Posts Tagged 'endocranial ontogeny'

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.

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.

Pan & Pan

Although geometric morphometrics is currently the most promising method to analyze endocasts, there are alternatives. Durrleman and colleagues propose an approach based on deformations between surfaces. This method can help with non-linearity of the ontogenetic processes, lack of morphological references, or continuity of the anatomical tissues. The approach is definitely more complex and less intuitive than geometric morphometrics. This may mean sometimes more analytical power, sometimes more analytical bias.  The case-study is the endocranial ontogeny in chimps and bonobos: some shared patterns, but interesting differences too.


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