Archive for the 'Skull' Category

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.

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.

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.

Skulls and brains

A recent study by José Luis Alatorre Warren and colleagues supplies one of the very few analyses on both skull and brain morphology from the same individuals, in humans, chimpanzees, and gorillas. They analyzed the shape variation of the brain and skull separately, and then at the same time. The study describes the spatial relationships between brain and skull, and the differences between species. The study confirms once more that there is a limited correspondence between cerebral and cranial anatomical boundaries and references. According to their results, the most striking difference between humans and chimpanzees is, in the former, a spatial change of the frontal lobe regions associated with language. They also found a larger parietal cortex in humans when compared with apes but, according to the model they use, these differences would be not so remarkable as suggested by most of the preceding studies. However, their geometrical model is very comprehensive and it includes many distinct and independent elements of the brain and of the skull, that are melted and averaged when all their coordinates are superimposed together. In these cases, punctual or local differences are spread onto the whole global variation, and can be hardly detected. Namely, these extensive shape registrations are excellent to analyze general covariation patterns, but can be tricky when trying to identify the contribution of local regions or of specific anatomical elements in a heterogeneous and multifactorial anatomical complex.

In my opinion, the article suffers, unfortunately, some important conceptual limitations. There is a confusion between Ralph Holloway’s theories on parietal evolution in australopiths (largely based on the position of the lunate sulcus) and my own ones on parietal evolution in modern humans (based on the size and proportions of the parietal lobe). It seems obvious to say, but we are dealing with distinct taxa (australopiths and modern humans) and distinct processes (early hominid evolution and late human specialization), and therefore there is no reason to look for common factors or mechanisms. For example, when taking into account the possibile influence of posture on brain shape, the evolution of bipedalism may be an issue for australopiths or early Homo, but not when comparing modern humans with Neandertals. Also, there seems to be a constant misunderstanding between parietal cortex enlargement (which involves parietal lobe absolute and relative size) and brain globularization (which refers to the rounded shape of the brain). It should be clear that these features can be influenced by (and due to) different factors, although they concern the same anatomical regions. Finally, there is a frequent confusion (and miscitation) between papers and results on parietal bones, and papers and results on parietal lobes. So, in my opinion, we are dealing with a study which supplies an amazing analysis, but a problematic discussion of the results. I think this study would have seriously benefited from a more cautious interpretation of the numerical outputs, and a more careful integration of the literature. Nonetheless, this exceptional database is there, and I hope it will supply in the future more information on the spatial relationships between brain and braincase.

Craniovascular traits

The vascular imprints on the endocranial surface, the diploic channels within the vault bones, and the emissary foramina of the cranial cavity, are used to make inferences on blood flow in archaeological and paleontological samples. Unfortunately, basic information on many of these craniovascular traits (variation, distribution, homology, development, influential factors and even functions) is still poor for our own species. It sounds unpractical to investigate a feature in a few broken bones of an extinct species, if the same information is lacking for many billions living individuals. A former study was focused on the parietal bone. Now we have published a comprehensive endocranial survey on these traits in two modern European populations, to supply more information on their variability and on the influences of skull size and proportions, asymmetry, or sex. Blood flow exchange within the endocranial cavity may be relevant for thermal regulation and brain cooling. The final aim is to establish what functional or structural factors are involved in the morphology of these vessels and of their bony traces, as to interpret differences in extinct species or past populations. Many of these features bridge interests in anthropology and medicine.

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.

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.

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.

Sulcal imprints

The fuzzy geometry of the brain surface shapes the endocranial wall, and endocasts can show traces and imprints of the cortical sulcal patterns. Individual variation is noticeable, and the precise mechanisms behind these folding schemes are not clear at all. Hence, it is not recommended to use this information in a simplistic “phrenological” fashion, as unfortunately it has been done in many evolutionary studies. At the same time, cortical morphology is the direct result of neurons growth and development, and therefore even the pretentious rejection of this information seems unwise. Many authors dismiss any result based on brain gross morphology, simply because it is “just brain form”. This is probably because they ignore the developmental processes behind that forms, and they don’t take into account that when we talk about “brain form” we are implicitly referring to those processes, and not to a crude geometrical appearance. At least, sulcal patterns are useful (and the only available macroscopic) boundaries to detect the absolute or relative extension of some cerebral districts or cortical areas. So, despite all the uncertainties, they are directly providing information on cortical proportions. Proportions means “some areas are larger and some others are smaller”. Size is not always a matter of more or less neurons, but it is however matter of more or less “something”. Whatever it is, it should be functional, and maybe even adaptive some way, associated with some specific histological factor, or with some indirect physiological consequence. This is why the issue is not trivial.

Sulcal imprints are generally more visible on smaller and younger skulls. A recent study investigates the expression of the sulcal traces in macaques. Anterior folds (frontal and temporal lobes) leave more traces than the posterior ones (parietal and occipital). There are no many differences among young ontogenetic stages but then, during aging, the expression of the traces decreases noticeably, and imprints become more blurred. Local anatomical differences in the barrier between brain and skull (meninges, vessels, etc) can have a role in this size-related differences. Nonetheless, probably it is a matter of growth. In earlier ages, the brain generates a constant pressure on the vault bones, shaping the bone surface. But in later ages, when brain growth is concluded, that intimate physical relationship is looser. During aging, the brain even undergoes a shrinkage of about 7-8%, and the contact is further lost. This study is simple and effective, a good paper to approach the topic. Between an uncritical phrenological approach and a snobbish rejection of the evidence, we should consider an intermediate approach, in which we evaluate what kind of information we can obtain from these traits. To do that, we have to investigate their phenotypic factors and their mechanical influences, their structural associations and their variability.

Vault and base

This week we have published a study on the integration between the parietal and temporal morphology in the human skull. Modern humans display large and bulging parietal lobes and bones, and large and projecting temporal lobes. It is possible that the latter (the anterior displacement of the temporal lobes) can be a spatial secondary consequence of the former (the enlargement of the parietal district). This survey on adult skulls suggests that this is not the case: parietal bulging and temporal displacement are apparently independent and not related. Nonetheless, when one of these districts undergoes a major variation (bulging in the case of the parietal bone, vertical stretching in the case of the middle cranial fossa), the other undergoes a spatial rotation: shape does not change, but the orientation varies according to the global cranial modification. The enlargement/reduction of the parietal bone have thus a major effect on head orientation, and it is also associated with facial proportions. Hence, it turns out that the general enlargement of the parietal district, a species-specific character of modern human brain and skull, has probably influenced the functional axis of the head, with possible consequences on body organization and posture. These results once more recall the importance of an integrated analysis between brain and braincase and, more generally, of a system-based approach to functional craniology.


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