A new analysis of 87 hominin skulls suggests that human brains grew larger over the past 2 million years not because natural selection repeatedly rewarded bigger brains, but because evolutionary constraints sometimes held them near a stable size—and then brief
Walk through a museum gallery and the story is easy to read in skulls: Neanderthals and Homo sapiens clearly carried larger braincases than earlier hominins. What’s harder to explain is why that growth happened at all.
A new analysis of fossil skulls challenges the familiar idea that evolution consistently “picked” for bigger brains over the past 2 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 at the University of Tübingen in Germany—but the pattern doesn’t appear to be driven by natural selection.
Instead, the work points to a different pressure at play. The evolution of the human brain may have been governed by a constraint that kept brains roughly the same size, with occasional releases—triggered, for unknown reasons, from time to time—that allowed braincase volume to expand.
Harvati and Mark Hubbe at the University of Tennessee, Knoxville, examined 87 hominin skulls. Of those, 24 were from recent members of our species, Homo sapiens. The rest came from extinct human species including Neanderthals (Homo neanderthalensis), Homo erectus and Homo habilis. For each skull, they collected measurements of its shape and size, concentrating on the braincase and the face.
From 2 million years ago to the present day, the results show human braincases became much bigger—making room for larger brains. At the same time, human faces became flatter, with less protruding jaws and smaller brow ridges above the eyes.
Harvati puts the debate in plain terms. The hypothesis has very often been that there’s selection driven by increased cognitive abilities. If cleverness offers an advantage, natural selection would favor the intelligent, and ancestors would accumulate ever-larger brains. The researchers tested that idea directly using a mathematical model built to compare six evolutionary scenarios.
One scenario assumed there was a gradual pressure from natural selection for brains to get bigger. Another treated changes as “neutral,” with skull traits shifting more or less at random. Three scenarios imagined the braincase hovering around an optimum design. with constraints of different strengths limiting how far evolution could roam. The final scenario was “punctuated equilibrium. ” where long periods of little change are interrupted by brief bursts when rapid change is favored.
The best fit for changes in braincase size and shape was neutral evolution. In that scenario. random mutations affect the size and shape. and changes accumulate over time without the new braincase designs being advantageous compared with older ones. The model also found evidence of stasis: evolution pressuring the braincase to stay roughly the same shape and size.
For faces, the picture is similar, but with a sharper edge. Human face changes were explained in comparable ways, except that the evolutionary pressure to stay the same seems to have been more intense.
The study is. as Amélie Beaudet at the French National Centre for Scientific Research in Poitiers puts it. a deep dive into the mechanisms by which evolution shaped our bodies. “That’s exactly what we need now.” But skulls can only tell part of the story, Beaudet cautions. Because the study uses skulls. it can only tell us about the overall size and shape of the brain — not the details of what the brain is doing. “It’s not only brain size; it’s also probably reorganisation of the brain.” Beaudet studies the imprints brains leave within skulls. which suggest that some key regions—like Broca’s area. known to be involved in language—have changed significantly over the past 2 million years.
Even with the model’s fit, Gerhard Weber at the University of Vienna in Austria warns against overconfidence. The sample size is too small to draw firm conclusions. Still. 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 says. arguing that in prehistoric societies there was likely some division of labour—people focusing on hunting or cooking or other tasks depending on their talents and inclinations. In that kind of culture. exceptionally high intelligence may not have been especially advantageous in the way the “bigger-brain” story assumes.
Weber is also drawn to the alternative mechanism suggested by the work: brains growing because constraints were released rather than because selection repeatedly rewarded size. “That is an interesting thought,” he says.
One proposed trigger for that release is cooking. Harvati suggests learning to cook food might have been a key shift. because the brain takes a lot of energy to run and cooked food has more calories than raw food. In her view, cooking may have provided the calorie boost needed to power larger brains.
For now. the study reframes a familiar question—did bigger brains win evolutionary battles?—into a more complicated one: did brains change mostly when the rules loosened?. The answer emerging from fossil skulls is less about constant advantage and more about periods of constraint. broken at intervals. when evolution found room to grow.
human evolution brain size Neanderthals Homo sapiens fossil skulls natural selection neutral evolution punctuated equilibrium hominin skulls Broca's area cooking hypothesis