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.
Chet Sherwood and Suzana Herculano-Houzel are editing a Frontiers volume entitled “The Human Brain’s Place in Nature: Evolution of Large Brains”, cross listed between Frontiers in Neuroanatomy and Frontiers in Human Neuroscience. There you can find review and research papers dealing with pros and cons of evolving large brains. Our article presents issues in functional craniology, with topics joining evolution and medicine. The article begins with an introduction to functional craniology and brain-braincase structural relationships. Features associated with sutures and brain spatial organization are interesting in evolutionary neuroanatomy and in surgery as well. Brain thermoregulation is a major factor in both fields, and modelling can help to test the influence of brain form changes in heat dissipation patterns. Changes in the frontal lobes proportions and position during human evolution are discussed as a probable background for visual impairment, in particular myopia, because of spatial conflicts between brain and orbits. The dilation of the parietal areas in modern humans and the complexity of the deep parietal elements are then integrated with information on neurodegenerative processes, in particular Alzheimer’s disease, in an evo-neuro perspective. Evolutionary neuroanatomy and medicine share information, tools, methods, and samples, being interested in the same characters and processes for different reasons and different targets. Functional craniology is the bridge we use to integrate these fields.
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.