Archive for the 'Endocasts' Category



Ontogenetic dilemma

Ponce de Leon et al 2016

Marcia Ponce de León and colleagues have published a comprehensive shape analysis on modern human and Neandertal early ontogenetic endocranial changes, as Philipp Gunz and his team did back in 2010. Interestingly, results are different. The previous study from the Max Planck Institute concluded that only modern humans have a species-specific postnatal stage in which the braincase bulges (globularization stage). In contrast, this new analysis, coordinated by Christoph Zollikofer, suggests that after birth Neandertals and modern humans share a similar pattern of endocranial shape change. In this case, any endocranial difference between these two species must occur before birth. The discrepancy between the two studies may be due to differences in the samples (which, recognizing the good samples used in these analyses, would reveal a problematic instability of most paleoanthropological studies) or to differences in the reconstructions of the specimens (which, recognizing the good experience of both teams, would reveal a problematic instability of most paleoanthropological studies). Nonetheless, we must also take into account that both articles rely on very complex statistical and algebraic passages, and methodological biases should not be ruled out. After all, also paleontology deals with the same limits of any science: we do not work with skulls or brains, but with models made of variables and parameters. Models that work well in some cases, and do a worse job in some others, depending on the questions involved. In this new study, the fact that endocranial shape differences between Neandertals and modern humans are prenatal is used to state that there are no cognitive differences between the two species. Of course, cognition is more than shape, so the relationship between the timing of these changes (before or after birth) and the statement on cognition is not particularly straight. Inferences on cognition should be made on multiple evidence, dealing with something that goes well beyond a surface analysis.

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Buia

Buia (Bruner et al 2016)This week we publish a morphometric analysis of the endocranial anatomy of Buia, a skull found in Eritrea and dated to 1 million years. The cranial capacity is 995 cc. The endocast is extremely dolichocephalic: very long and narrow. Nonetheless, it shows all endocranial traits that are commonly described in “archaic humans“. The bulging occipital lobes and the vascular system resemble the Chinese specimens from Zhoukoudian. Its pronounced parietal bosses are due to a narrow cranial base and temporal areas, and not to a real enlargement of the parietal lobes. Actually, the cranial base in Buia is very narrow and flexed, and it may have influenced both the neurocranial and splanchnocranial proportions (bulging parietal surface and tall facial block). At present, there is no reason to exclude this specimen from the Homo ergaster/erectus group. The skull from Daka show a similar chronology and a similar geographic origin, although it displays much more brachycephalic proportions. If all these Afro-Asiatic archaic specimens belong to the same species, the variability is notable. It remains to be established whether the evolutionary roots of more derived taxa (like Homo heidelbergensis) can be traced back to these archaic populations, or else if Buia and Daka are still part of an undifferentiated phylogenetic group.

Parsimony

Mounier et al 2016Paleoneurology is rarely used to test taxonomic or phylogenetic hypotheses, at least for three reasons. First, the biology and variation of many endocranial traits are not even known for living humans. Second, plesiomorph traits, parallelisms, large intra-specific variability, and subtle inter-specific differences, make this issue very difficult to test through robust quantitative approaches. Third, the paucity and fragmentation of the fossil record often hamper meaningful statistical inferences. Of course these same problems concern many other anatomical districts, and that’s why probably morphology is not always recommended as a good and reliable source of taxonomic and phylogenetic information. Despite these limits, Aurelién Mounier and his coauthors have now tried to apply cladistics to paleoneurology, taking into consideration neurocranial and endocranial traits. According to their results, at least modern humans and Neandertals can be properly characterized in terms of braincase morphology, suggesting the existence of an actual phylogenetic signal behind the patterns of endocranial variation.

Hominoids

Scott et al 2014After 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.

Amud

Amano et al 2015 - AmudA new reconstruction of the Neandertal skull and endocast of Amud has been published by a team coordinated by Naomichi Ogihara, at the Keio University (Yokohama). They applied mathematical models to align the surfaces of the original fossil fragments (surface extrapolation). Thin-plate spline was then used to integrate the available anatomy from Amud with other Neandertals, namely La Chapelle-aux-Saints 1 and Forbes’ Quarry 1 (shape interpolation). Modern skulls were used to smooth all together. This new reconstruction shows a skull that is shorter and wider than the former one. The basicranial areas, largely missing in this specimen, were the most difficult parts to interpolate, because of their complex morphology influenced by different independent factors. Amud is dated to 50-70 ka, with a massive cranial capacity: around 1740 cc.

Victoriapithecus

Victoriapithecus (Gonzales et al 2015)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.

Warping brains

Ogihara et al 2015The team coordinated by Naomichi Ogihara has published an analytical review on computed reconstruction of fossil crania and interpolation of their brain morphology. The article presents and discusses the applications of biomedical imaging in paleontology, including technical and algebraic details. Automated assembling of fossil fragments is approached following geometric similarity, fracture surfaces, pattern matching, smoothness, and anatomical correspondence. Skulls, endocasts, and brains are integrated mixing information from computed tomography and magnetic resonance, and spatial deformation functions are used to interpolate brain morphology in fossil species. This is a very useful paper both for the technical issues and for general perspectives in digital anatomy and computed morphometrics in paleoneurology. Additional information on this topic can be found in the paper by Gunz and colleagues on virtual reconstruction and in the review by our team on functional craniology.


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