Larger Brain, Smaller Face: Human Evolution Took a Different Course Than Previously Thought

The genus Homo, which is represented today solely by modern humans, emerged about 2.5 million years ago. “With few exceptions, the evolution of the various Homo species was characterized by an increase in brain size as well as a decrease in the size and robustness of the face and jaws,” explains Prof. Dr. Katerina Harvati of the Senckenberg Centre for Human Evolution and Palaeoenvironment (SHEP) at the University of Tübingen. “At the same time, significant behavioral changes occurred: stone tools were used more intensively, food was obtained and processed in increasingly diverse ways, populations spread across significantly larger geographic areas, and more complex social structures presumably emerged.”

For decades, researchers generally assumed that these changes were the result of sustained, directed natural selection; that larger brains were favored because they improved cognitive abilities, while smaller faces provided energetic advantages as tools increasingly took over the task of chewing.
In their new study, Professor Mark Hubbe of the University of Tennessee-Knoxville and Harvati examined how well the morphological changes within the genus Homo fit various evolutionary models, and they found that the data tells a different story.

“While our analyses confirm the well-known evolutionary trends of cranial growth and facial reduction, they show that the differences within our genus can be explained much more effectively by neutral evolutionary processes and long periods of evolutionary stasis,” explains Hubbe. In other words: The gradual enlargement of the brain and reduction in facial size does not appear to be the result of a steady, progressive path toward the modern human form. Instead, random genetic mutations, stabilizing selection, and biological and ecological constraints likely played a much greater role than previously assumed.

Hubbe and Harvati analyzed data from three-dimensional skull measurements of 87 fossils of the genus Homo – ranging from early species such as Homo habilis and Homo rudolfensis to Homo erectus and Homo heidelbergensis and ultimately to Neanderthals, as well as early and modern populations of Homo sapiens. The dataset encompasses the majority of well-preserved hominin fossils from the past two million years, making this one of the most comprehensive studies of evolutionary changes within our genus. “We compared this exceptional dataset with six different evolutionary models using statistical analyses to assess which model most accurately explains the observed changes in head and facial morphology within the genus Homo,” says Harvati. The models included the well-known evolutionary process of natural selection as well as other extensively studied mechanisms found in nature – including neutral evolution, prolonged phases of barely perceptible change, and the “punctuated equilibrium” model, which posits that species remain relatively stable over long periods of time before undergoing rapid evolutionary changes during comparatively short phases.

The study’s results suggest that a combination of neutral and limited evolution played a role in the evolution of the Homo lineage. This provides opportunities for a more detailed examination of key time periods during which conditions appear to have changed.

Major phases of brain enlargement – such as in Homo heidelbergensis and, later, in Homo sapiens and Neanderthals – likely occurred during periods when these evolutionary constraints were temporarily less severe. The researchers point to a combination of possible factors, including developmental biology, metabolic and energetic conditions, and – most importantly – cultural innovation. “In many ways, culture acts as a buffer: It enables us to utilize new habitats and access more resources. This reduces the pressure on certain physical structures because they need to be less strictly adapted to environmental conditions,” explains Hubbe, and he continues, “In this way, periods of intensified technological and cultural innovation can trigger rapid evolutionary changes. Such changes were clearly of great significance for the evolution of the genus Homo, as they enabled our ancestors to meet the nutritional demands of larger brains and to fully exploit the benefits of higher cognitive abilities.”

Similar mechanisms could also explain the evolutionary changes that distinguish modern humans from earlier Homo species, according to the research team. For example, Neanderthal facial morphology appears to have remained more restricted over long periods of time, while the faces of modern humans are significantly smaller than those of other genetic lineages. “It is possible that these later changes were also linked to particularly profound behavioral shifts that accompanied the emergence of our species,” adds Harvati.

The study provides no evidence that natural selection has been insignificant in human evolution, but it highlights the need to determine the most productive questions to ask. “Our findings shift the focus,” Harvati concludes. “Instead of asking why humans have continuously evolved toward larger brains and smaller faces, it would make more sense to investigate under what conditions human populations were able to break free from existing constraints and develop new traits. This approach could be particularly well-suited toward better understanding the evolution of our genus.”

Publication: Hubbe, M., Harvati, K. Evolutionary drivers of encephalization and facial reduction in the genus Homo. Nat Commun17, 5625 (2026). https://doi.org/10.1038/s41467-026-74739-w