Human Brain Size Expansion Driven by Relaxed Evolutionary Constraints, Not Selection, Study Finds

Our unusually large brains are one of the defining features of our species. However, a recent analysis of fossil skulls suggests that evolution did not strongly favor larger brains during the past two million years.

“There is indeed an increase in brain size through time, and there is indeed a reduction of facial size with time,” says Katerina Harvati of the University of Tübingen in Germany. This trend, however, does not appear to have been driven by natural selection. Instead, the evolution of the human brain seems to have been governed by a pressure to maintain a relatively constant size, with occasional relaxations of this constraint that allowed the brain to enlarge for unknown reasons.

In the study, Harvati and Mark Hubbe of the University of Tennessee examined 87 hominin skulls. Twenty‑four of these belonged to recent Homo sapiens, while the remainder represented extinct lineages including Neanderthals, Homo erectus and Homo habilis. The researchers documented the braincase and facial dimensions of each specimen.

Between two million years ago and the present, hominin braincases grew considerably, providing room for larger brains, while faces became flatter, with less protruding jaws and reduced brow ridges.

The prevailing hypothesis has long been that natural selection favored greater cognitive abilities, driving a steady increase in brain size. To test this idea, Harvati and Hubbe employed a mathematical model that evaluated six evolutionary scenarios: (1) gradual selective pressure for larger brains, (2) neutral random change, (3‑5) stabilizing selection with varying constraints, and (6) punctuated equilibrium.

The model indicated that neutral evolution best explained the observed changes in braincase size and shape, implying that random mutations accumulated without conferring a selective advantage. The data also revealed periods of stasis, where evolution appears to have pressured the braincase to remain roughly constant.

Facial morphology followed a similar pattern, although the constraints that limited change were stronger for the face than for the braincase.

“The hypothesis has very often been that there’s selection that is driven by increased cognitive abilities,” says Harvati. She adds that the study’s findings provide a more nuanced view of how our crania have changed over deep time.

The study really drills into the mechanisms by which evolution has shaped our bodies, says Amélie Beaudet of the French National Centre for Scientific Research in Poitiers. “That’s exactly what we need now.”

Because the study uses skulls, it can only tell us about the overall size and shape of the brain, and this is only part of the story, says Beaudet. “It’s not only brain size; it’s also probably re‑organization of the brain.” She studies the imprints brains leave within skulls, which suggest that some key regions—such as Broca’s area, known to be involved in language—have changed significantly over the past two million years.

The sample size is too small to draw firm conclusions, says Gerhard Weber of the University of Vienna in Austria. Nevertheless, he suspects Harvati and Hubbe are right that evolution often drove our ancestors’ brains to stay roughly the same size. “We are social animals,” he notes. “Even prehistoric societies probably had some division of labour, with people focusing on hunting or cooking or other tasks depending on their talents and inclinations. Being exceptionally intelligent may not have been especially advantageous in such a culture.”

Weber finds the idea that our brains grew not because natural selection encouraged it, but because constraints were released, “an interesting thought.”

Learning to cook food might have been a key shift, says Harvati. The brain takes a lot of energy to run and cooked food has more calories than raw food, so the invention of cooking may have provided the calorie boost needed to power larger brains.

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