Amélie Beaudet and colleagues have published a comprehensive and detailed paleoneurological study on South African fossil cercopithecoids. The paper supplies three main advances. First, it provides key information on primate paleoneurology, in particular on Plio-Pleistocene monkeys, belonging to the genera Theropithecus, Parapapio, and Cercopithecoides. Paleoneurology is often more focused on humans and hominoids than on monkeys, and therefore this article is particularly welcome. Furthermore, the study is based on a surface-based method, that compares the rough geometry of the object. Surface analyses can represent an additional and interesting alternative for computing endocast comparisons. There are many complex techniques currently available in shape analysis, and we should always carefully consider that their results depend upon their specific criteria and constraints. Morphometric outputs are “ordered representations” of a given sample variation according to specific numerical and logical assumptions. Consequently, methods are crucial in determining the comparative framework. Different methods, different criteria. For example, surface analysis is not constrained by anatomical correspondence, but it is only sensitive to geometrical correspondence. Hence, the approach misses the information on anatomical boundaries between different elements and areas, distributing variation all through a homogeneous and undifferentiated object.This can be an advantage when taking into consideration form alone, or a disadvantage if one want to investigate the contribution of specific anatomical components. Finally, this study presents a semi-automatic approach for sulcal detection, that is a geometry-based method for the identification of surface relieves, curvature lines, and topographical variations. This approach may seriously represent a major advance in paleoneurology. Nonetheless, it should be taken into account that we still ignore many mechanisms behind cortical folding, and that folding patterns could be the result of passive biomechanical constraints with uncertain phylogenetic or functional meaning.
Archive for the 'Primatology' Category
Tags: Cercopithecoids, cortical folding, shape analysis
Tags: chimpanzee, parietal lobes, precuneus
We modern humans have larger parietal bones and large parietal lobes when compared with extinct human species and living apes. We also have an ontogenetic parietal bulging stage, a stage which is absent in apes and Neandertals. Interestingly, a main factor of variability in our brain morphology is the size of the precuneus, in terms of proportions and cortical surface area. Now we have compared human and chimp brains, and here it goes again: the main difference is a much larger precuneus in our species. It doesn’t look like an allometric issue, being possibly associated with that bulging stage specific of modern humans, and even absent in large-brained Neandertals. The precuneus is essential in visuospatial integration, coordinating brain, body, and environment, and bridging the somatosensorial experience with simulation and self-awareness. It is a key element to integrate space, time, and social perception. It is also worth noting that parietal lobes are particularly vascularized in our species, and the precuneus is a high-metabolic and heat-accumulating element. This may be interesting when considering that it suffers early metabolic impairments in Alzheimer’s disease, a pathology particularly associated with our species. The precuneus is also a central hub of the default mode network. Interestingly, at least in adult modern humans the size of the parietal lobes is inversely correlated with the size of the frontal and temporal lobes, introducing some phylogenetic issues on the evolution of the fronto-parietal system.
For a long time we have been looking for subtle differences between human and ape brain. This one looks not that subtle. Any functional or histological change behind this expansion, at inter-specific and intra-specific level, is still to be investigated. But most of all, it remains to be established the nature of such morphological variations. Genetic factors and selective processes cannot be excluded, but these areas are also particularly sensitive to environmental influences, including training and cultural effects.
Tags: brain size, heritability, phenotypic plasticity, sulcal patterns
Brain evolution involves changes in size and morphology, but also changes in the capacity to be changed. Plasticity refers to the range of phenotypic variation allowed within a given genetic structure. Environment is a major factor influencing the phenotypic expression, and we humans have a special additional environmental component called “culture”. The ecological, cultural, and social niches, shape each others, with dynamics which are far from being understood. Aida Gómez-Robles and colleagues have now published a morphological analysis of human and chimp brain, taking into consideration heritability. In both species cranial capacity is highly heritable, more for humans than for chimps. Also the general dimensions of the main brain areas show in both species an apparent genetic component. The situation is different when dealing with sulcal morphology, which are still heritable for chimps but not that much for humans. This means that brain morphology in chimps has a stricter genetic program, while humans are more sensitive to non-genetic factors and individual responses. Environmental influences are supposed to be the key, mostly when considering the altricial condition and heterochronic changes associated with the human brain growth and development. In a recent review on the evolution of visuospatial integration with Atsushi Iriki we focused on the necessity to understand to what extent brain changes associated with human evolution are due to genetic, epigenetic or environmental factors. We pointed to the sensitivity of the brain to be “trained” through feedbacks between biology and culture as a crucial variable targeted by selection. This new study stresses further the possibility that selection can act on the capacity to change, more than on the change itself.
Tags: apes, endocranial ontogeny
After shape analysis of the endocranial growth and development in modern humans, chimps, and Neandertals, the team from the Max Planck Institute has published a study on apes endocranial ontogeny. In their former articles they evidenced a shared trajectory of form change in humans and chimps. The only exception is the “globularization stage” in modern humans, an early postnatal stage associated with parietal and cerebellar enlargement. This study now includes also gorillas, orangs, and gibbons, confirming that after eruption of the deciduous dentition all hominoids share a similar pattern of form variation. Differences among species are largely a matter of degree of change, but within a shared set of rules. This implies that most of the observed differences among their endocranial forms take place before, in prenatal stages.
Tags: frontoparietal system, parietal lobes
Roberto Caminiti and his colleagues have just published a very large and detailed review on the fronto-parietal network, comparing functional anatomy, histology, and connectivity in humans and macaques. The organization of the fronto-parietal system is similar in these two taxa, suggesting a shared conservative structure rooted in a long evolutionary history. However, there are also discrete differences, most of all at the intraparietal sulcus and in the precuneus. Because such differences were apparently put forward on a shared background, they support the hypothesis of Fred Coolidge about exaptation of the parietal lobes, as reuse of primitive characters to achieve new functions. Visuo-spatial integration and the eye-hand system are of course central in this perspective, but those parietal elements are also involved in different kind of processes ranging from consciousness to numerosity. As usually, we are supposing that macaques show a primitive organization, and humans a derived one. As recently discussed, such assumption is very general and it has no logic or experimental support, and caution is recommended in this sense. In fact, both macaques and humans could display derived characters, evolved independently. The review carefully considers also the human paleoneurological evidence, supplying a very complete image which effectively synthesizes at once more than ten years of works published by myself and by Simon Neubauer and Philipp Gunz.
Tags: Cercopithecoids, cerebral complexity, endocranial volume, olfactory bulb
Macaques and chimps are still used in anthropology and neuroscience as “primitive models” for human evolution. This is of course a non-sense: all living species, after the divergence from a common ancestor with modern humans, have evolved and changed as humans did. The genus Macaca is as young as the genus Homo, and living macaques and living humans are recent species in evolutionary terms, approximately with a comparable age. The problem with chimps is that we miss fossils, so we ignore how and how much their lineages has changed. But we have more information on macaques, and in general on fossil cercopithecoids. A very detailed and informative study on the endocast of Victoriapithecus has been recently published, definitely a stimulating and comprehensive article for primate paleoneurology. This Old World monkey, dated to 15 Ma, had a small cranial capacity and large olfactory bulbs, but a sulcal pattern similar to modern cercopithecids. This suggests two major points. First, in Old World monkeys sulcal complexity evolved before brain size increase. Second, brain morphology evolved in cercopithecoids and hominoids through distinct processes, mixing primitive traits, different mechanisms, specific adaptations, and some convergences. These results stress further the necessity of caution and of a proper evolutionary perspective when dealing with comparative primatology and human brain evolution: macaques (and chimps) are derived species as we are, with their own independent evolutionary histories. They can provide information on biological factors which are shared among our respective lineages, but it would be an error to think that their anatomy, physiology, or genetics, represent an ancestral condition.