The remarkable, yet not extraordinary, human brain as a scaled-up primate brain and its associated cost.
Level 5 - mechanism / opinion, no new human data
Narrative review and comparative neuroanatomical synthesis with no primary clinical or human trial data
PubMed 22723358 · doi:10.1073/pnas.1201895109
What was done
The author reviewed recent quantitative neuroanatomical data obtained from isotropic fractionation cell-counting methods to evaluate long-standing assumptions about human brain cellular composition, relative cerebral cortex size, and metabolic cost compared to other primates.
What was found
The review notes that the human brain contains approximately 86 billion neurons and roughly an equal number of nonneuronal cells, contrary to traditional claims of 100 billion neurons and 10- to 50-fold more glial cells. Cellular scaling and the consumption of 20% of total body energy for 2% of body mass match expected values for a scaled-up primate brain rather than an evolutionary anomaly.
Why it matters
This work reframes human cognitive specialisation as the result of a large absolute number of neurons conforming to primate scaling laws, dispelling longstanding neuroscientific myths of unique cortical overdevelopment.
Limits
The abstract describes a narrative review without reporting primary empirical data, sample sizes, donor demographics, measurement variance, or confidence intervals.