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.

Karl Zilles

Karl Zilles (1944–2020):
a personal tribute

by Laszlo Zaborszky

Karl Zilles, founding co-editor-in-chief of this journal [Brain Structure and Function], died on April 26, 2020. He was born in Würzburg, a small city in Bavaria with famous baroque architecture that was heavily damaged by allied bombing during WWII; he often visited his parents in this town. We first met in the mid 1970s, while I was on leave of absence from Semmelweis University Hungary, and I was working in the University of Würzburg anatomy department, located in the Koellikerstrasse, named after the famous anatomist whose hand was X-rayed by Roentgen, another famous professor at that university. We also met in many of the annual deliberations of the German Anatomical Society, of which he was president in 1996/1997. He followed the best tradition of German neuroanatomical research in the cortex, but he realized the shortcomings of Brodmann’s map (Brodmann 1909) that needed to be solved to be able to interpret the burgeoning in vivo human imaging data: (i) lack of observer-independent quantification, (ii) lack of data on the intrasulcular surface that makes up 2/3 of the cortical surface, (iii) lack of knowledge on interindividual differences, and (iv) lack of conversion from 2D to 3D …

[Keep on reading this article on Brain Structure and Function]

 

Brain growth and pressure

A recent study by Jimena Barbeito‑Andrés and colleagues takes into consideration the effect of brain growth on the cranial bones, according to a model of equal pressure (homogeneous growth) and to a model of differential pressure (real brain form changes) between 0 and 14 years of age. Main growth surfaces were identified in the frontal, parietal, occipital and cerebellar regions. According to their results, a homogeneous brain growth would exert a main pressure at the midsagittal (metopic) frontal region, and at lambda. Instead, a real growth pattern concentrates expanding pressures only at bregma and lambda. Is this a cause or a consequence of suture organization? These models are necessarily simplified, because they assume, for example, a passive structural role of bones, sutures, and connective tissues. Nonetheless, they are extremely useful to “think about” the brain-braincase biomechanical system, and to promote a proper quantitative framework to test hypotheses on brain and endocast morphological variations. Here another post on modelling brain-skull interface, a study on brain structure and topology, and a recent article on mechanical loads in dural folds and calvaria.

3D parietal lobe evolution

This week we have published a 3D shape analysis of the parietal lobes in Neandertals and modern humans. Parietal cortex is larger and more complex in humans than in other primates, and it is associated with visuospatial functions ranging from visual imaging to brain-body-environment integration. When compared with Neandertals, modern humans display, on average,  larger and expanded parietal lobes. According to this new shape analysis, two regions may be particularly responsible for this morphological differences, namely the dorsal and posterior region (the posterior part of the precuneus, on the superior parietal lobule, involved in the most visual part of body-environment integration) and the Jensen region of the inferior parietal lobule (a branch of the intraparietal sulcus, involved in eye-hand integration). Such differences in terms of brain development and proportions may be associated with differences in the visuospatial behaviours of these two human species. Here more details, and a 2018 review on the evolution of the parietal lobes in the human genus.

Temporal lobes and primate paleoneurology

Humans display relatively larger temporal lobes when compared with other primates, and this suggests an evolutionary expansion of their areas and functions in our lineage. In paleoneurology, the size and proportions of the temporal lobes are indirectly extrapolated from the morphology of the middle cranial fossa. However, the middle cranial fossa may not properly represent the volume of the temporal lobes, because only a portion of the temporal cortex is housed in its space. Furthermore, the morphology of the middle cranial fossa is constrained by many structural (non-neural) factors associated with the biomechanics of the mandible, with the position of the facial block, and with the balance of the cranial base flexion. We have now published a study on the correlation between temporal lobe dimensions and middle cranial fossa in primates. Results show that the correlation is pretty strong, therefore suggesting that the size of the middle cranial fossa can be actually used as a good proxy for the size of the temporal lobes in extinct species. Here more details on this study. It is worth remembering that, anyway, “lobes” are but conventional regions, with no biological or functional meaning. What we call “a lobe” is a mix of areas with distinct and partially independent functions, that have evolved according to different reasons and mechanisms, with different combinations of features in different lineages. Accordingly, the enlargement of a “lobe” can be due to many distinct evolutionary factors, involving different parts, distinct functions, and several elements (grey matter, white matter, glia, blood etc.). “Lobe evolution” must be therefore intended as a crucial but very general information, a first step to focus the attention on aspecific brain region, that is nonetheless formed by a heterogeneous system of cerebral components.

Australopithecus afarensis

Amazing paper by Gunz and colleagues on the endocasts of Australopithecus afarensis. Good reconstructions, a lot of brain anatomy, and human-ape comparisons. They conclude that australopiths may have had a prolonged brain growth, but the general macroscopic organization (the sulcal pattern and major cortical proportions) was definitely similar to apes. Or, at least, possible differences are apparently not visible in terms of gross anatomical features.

Endomaker

Antonio Profico and colleagues have just published a paper introducing Endomaker, an algorithm for automatic extraction of cranial endocasts and the calculation of their volumes. This is an implementation  based on AST-3D that  recognizes the endocranial cavity without the user intervention. A bounding box is defined around the mesh of the skull, and then converted into a three-dimensional grid of binarized voxel. High density regions (bone) are then separated by low density regions (empty spaces) through a count of surrounding voxels. The larger volume of contiguous empty voxels is then identified, and AST-3D algorithm is employed in order to produce the digital endocast. In this article, this approach is tested on Homo sapiens, Australopithecus africanus, Macaca mulatta and Lemur catta, as well as in a rat, a partridge, a dolphin and a dog. Have a look!

 

 

Meganalyses

In the last decades, computed and digital methods are providing amazing tools for anatomy, a field that is enjoining new challenges, but most of all a field that is rediscovering many old – unresolved – questions. In general, macroscopic anatomy still suffers from a profound lack of basic knowledge, and we don’t know the functions and variability of many morphological features, including for our own species. That’s why every small information, in this sense, can be outstanding. The current bubble of science marketing, however, is not sufficiently fuelled by small or isolated chunks of information, and it requires “sexy” scoops and massive products. As a result, in the gold rush to financial support and social recognition, researchers and institutions are aiming at providing, in one single article, the largest amount of data. Such huge studies often involve never-ending lists of methods and techniques that no single person can entirely control and guarantee (sometimes with patent consequences in their technical reliability or scientific interpretation). Paradoxically, because the product must be easily sold to the information agencies, it must also be fast and short. As a consequence, many of those entangled methods and results end up buried and forgotten in remote supplementary files, or quickly mentioned in a synthetic figure caption. Many of those little pieces of these mega-analyses (like a correlation plot, an average, a group comparison, a regression parameter, a coefficient of variation or a punctual observation) are crucial, illuminating and revealing, but they serve to a greater campaign, and must be sacrificed as minor passages for the sake of the big scoop. As transitory and secondary parts of the game, they will pass unnoticed, undiscussed, and uncomprehended. What a pity!