One year ago we showed that a main source of variation among adult human brains is due to the proportions of the precuneus. This seems to be a stand-alone feature, not integrated with other patent morphological changes of the brain form. The spatial pattern associated with the dilation/contraction of the precuneus is particularly similar to the parietal bulging characterizing the brain of our species in evolutionary terms. Now we have published a study of the anatomical factors associated with this shape change, namely an analysis of the whole precuneal volume (cortical surface and cortical thickness) in a sample of adult humans. The results suggest that the observed changes of precuneal shape and proportions are associated with actual changes in precuneal surface area. Therefore, it is a matter of absolute cortex volume, and not just of relative size. There are no differences in cortical thickness. The precuneal volume increases with positive allometry as brain size increases (that is, it increases more than the rest of the brain, as brains get bigger), but the individual differences – as well as the differences between hemispheres – are important. What is the cellular reason of such morphological variation? Number of neurons, connections, or other components?
Interestingly, such marked anatomical variation seems not influencing any standard psychometric variable. It can be hypothesized that traditional psychometric performances are not adequate to quantify the functions of the precuneus. This is likely most of all when taking into consideration its importance in the default mode network, which functions are not easy to capture with task-based metrics.
Published December 7, 2014
Simon Neubauer has recently published a review on endocasts. There is a general and effective introduction to functional craniology and paleoneurology. Then, some debated case-studies in paleoanthropology are presented and discussed, following a welcome and sensate objective approach. Computed tools and methods are also briefly introduced. Finally, evo-devo concepts and principles are applied to current paleoneurological data, integrating life-cycles and morphogenesis. This is a very useful article for teaching, and for those who are looking for a quick updating on applications and perspectives in paleoneurology. You can find another recent review of Simon Neubauer in the book Human Paleoneurology, a chapter entitled “Human brain evolution: ontogeny and phylogeny”.
Despite most than one century of studies, the taxonomic and phylogenetic status of Homo erectus is still largely debated. There is no agreement whether or not the African and Asian specimens belong to the same species, or on the meaning of the variation within the Asian group. The relevant influence of a shared allometric component, the large geographic and chronological span, the marked individual and idiosyncratic variability and (most important) the small sample size, hamper any definitive conclusion. Because of all these actual limits, we should seriously consider if our insistence in searching fixed and stable taxonomic certainties represents a necessary and useful effort. Because of these limits, probably paleoanthropology should rely on a different approach to taxonomy, more centred on the actual information available than on hypothetical and conceptual schemes. In the meanwhile, this week we publish a general review on Homo erectus paleoneurology. We describe the general morphology of the African and Asian Homo erectus endocasts, providing a quantitative perspective of their variation and variability by means of traditional endocranial metrics. As expected, no patent differences are evidenced among different geographic groups, being size and allometry the main source of variation. Of course, the absence of morphological differences in the endocasts does not necessarily means the absence of differences in brain organization, and it does not give information on the underlying taxonomical structure. The limits of the sample size are evident: a power analysis suggests that, beyond the issue of biological representativeness, for a simple variable like cranial capacity groups of at least 40 specimens would be necessary to deal with the statistical uncertainties! Nonetheless we can now state that, at least according to the current metric information, all the possible taxa included in the Homo erectus hypodigm share similar endocranial proportions.
Neuroanatomical evidence suggests that we have relatively larger temporal lobes when compared with the apes’ allometric brain variation. Actually, there are also some form differences in our middle cranial fossa, housing the temporal lobes. However, the morphology of the middle endocranial fossa is influenced by many factors involved in the cranial base phylogeny and ontogeny, and we can wonder whether it strictly represents, in terms or direct linear variations, corresponding changes of the temporal lobes. The structural relationship with the underlying mandible is just one of the many non-neural influences of the middle endocranial area. Nonetheless, the middle endocranial surface can also provide information on the sulcal pattern of the temporal cortex, now further investigated by Antonio Rosas and Markus Bastir. In this case, the resulting morphology is more likely to be the direct consequence of brain morphogenesis and cortical organization, being less influenced by structural cranial constraints. That is, possible species-specific differences in the sulcal pattern can be more easily interpreted in terms of intrinsic brain factors (independently upon their functional meaning), more than in terms of extrinsic secondary consequences of the complex spatial dynamics of the endocranial base.
One of the main achievements in anatomy and morphometrics has been the introduction of the concepts of integration and modularity. Characters (and genes) are no longer interpreted as individual and independent units, but integrated into structural and functional systems. This does not mean that everything is integrated, and we should recognize that integration and modularity are based on both continuous and discontinuous hierarchies presenting many different degrees of relationship. There may be distinct combinations, and very different situations. Analyzing the structure of covariance of the endocranial base in modern humans, I suggested that local influences can be more relevant that general and long-range factors, in shaping the endocranial districts. An admixture of effects from brain, face, posture, muscles, physiology and biomechanics, makes local factors decisive to mould the specific endocranial areas. Similar results were obtained when analyzing the covariance patterns of the midsagittal brain morphology. Now Aida Gómez-Robles and colleagues have published a decisive analysis: the whole brain in three dimensions. The integration among brain parts is modest, and largely based on spatial proximity. Local factors are crucial in moulding the brain areas, at least in terms of their morphology and position. Initially one can be deceived if expecting to find a more integrated system. But at the same time such an independent organization suggests that local form changes can be analyzed considering the local context, the morphology being less contaminated by external (long-range) effects. Interpretations are easier if only local factors must be evaluated. This is something extremely relevant when dealing with evolutionary neuroanatomy, paleoneurology, and functional craniology. And then there is an issue on evolvability: the authors suggest that such limited integration can facilitate evolutionary changes.
The book “Human Paleoneurology” is now available on the website of the Springer Series in Bio-/Neuroinformatics. There is an introduction by Ralph Holloway, evidencing some open questions on endocasts. Laura Reyes and Chet Sherwood discuss current issues in evolutionary neuroscience. Philipp Gunz talks about computed methods and digital tools used to reconstruct and compare brain forms. I present a review in functional craniology, namely on the structural relationships between brain and braincase. Simon Neubauer focuses on brain evolution in ontogeny and phylogeny, dealing with the variations in brain size and shape. Natalie Uomini provides an archaeological perspective on behaviour. Erin Hecht and Dietrich Stout supply a detailed description of methods and topics in neuroarchaeology. Fred Coolidge, Tom Wynn, Leee Overmann and Jim Hicks present an overview in cognitive archaeology. A set of images by José Manuel de la Cuétara shows cranial and endocranial digital reconstructions of living apes and extinct hominids. Here a list of chapters and authors. This is a comprehensive collection of papers useful for anyone interested in approaching this field, good for teaching and helpful to present the current state-of-the-art of this discipline.
Together with Marina Lozano (IPHES), this week we have published a JASs Forum on a speculative hypothesis concerning the use of the mouth in support to praxis and handling in Neandertals and their ancestors, as evidenced through the analysis of their dental marks. This behaviour, very common in Homo neanderthalensis and Homo heidelbergensis, is not so frequent in modern hunter-gatherer. According to the theory of extended mind, cognition is the result of the interaction between brain and environment as mediated by the experience of the body. The main “ports” of such interface are the eye (input, from the world to the brain) and the hand (output, from the brain to the world). Modern human brain displays a peculiar dilation of the deep parietal areas, which are particularly involved in visuo-spatial integration, which includes the management of the eye-hand system, the integration with memory, and the integration with frontal executive functions. Hence, we suggest that the necessity of a further additional element (the mouth) may be necessary when the standard anatomical elements are not sufficient to integrate the body relationships with the cultural complexity. A mismatch between the biological substrate (neural system/body interface) and cultural substrate (complex tools and behaviours) could have been the backstage of a risky involvement: the mouth as integrative body support. The investment is not safe, considering the importance of the mouth in different and relevant functions, and it sounds like an extreme solution. Neandertals do not show a similar enlargement of the parietal areas, when compared with Homo sapiens. Although we ignore the exact relationship between brain form and function, the fact that these areas are crucial for visuo-spatial integration is, at least, intriguing. Needless to say, a possible mismatch between neural and cultural systems in Neandertals should not be interpreted as an “intermediate” condition between archaic and modern forms, but else as a lack of proper coordination associated, as far as we know, with an evolutionary blind alley.
The hypothesis has been commented by Lambros Malafouris, Marco Langbroek, Thomas Wynn, Fred Coolidge, and Manuel Martin-Loeches. Next issues to be considered: details of the hand anatomy and hand management, early modern humans associated with Mousterian tools, and functional behaviours in those modern populations that use mouth and teeth for praxis. Hypotheses in cognitive archaeology are necessarily speculative. But we can try nonetheless to supply multidisciplinary evidence to integrate paleoneurological and archaeological data, providing at least a logical framework. In this case the next step is clear: to evaluate further this hypothesis we have to investigate more visuo-spatial behaviours in these extinct forms.
[You can download here the whole forum]