Archive for the 'Paleoanthropology' Category

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!

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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.

Australopithecus africanus

Amélie Beaudet and coauthors have published a shape analysis of the endocranial surface of two specimens included in the hypodigm of Australopithecus africanus, namely Sts 5 and Sts 60. They used a deformation-based approach, as to quantify the global endocranial surface variations. They compared the endocasts of the two specimens with modern humans (Homo sapiens) and chimps (Pan troglodytes and Pan paniscus). The endocranial geometry of the two piths is similar to the shape displayed by the apes. The incomplete Sts 60 lies right in between the two chimpanzees, while Sts 5 is slightly displaced toward the human form. The main difference between humans and the two australopiths can be localized in the parietal surface (larger in our species). Nonetheless, piths may present some minor shape difference on the frontal (orbital) cortex, when compared with chimps. Back in the ’20s Raymond Dart stated that, if extant African apes show a primitive scheme, a displacement of the lunate sulcus in Australopithecus might suggest a relative enlargement of the parietal cortex in these early hominids. Unfortunately, there are still disagreements on this point, because of the uncertainties associated with the identification of sulcal landmarks on fossil endocasts. Also frontal changes have been already proposed for other australopithecines. However, in this case the spatial interaction between frontal cortex and the facial system (orbits and ethmomaxillary block) may hamper a straight separation of the effects due to actual brain changes and those due to cranial constraints.

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.

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.


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