Study finds human brain growth driven by chance, not natural selection

A new study published Monday in Nature Communications challenges one of the most deeply held assumptions in paleoanthropology: that the steady enlargement of the human brain over two million years was driven by natural selection favoring greater intelligence. Instead, the research finds that random genetic changes and long periods of evolutionary stasis better explain how human skulls evolved.

Randomness Over Direction

The study, led by paleoanthropologist Katerina Harvati at the University of Tübingen and co-authored by Mark Hubbe, analyzed three-dimensional landmark data from 87 fossil skulls spanning the genus Homo, from early representatives to recent Homo sapiens, including Homo erectus, Homo heidelbergensis, and Homo neanderthalensis. The researchers tested six mathematical models of evolutionary change against the fossil record to determine which best explained the patterns of increasing brain size and facial reduction observed over time.anthropology+1

The results favored neutral evolutionary processes — an unbiased random walk and evolutionary stasis — over directional natural selection in nearly every comparison. Directional selection, the model that would support the long-standing narrative of smarter hominins steadily outcompeting their peers, "came in a distant also-ran," according to an analysis by Anthropology.net.anthropology

"Our analyses confirm the known evolutionary trends of neurocranial growth and facial reduction, but show that variation within our genus is much better explained by neutral evolutionary processes and long phases of evolutionary stasis," Hubbe explained in a statement from the University of Tübingen.uni-tuebingen

Reframing Human Evolution

The findings do not dispute that brains grew larger or that faces became smaller — both trends are well documented. What the study challenges is the mechanism. Rather than constant selective pressure pushing brains toward ever-greater size, the statistical signature instead resembles a trait wandering within constraints that were occasionally relaxed, allowing growth to occur.uni-tuebingen+1

According to the University of Tübingen, the gradual enlargement of the brain and reduction of the face "does not appear to be the result of a steady, progressive path toward the modern human form." Instead, random genetic changes, stabilizing selection, and biological and ecological constraints likely played a far greater role than previously assumed.uni-tuebingen

"Our findings shift the focus," Harvati said. "Instead of asking why humans have continuously evolved toward larger brains and smaller faces," the question becomes what constraints were periodically lifted to allow such changes.youtube

A Mixed Evolutionary Picture

The study points to a combination of neutral evolution and constrained change operating across the Homo lineage. Facial reduction showed an even stronger lean toward strict stasis than brain size, suggesting that the face was essentially locked in place for long stretches, with variation amounting to noise around a fixed mean.anthropology+1

The paper, titled "Evolutionary drivers of encephalization and facial reduction in the genus Homo," appears in Nature Communications volume 17.topontiki+1