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The brain does not follow the head

The human brain is about three times the size of the brains of great apes. This has to do, among other things, with the evolution of novel brain structures that enabled complex behaviors such as language and tool production. A study by anthropologists at the University of Zurich now shows that changes in the brain occurred independent of evolutionary rearrangements of the braincase.

The brain does not follow the head
CT/MRI datasets of a human (left), chimpanzee (center), and gorilla (right). Surface reconstructions of bony structures
were derived from CT data, while volume renderings of brain segmentations were obtained from
postprocessed MRI data [Credit: J.L. Alatorre Warren, UZH]
The human brain is like a fish in an aquarium, floating inside the liquid-filled braincase - but filling it out almost completely. The relationship between the brain and the braincase, and how they interacted during human evolution, has been occupying the minds of researchers for almost a century. They addressed this question by studying brain-braincase relationships in our own species, and in our closest living relatives, the great apes.

Quantifying spatial relationships between brain and cranial structures

Jose Luis Alatorre Warren, researcher at the Department of Anthropology of the University of Zurich, tackled this question using computed tomography (CT) and magnetic resonance imaging (MRI) data from humans and chimpanzees.

By combining CT/MRI data, he was able to quantify the spatial relationships between brain structures such as gyri (convolutions) and sulci (furrows) on the one hand, and cranial structures such as bony sutures on the other.

The results show that the characteristic spatial relationships between brain and bone structures in humans are clearly distinct to those in chimpanzees. While the brain and its case continued to evolve side by side, they did so along largely independent evolutionary paths.

Bipedalism leads to changes in braincase

For example, brain structures related to complex cognitive tasks such as language, social cognition and manual dexterity expanded significantly in the course of human evolution. This becomes visible as a shift of the neuroanatomical boundaries of the frontal lobe of the brain.

Transformation of braincase (black) and brain (red-to-blue) structures from chimpanzee to human, 
and back to chimpanzee [Credit: J.L. Alatorre Warren]

This shift, however, did not affect the bony structures of the braincase. Instead, changes in the braincase largely reflect adaptations to walking upright on two legs, or bipedalism. For example, the opening at the skull base for the spinal cord moved forward during human evolution in order to optimize balance of the head atop the vertebral column. However, these evolutionary changes to the braincase did not have an effect on our cerebral structures.

Point of reference for future research

"The brain followed its own evolutionary path of neural innovation while freely floating in the braincase," summarizes Alatorre Warren. "The position and size of braincase bones thus don't enable us to draw conclusions about evolutionary changes in the size or rearrangement of adjacent brain regions." Co-authors Marcia Ponce de Leon and Christoph Zollikofer believe their study's data provide an important point of reference for future research: "Having answered the brain-braincase question for humans and great apes, we can now take a fresh look at the braincases of fossil hominids."

The study is published in Proceedings of the National Academy of Sciences.

Source: University of Zurich [October 15, 2019]


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1 comment :

  1. Thanks for this. In several publications I've been arguing that the external brain-skull form has nothing to do with the internal brain organization (brain functioning), e.g. 1995 Med.Hypoth.44:409-413 "Aquatic ape theory, speech origins, and brain differences with apes and monkeys".
    Comparative evidence suggests that the platycephalic skull of archaic Homo (the very low, long & flat brain-skull of H.erectus & H.neanderthal.) was a hydrodynamic adaptation for collecting shallow-aquatic foods when early-Pleistocene dispersed intercontinentally along coasts, islands & rivers: the rich aquatic foods contained lots of brain-specific nutrients (esp. DHA) necessary for evolving a larger brain, google e.g. "coastal dispersal of Pleistocene Homo 2018 biology vs anthropocentrism". But platycephaly did not change the internal connections of the brain inside.
    Different sorts of evidence (isotopic, paleo-environmental, comparative-anatomical etc.) suggest that late-Pleistocene H.sapiens reduced diving and began more wading bipedally for shallow-aquaic foods, hence the loss of the archaic pachyosteoslerosis & of platycephaly and the appearance of our higher forehead, rounder skull form, extreme basi-cranial flexion etc. ("gracilization").
    For a more thorough discussion, please google "two incredible logical mistakes 2019 Verhaegen" + refs therein, e.g. 2013 Hum.Evol.28:237-266 "The aquatic ape evolves: common misconceptions and unproven assumptions about the so-called Aquatic Ape Hypothesis".


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