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.
Tags: Homo erectus, Homo ergaster
Tags: Antonio Rosas, functional craniology, Markus Bastir, middle cranial fossa, Temporal lobes
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.
Tags: Aida Gómez-Robles, Modularity, Morphological integration, Neuroanatomy
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.
Tags: embodiment, extended mind, Homo heidelbergensis, Neandertals, parietal lobes, visuospatial integration
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]
Tags: Brain shape, geometric morphometrics, Landmarking
There is an interesting analysis by Madeleine Chollet and colleagues on brain landmarking. They analyzed and quantified intra- and inter-observer error when collecting coordinates from 3D digital brains. Results are encouraging, evidencing average errors of 1.9 and 1.1 mm, respectively. However, values are much variable, depending upon the specific brain area. Generally, midsagittal landmarks show less uncertainty than parasagittal ones, and subcortical landmarks are more reliable than cortical ones (good for me: that’s why most of my papers are on midsagittal and subcortical morphology!). The analysis has been performed only on 10 specimens, and larger samples can surely add to the current results. Furthermore, in this study only the left hemisphere has been considered, in which sulci and gyri are generally easier to recognize. Despite these limits, the paper supplies clear results and useful comments. These kinds of analyses are necessary to promote quantitative perspectives in landmark-based morphometrics. For an excellent example of quantitative approach in paleoneurology I recommend to see the article by Simon Neubauer and colleagues on australopiths’ brain size estimations.
Back in 1863 Thomas Huxley stressed the relevance of the cranial base to compare cranial form among human populations. He even performed a pioneering hand-made superimposition of midsagittal profiles (see the side image of this post) by using references from the cranial base angle. The cranial base represents the central structural node of the cranium, integrating face and vault from one side, head and body from the other. Debates on its role and changes during human evolution are still constrained by limited information on its normal variation, growth and development, functional relationships, and homology among primates. We have now published a basic review on the skull base, introducing issues in medicine and anthropology. It may be a useful text to teach topics in human anatomy. This didactic article is part of a series of collaborations published in Child’s Nervous System in the last years, including similar review papers on sutures, middle meningeal artery, and cranial morphology.