Human brains grew via neutral evolution, not selection
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- Katerina Harvati at the University of Tübingen and Mark Hubbe at the University of Tennessee, Knoxville analyzed 87 hominin skulls — 24 from Homo sapiens and the rest from extinct species including Neanderthals, Homo erectus, and Homo habilis — and found the best-fitting model for braincase change was neutral evolution, not selection for larger brains.
- The study tested six evolutionary scenarios with a mathematical model, and the changes were best explained by random mutations accumulating without offering advantages, alongside stasis — pressure keeping brains roughly the same size until constraints eased.
- Harvati proposes that the invention of cooking may have supplied the calorie surplus needed to fuel larger brains when those constraints were temporarily released, since raw food alone couldn't power the brain's high energy demands.
- Amélie Beaudet at the French National Centre for Scientific Research praised the mechanistic focus but cautioned the skull data captures only overall size and shape, missing internal reorganization — such as shifts in Broca's area tied to language.
- Gerhard Weber at the University of Vienna called the 87-skull sample too small for firm conclusions but said the stasis argument is plausible, noting that in social species with division of labor, exceptional intelligence may not have conferred much advantage.
- Human facial structure evolved under similar pressures, with stasis proving even more intense for faces than for braincases.
Why it matters: This reframes one of evolution's signature narratives — that natural selection steadily rewarded bigger, smarter brains — as largely passive drift punctuated by constraint releases, with cooking as a candidate trigger for those releases.
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