This week, with a team coordinated by Michael Masters (Montana Tech), we have published a correlation analysis to evaluate the relationships between eye, orbit, and brain, in adult modern humans. As already evidenced in other studies in anthropology and primatology, the correlation between eye size and orbit size is very modest. Therefore, the orbit is really a poor predictor of the eye morphology, at both evolutionary and species-specific level. There is also a minor size correlation between the eye and the occipital cortical areas, probably because of their shared visual functions. However, there is also a similar (and even higher) correlation between eye and frontal lobe. In this case there is a structural issue: the frontal lobes lie just above the orbits, generating a spatial interaction between facial and neurocranial elements. Within hominids, this spatial proximity between prefrontal cortex and eyes is generally observed only in modern humans and Neandertals. These two taxa, possibly because of such vertical constraint, enlarged their frontal lobes mainly laterally. These correlations between soft and hard tissues, when dealing with inter-specific trends, can be useful to make inferences on brain proportions based on osteological evidence, providing an heuristic tool for indirect paleoneurology.
So, back to modern human evolution, the situation of the eye was pretty difficult: large eye (due to brain size increase), small orbit (due to facial reduction), upper constraints (the frontal lobes right on the orbital roof), posterior constraints (larger and closer temporal lobes). And, in industrial Countries we can also add more fat between eye and bone. Hard times for the eyeballs, forced to minor deformations blurring images on the retinal screen: myopia. Luckily for us crossing the 40s, the brain stops growing, but the face does not: it grows bigger, giving more space to the eye, which can enjoy a more comfortable environment year by year.
Paleoneurology gets new tools. By using living species, Eiluned Pearce and colleagues look for correlations between neural and non-neural anatomical elements, and then apply their results to fossils. This approach opens up an incredibly large set of new research lines. They find a correlation between orbit size and visual cortex which, after a delicate series of normalization processes, suggests that Neandertals had larger visual areas of the brain. At the expense of the parietal ones. This new information seems to provide further support for the hypothesis of relative dilation of the parietal lobes in our species. The long chain of numerical transformations the authors applied to control any possible scaling factor is really efficient and successful. Evidently it also introduces many assumptions and estimations which call for caution when dealing with strict interpretations of the final outputs. Also, we know that only in modern humans and Neandertals the prefrontal brain areas lie on the orbital roof, generating complex constraints between brain and eyes that may produce departures from the schemes of other species. Nonetheless, with these limits in mind, we must recognize that this study supplies new and fresh ideas to our field, and relevant new information on Neandertals. It is also useful in reminding us once more that different lineages may undergo different cognitive evolution, which in the case of the brain means improving or demoting different behavioural skills. This recalculation of the different cortical areas in fossil hominids also supplies new estimations of parameters associated with primates’ biology and social structure, such as the average group size.
Many people are warning against “paleo-phrenology”, but this should not be the case. This is a correlation study. A theory can explain a correlation or fail to do it, but the correlation is there, and it can reveal underlying information. Caution is recommended, but this is always recommended in science. It is strange how we are critic against a lots of details for complex models in evolution, yet we accept overly simplistic alternatives like those associating cognition with a single molecule or gene …
A comment on National Geographic.
Paleoanthropologists are now fairly convinced about the importance of integration in biology and evolution. It is a rare pleasure to see such perspectives successfully applied to every-day life problems. Michael Masters, with a very well documented study on human evolution and functional craniology, suggests that myopia (the primary source of reduced vision throughout the world) may be the consequence of our recent anatomical evolution. Large brains placing the frontal lobes on the orbital roof and constraining the orbital space, while at the same time facial reduction provides further structural limitations. That is, in our species the orbit cannot acknowledge properly the morphological requirements of the eye. Brain and eye compete for space, and the advantages associated with the former are paid with the problems associated with the latter. The consequent packing deforms the eye, leading to vision problems. Allometric and brachycephalic proportions make the situation even a bit more difficult in women and East Asian populations. This hypothesis is an excellent example of interchange between evolutionary biology and medicine. Until now, myopia has been mostly studied considering the eye like an isolated unit. Masters has now provided a very effective example of how induction and deduction can be improved mixing fields, in this case integrating medicine with functional craniology and paleoanthropology. Interestingly, similar deformations associated with the frontal lobe spatial packing have been also described for some psychiatric disorders.