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.

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.

Surfin’ endocasts

Endocasts and brains are difficult to analyze through traditional anatomical landmarks, because of the smooth morphology, blurred boundaries, and a noticeable individual variation. Currently, semilandmarks and surface analyses are good alternatives. Nonetheless, these two methods analyze the geometry of an “object”, ignoring its anatomical nature. If such geometrical modelling is interpreted too strictly, it may generate speculations and even incorrect conclusions. Numerical transformations behind spatial and geometrical models can be very complex and entangled, and the long chains of algorithms cannot be disentangled in any research article (the same occurs in any other field, like molecular biology, where long chains of reactions and engineering processes can’t be resumed in detail in every single paper and, necessarily, we must blindly rely on their proper functioning). In those many numerical steps, we must be aware that there may be incorrect passages, or simply algebraic assumptions that are not consistent with the real biological and evolutionary processes. More importantly, the brain is formed by so many independent elements, histological components, and cortical areas, and a pooled geometrical analysis can generate hybrid results. Anatomical landmarks are still necessary to mark boundaries and proportions, as to evaluate the real contribution of each element. Of course, anatomical landmarks are difficult to assess, they require experience, and they require inferences: as in every experimental paradigm, as in every field of science. Shape analyses deals with models, not with real anatomical entities. And models only take into considerations some specific properties of those anatomical systems, following algebraic rules that, right or wrong, represent conventional and operational assumptions. Here an opinion paper on all these issues.

At Holloway’s

Ralph Holloway is at the Columbia University (New York) since 1964. More than half century dedicated to paleoneurology, brain evolution, fossils and endocasts. Some weeks ago I was visiting his laboratory, an amazing place, full of books, experience, and history. And collections. Endocasts are everywhere, witnessing at once the evolution of the human brain and the evolution of the moulding techniques. So I took the opportunity to ask Ralph some quick comments for this blog …

Why do we still need physical casts? (but do we?)

I think for the most part, we can do very well with virtual endocasts (as long as these don’t get hacked…), although these can never provide the same haptic experience as a true cast, even if it is a good 3D print. At the moment, I am working on LES1, Homo naledi, and while I have a 3D print from scan data of the endocast surface, and good images provided by Heather Garvin, I am making an endocast from a 3D print of the cranial portion, as I need the best resolution I can get of the occipital portion in particular. Even micro-CT scanning doesn’t always provide the subtle variations on the endocast surface that are critical for correctly identifying convolutional details. The downside of course is possible damage to original specimens, and lack of sharing with colleagues at other institutions unless they visit the lab where these are made. Furthermore, the accuracy of virtual endocasts depends on the software, the researcher’s experience, expertise, and whether the algorithms used to correct for distortion, etc, are accurate.

What is the main current challenge in paleoneurology?

The major challenge is to synthesize the overall size data (ECV’s) with whatever sulcal and gyral information (e.g., lunate sulcus, fronto-orbital sulcus, Broca’s cap regions, etc) is available with morphometric analyses for each specimen with temporal and archaeological evidence, so that actual hypotheses can be generated than can be tested within (or even beyond) the paleoneurological community. This requires researchers fully cognizant of anatomical details, and both nonhuman primate and human neuroscience. Needless to say, but many more hominin and hominid endocasts need to be found and studied if paleoneurology is to become a better science.

Advices to those who begin working in this field …

Know your neuro- and cranial anatomies! Stay humble, lose your hubris, and keep in mind how rare endocasts are, and how imperfect these usually are, and how difficult, if not impossible it is to really know what the brain was like when the hominin was alive, and realize that you will probably never see an endocast that fully captures all the convolution details that were part of that once throbbing brain. A lack of hubris will be essential for good science, and don’t dismiss earlier works in paleoneurology simply because these are not modern or based on the last decade of morphometric advances. Staying up to date, or being current with the findings coming out of neuroscience will be particularly difficult.

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.


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