Human brain growth wasn't driven by natural selection
Get the Health newsletter
Daily health & science — research, biotech, public health, the studies worth knowing. Free.
- 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 63 from extinct species including Neanderthals, Homo erectus, and Homo habilis — testing six evolutionary scenarios against the data.
- The best-fitting model was neutral evolution, meaning random mutations accumulated in braincase size and shape over time without the new designs offering any advantage over older ones.
- Researchers also found evidence of stasis, an evolutionary pressure keeping braincases roughly the same size, with brains only occasionally growing when this constraint eased for unknown reasons.
- Human facial structure changed along similar lines but under even stronger pressure to remain stable, the study found, with faces becoming flatter and brow ridges shrinking.
- Amélie Beaudet at the French National Centre for Scientific Research cautioned that skull measurements only capture overall brain size and shape, not internal reorganization — pointing out that regions like Broca's area, linked to language, likely changed substantially.
- Gerhard Weber at the University of Vienna called the 87-skull sample too small for firm conclusions but agreed the social nature of early humans — with division of labor across tasks — may have meant exceptional intelligence wasn't strongly selected for.
- Harvati suggested the invention of cooking could have provided the calorie surplus needed to grow larger brains whenever evolutionary constraints loosened.
Why it matters: This challenges the textbook 'brains grew because being clever was advantageous' narrative central to paleoanthropology: if neutral drift drove brain expansion, the cognitive capabilities defining Homo sapiens may be partly a byproduct of constraints releasing (possibly via cooking) rather than direct selection for intelligence, reshaping how researchers interpret what makes humans unique.
