Archive for the 'Endocasts' Category

Monkeys’ parietals

Parietal lobes are specialized in primates, and particularly in humans. Nonetheless, the information on their anatomical variation is still scanty. In non-human primates, parietal cortex is investigated only in few species (generally, macaques) and mostly at the histological level. Now we have published a morphometric analysis on the parietal lobes of 11 cercopithecid genera. The study was performed on endocasts, as to broaden the conclusions to the fossil record too. Parietal differences among the main subfamilies have been described before, even in fossils, but without a detailed quantitative analysis. The main shape changes separate genera with large occipital lobes and small parietal lobes (cercopiths) from species with large parietal lobes and small occipital lobes (colobuses and baboons). Allometry is apparently not involved in this feature, and size increase is only associated with taller endocasts (probably due to cranial – not cerebral – factors). These different parietal-occipital proportions are supposed to be related to distinct cognitive organization, hypothetically influenced by diet and locomotion. It would be hence interesting to test the effect of different parieto-occipital ratios on specific behaviors and cognitive capacities. More body or more vision? Different views of the world …

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The Pit

Eva Poza Rey and colleagues have now published a detailed paleoneurological survey of the endocasts from Sima de los Huesos, now dated at 430 ky. The study includes anatomical descriptions and a multivariate analysis of endocranial diameters. Fifteen years have passed since that early article on Sima 4 and Sima 5, showing that these two endocasts had an archaic phenotype, apparently missing any Neanderthal derived trait. This new article definitely increases the sample size, including a total of 16 endocasts. Results suggest that the endocasts from Sima de los Huesos display an intermediate morphology, between the human plesiomorphic brain form (like in Homo erectus or Homo heidelbergensis) and Neanderthals. The difference from an archaic condition would be in the posterior an inferior brain regions (posterior temporal, inferior parietal, and anterior occipital cortex), larger and wider in the Sima brains. Their endocranial size further supports the hypothesis that in Neanderthals encephalization was a gradual process. I think in the article there is probably too much space dedicated to asymmetries. Taking into considerations that all human species display a similar pattern of gross asymmetries, that differences between humans and apes could be a matter of allometry, that hemispheric differences can be very subtle and hence would require huge samples to be properly tested, and that in fossils we can only observe the superficial cortical dimensions with no information on the anatomical factors involved, I frankly can’t understand why this topic keeps on deserving so much attention in many paleoneurological papers.

The study is comprehensive and convincing, although I personally miss two points. First, there is no mention on the overall morphology of the frontal lobes, except some minor comments on the orbital gyri and frontal length in Neanderthals. Although there is no clear evidence of frontal expansion in the evolution of the human genus, Neanderthals and modern humans display relatively wider frontal cortex, probably because of a spatial constraint with the underlying orbits. In this aspect, the Sima endocasts show an archaic morphology, with narrow frontal lobes. Second, I would be really interested in a comparison with the endocast of Maba (China), which combination of traits is remarkably similar to Sima de los Huesos, showing Neanderthals features in the face but an archaic brain form. Convergence, same taxon, or shared ancestors?

Ileret

I had missed this amazing study (2018) from Neubauer and colleagues on the skull and endocast KNM-ER 42700 from Ileret, Kenya, with a chronology of 1.5 million years. They performed a set of reconstructions for the skull and endocast, and compared these figures within the diversity of early hominids. Their results are quite convincing, and confirm that the specimen is definitely out of the range of variation of adult Homo erectus. Overall, the endocast is somehow similar to the endocast of KNM-ER1470 (H. rudolfensis). However, its morphology also lies midway along an ontogenetic trajectory going from Mojokerto to adult H. erectus. So, the taxonomy of KNM-ER 42700 can be uncertain, but the most likely explanation is a H. ergaster/erectus of a young age, much younger that predicted on the basis of other cranial features. Taking into consideration the approach based on multiple reconstructions, the multivariate shape toolkit, and the disclosure of the ontogenetic trajectory of H. erectus, this paper is definitely a great paragon in paleoneurology. A review on metric endocranial variation in H. erectus was published some years ago here.

Little Foot

The endocast of the australopith StW 573 is pretty complete, and now Amélie Beaudet and colleagues have published a very detailed and comprehensive anatomical analysis of its features. For many paleoneurological traits we still miss a reliable knowledge on intra- and inter-specific variation but, according to what we can currently see in Australopithecus, Paranthropus and chimpanzees, StW 573 does not display derived sulcal patterns in the frontal and parietal regions. Its overall endocranial form resembles the morphology of some Paranthropus specimens, although in this case there are still some issues on deformation and possible taphonomic effects (specially at the frontal bone). The study supplies a careful description of the vascular patterns, in particular for the middle meningeal artery. In humans, only our species has generally a complex vascular network, while vessels are more scarce and less connected in extinct human taxa. Nonetheless, these same vessels (or, at least, their analogous networks) are more developed in apes. Therefore, australopiths are a key group to understand what happened with these traits, and to assess the polarity of these features in the evolution of distinct hominoid branches.

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.


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