal pattern. The temporal lobes and the frontal regions are the ones with more information. The dorsal regions are instead where endocasts are less reliable, probably because of problems associated with brain deformation, gravity, vessels, or cerebrospinal fluid. The region of the superior sagittal sinus is the most problematic, and the superior parietal surface is often void of traces too (in this article we found the same problems with landmarking). The paper addresses some early disagreements on whether endocasts are a good proxy for brains, discusses major difficulties associated with this field, and reviews new methods for automatic sulcal detection. The study is based on 5 individuals only, but the results are definitely stimulating.

Homo erectus’ temporal lobes

Modern humans are supposed to display relatively larger temporal lobes when compared with other primates, but little information is available on extinct humans. The middle cranial fossa is apparently a good proxy to investigate temporal lobe size in fossil human species, and this time we have checked the situation in some available specimens associated with Homo erectus and Homo ergaster. Apparently, they both have relatively smaller temporal lobes when compared with our own species. Also, the Asian specimens show larger lobes when compared with the African ones although, in this case, the fossil record is too poor to provide significant evidence. Actually, the middle cranial fossa is extremely fragile, and therefore rarely preserved in paleoanthropology. Its floor bones are not so robust as the vault bones, and the spatial constraints with the facial block and mandible reduce further its thickness. Here a post with more information.

Asymmetric brain boxes

A new paper by Melchionna and colleagues provides new insights on the never-ending issue of the evolution of human brain asymmetries. The study supplies a comprehensive shape analysis of endocasts from many hominoid taxa, including fossils. Differences that go beyond the allometric consequences of a larger brain can be detected in Homo sapiens, Homo heidelbergensis, and Homo neanderthalensis. The brain is asymmetric in its anatomy and functions and, after more than a century of debate, we still don’t know if our degree and scheme of asymmetry is due only to a larger cortex, or else if there are some specific evolutionary adaptations. Of course, a unique (all hominoids) trend is not necessarily expected, and each species can have had its own, independent and peculiar story. Indeed, we should not forget that there is a major limitation in paleoneurology: if things are not clear for living primates and living brains, little can be probably done with fossil species and gross endocranial moulds. The anatomical and statistical uncertainties are, at best, unquestionable. In this sense, there is still a major biological issue which is largely unresolved: we ignore the nature of the endocast asymmetries. White matter or grey matter? Are neurons, glia or blood involved? Skull- or brain-induced? It is a physiological or biomechanical issue? Unless we have clear evidence on these aspects, the information on “how much” and “where” a cast is asymmetric may be but a secondary information.

Diploic growth

After our first survey on diploic channels in humans, here a second one on their growth and development. We have analyzed the ontogenetic variation in their length, lumen size and volume, in the frontal, parietal and occipital bones, and their correlation with skull size and bone thickness. Interestingly, there is a gradual increase of the vascular complexity, but a noticeable and outstanding spurt only in the adult stage. The development of these vessels is probably constrained by the thickness of the cancellous bone, and that’s why only in the adult stage we can observe a marked increase of their network complexity, as well as a marked increase of the individual diversity. If these vessels are involved in the thermal regulation of the endocranial cavity, their role is not patent until adulthood, at least if we consider their largest branches. Studies on the smaller ones are in course. It is worth noting that a large and complex diploic network is only observed in Homo sapiens, and not in living apes or extinct hominids. This may suggest some recent evolutionary adaptations. Here another post with more details.

Here a couple of recent reviews on craniovascular traits and anthropology and on craniovascular traits and human evolution. One paper specifically on the parietal bone, and a large survey of the prevalence of these features in modern populations.


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