Study Finds Human Brain Growth and Facial Reduction Driven by Neutral Evolution, Not Just Natural Selection

A new study published in Nature Communications challenges the long-held belief that the increase in brain size and reduction in facial and jaw size in human evolution were primarily driven by natural selection. Researchers from the University of Tübingen and the University of Tennessee analyzed 3D measurements of 87 hominin skull fossils spanning two million years, including Homo habilis, Homo rudolfensis, Homo erectus, Homo heidelbergensis, Neanderthals, and early and modern Homo sapiens.

The team, led by Prof. Katerina Harvati and Prof. Mark Hubbe, found that while the trends of brain enlargement and facial reduction are clear, these changes are better explained by neutral evolutionary processes and long periods of evolutionary stasis rather than continuous directional natural selection. Random genetic mutations, stabilizing selection, and biological and ecological constraints likely played a larger role than previously thought.

The study suggests that major brain size increases occurred during times when evolutionary constraints were temporarily relaxed, possibly due to developmental biology, metabolic and energetic conditions, and especially cultural innovations. Culture acted as a buffer, allowing humans to exploit new habitats and resources, reducing physical adaptation pressures. This enabled rapid evolutionary changes during periods of technological and cultural advancement.

The researchers also note that differences between modern humans and earlier hominins, such as the smaller faces of Homo sapiens compared to Neanderthals, may relate to profound behavioral changes accompanying our species' emergence. The findings shift the scientific focus from why humans evolved larger brains and smaller faces to understanding the conditions that allowed populations to overcome constraints and develop new traits.

Prof. Hubbe emphasized that this approach could improve our understanding of human evolution by investigating when and how populations escaped evolutionary limitations rather than assuming constant directional selection.