Princeton-Duke team uncovers new insights into human brain evolution

The human brain is 238% larger than any other primate of similar body mass. How evolution brought about this situation has been a focus of physical anthropologists for more a century. 

Recently a trio of scientists from Princeton, Duke, and the University of Durham in England have provided some new ideas. Their results appear in a paper published in the online journal eLife in January 2019. 

For a long time it was thought that the increase in human brain mass could be explained by a greater growth of the neocortex, the portion of the brain which handles sight, hearing conscious decision-making and language. 

But careful comparisons of brain size in human and living primate species, as well as of skull size in early primates, including Neanderthals, show that the increase in human brain capacity did not involve a disproportionate growth of the neocortex. 

“Contrary to widespread assumptions, we found that the human neocortex is not exceptionally large relative to other brain structures,” the authors write. 

Cerebellum a key factor

Rather, there were size increases in many parts of the brain, especially the cerebellum, which is important for balance and movement. 

The larger cerebellum could have developed as early hominins moved from trees to ground, requiring greater balance and navigational skills.

Using statistical methods of comparative analysis, the authors report “we identified that a distinct shift in brain-body scaling occurred as hominins diverged from other primates and again as humans and Neanderthals diverged from other hominins.” 

‘A positive feedback process’

The exceptional size of the human brain may have evolved over several million years. 

“Within hominins we detected a pattern of directional and accelerating evolution towards larger brains, consistent with a positive feedback process in the evolution of the human brain,” they wrote. 

The pattern of accelerating brain size suggests what the authors call “a co-evolutionary positive feedback process.” This can include feedback between brain size and culture or language, as some have suggested, or as a result of competition for mastery of skills (“a socio-cognitive evolutionary arms race”) as others have proposed. 

Interestingly, Neanderthals came in a close second in respect to relative brain size, but were still outclassed by modern humans. 

As to further work, the authors suggest that use of “more fine-grained neuro-anatomical data” could help to “tease apart” the role of different brain components. 

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Ian Miller et al. Quantitative uniqueness of human brain evolution revealed through phylogenetic comparative analysis. eLife (2019). DOI: 10.7554/eLife.41250