Parietal cortex

In November 2017 Ashley Morhardt organized a Karger Workshop at Hyattsville (USA), entitled “From fossils to function: integrative and diverse approaches to vertebrate evolutionary neuroscience“. The workshop was included in the activities of the J. B. Johnston Club, and papers are  now published in Brain Behavior and Evolution. My contribution is a review on the evolution of the parietal cortex in the human genus. Articles will be freely accessible for the next six months. Have a look!

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Neanderthal’s visual cortex

A recent study published by Antonio García Tabernero addresses issues about the morphology of the occipital lobes in Neanderthals. It is an anatomical description of a new occipital fragment from El Sidrón, and a morphological analysis of the Neanderthal occipital variation. Neanderthals display more occipital asymmetries than other hominids, including in the vascular pattern of the venous sinuses. They have been also hypothesized to have larger occipital cortex when compared with modern humans. It remains to be evaluated whether differences in the occipital lobe morphology can suggest distinct functional capacity, and here is a stimulating article right on this topic. However, it should be considered the possibility that occipital proportions in Neanderthals are not a derived feature, but a plesiomorph human (Homo) condition, being modern humans the ones who depart from this scheme.

Electrodermal archaeology

After all those surveys on parietal lobes and parietal evolution, some years ago we began investigating some functions particularly associated with the parietal cortex, and generically labeled as visuospatial integration. Some visuospatial behaviors can be inferred in fossils, according to anatomical and archaeological evidence. In my lab, we are interested in aspects bridging cognition, body, and tools. In a recent paper published in Progress in Brain Research we have applied electrodermal analysis to investigate the cognitive response during a haptic experience with stone tools. Electrodermal signals have been employed here to evaluate changes in emotion and attention during stone tool manipulation, as to evidence whether different tools exert different cognitive responses when handled. New methods for cognitive archaeology!

 

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The paper is part of a volume entitled “Cerebral Lateralization and Cognition: Evolutionary and Developmental Investigations of Behavioral Biases“, edited by Gillian Forrester, William Hopkins, Kristelle Hudry and Annukka Lindell.

Naledi

Ralph Holloway and colleagues have just published a paleoneurological study of Homo naledi. They used seven cranial portions from at least five individuals to provide a general view of an endocast of this species. The study is comprehensive and very detailed, indeed. It turns out that, despite the very small endocranial volume (about 500 cc), the brain general organization is very similar to all the other human species. Beyond some particular features in Neanderthals and modern humans, all human (Homo) species display the same general sulcal pattern. If there were differences in their sulcal organization, these should have been pretty minor or hardly recognizable on an endocast, at least according to what we can test with the small samples generally available in paleoanthropology. So, it is not surprising that Homo naledi has a Homo brain form. But the interesting thing is the association between a human brain morphology and a small brain size, as suggested by this current study. If true, we have two main conclusions. First, our brain cortical complexity and our large brain size are two independent features. They have evolved together in many cases, but not in others. Second, our human cortical folding scheme is not simply an allometric (scaled) version of the apes’ one. Cortical folding is largely influenced by mechanical factors, most of all size-related effects, so one could think that our brain morphology, although distinct from apes, is a secondary consequence of having a big brain. The results presented in this study suggest that this is not the case. We humans have a specific cortical organization and, furthermore and additionally, a big brain too. Reasonably, both features have an influence on our cognitive capacities.

Of course, these results must be confirmed on a larger perspective. Remember that here we don’t have a “brain”, but some scattered endocranial surfaces of a few specimens. That’s not sufficient to reach detailed and reliable conclusions on the brain itself, not to say on cognition. Also, the species Homo naledi (and its chronology) is at present strictly associated with one specific site and needs further corroboration from a wider geographical scenario before supporting firm or generalized statements. Its striking feature is the very small brain size. In this sense, it is worth noting that we often use to mention “average” values, sometimes forgetting about their associated variation and variability. We modern humans have a normal cranial capacity spanning a range of more than 1000 cc. In this paper, Holloway mentions the case of Homo erectus, spanning from 550 cc to 1200 cc. Therefore, caution is still necessary when interpreting the small brain size of these individuals. Of course, the fact that this species (as the Flores hominid) could have undergone brain size reduction or small brain retention does not point against the importance of brain size and encephalization. According to the available fossil record, most human species bet on big brains. Exceptions are expected, but do not break the rule.

I want to focus on one more aspect of this article. Although the topic was definitely “sexy”, the authors avoided any speculation on cognition or phylogeny. Such attitude is so professional and definitely welcome, thank you!

Neanderthal brains

After their chapter on the book Digital Endocasts, Kochiyama and colleagues have published this week a comprehensive reconstruction of a Neanderthal brain. An outstanding example of quantitative paleoneurology, indeed! They deformed our modern human brain into a Neanderthal endocranial cavity, as to allow an estimation of cortical volumes and proportions. They confirm that modern humans have larger parietal lobes and larger cerebellum, and that Neanderthals could have had larger occipital lobes. They also confirm that early modern humans did not display a modern human brain form. Of course, this simulation is based on the assumption that no specific and localized cortical changes have occurred along both modern and Neanderthal lineages since their separation. The assumption is a reasonable simplification, and is necessary to provide a shared comparative framework. Nonetheless, if specific and localized changes have occurred in one or both lineages, that one-to-one spatial fitting will lost local predictive power. In terms of brain anatomy, local cortical changes can actually occur as genetic adaptations to selective processes or else as induced plastic feedbacks in response to environment (including culture). Also, we must always consider that many brain regions (the cerebellum is one) have a gross morphology that is in part influenced by cranial constraints. It may be hence difficult, in some specific endocranial districts, to distinguish between brain cortical variations and cranial effects.

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.

Digital Endocasts

A new Springer book: Digital Endocasts: from skulls to brains. Chapter 1 (Holloway) is an introduction to physical casting. Chapter 2 (Ogihara et al.) deals with digital reconstructions of Neandertals and early modern humans’ endocasts. Chapter 3 (Kobayashi et al.) is about inferences on cortical subdivision from skull morphology. Chapter 4 (Beaudet and Gilissen) introduces paleoneurology on non-human primates, and Chapter 5 (Walsh and Knoll) is on birds and dinosaurs. Chapter 6 (Rangel de Lázaro et al.) reviews  craniovascular traits. Chapter 7 (Bruner) is on functional craniology and multivatiate statistics. Chapter 8 (Gómez-Robles et al.) concerns brain and landmarks, and Chapter 9 (Pereira-Pedro and Bruner) concerns endocasts and landmarks. Chapter 10 (Dupej et al.) is on endocranial surface comparisons. Chapter 11 (Kochiyama et al.) presents computed tools to infer brain morphology in fossil species. Chapter 12 (Neubauer and Gunz) deals with brain ontogeny and phylogeny. Chapter 13 (Bruner et al.) is on an application of network analysis to brain parcellation and cortical spatial contiguity. Then, there are chapters dedicated to the evolution of the frontal lobes (Chapter 14 – Parks and Smaers), of the parietal lobes (Chapter 15 – Bruner et al.), of the temporal lobes (Chapter 16 – Bryant and Preuss), of the occipital lobes (Chapter 17 – Todorov and de Sousa) and of the cerebellum (Chapter 18 – Tanabe et al.). The aim of the book is to provide a comprehensive perspective on issues associated with endocasts and brain evolution, and to promote a general overview of current methods in paleoneurology. The book has been published within the series “Replacement of Neanderthals by Modern Humans“. Here on the Springer webpage.


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