Why Human Brains Grew and Faces Shrank: New Evidence Challenges the Traditional View of Natural Selection

For decades, the standard story of human evolution went like this: natural selection directly favored bigger brains because they conferred cognitive advantages, and as brains ballooned, faces conveniently shrank. But a wave of recent studies — from Nature Human Behaviour, PNAS, Nature Communications, and others — is upending that narrative.

A 2024 evolutionary–developmental (evo-devo) modeling study published in Nature Human Behaviour found that hominin brain expansion was not caused by direct natural selection for brain size . Instead, the model showed that brain enlargement arose from a genetic correlation between brain size and developmentally late preovulatory ovarian size . This challenges the longstanding assumption that larger brains were directly and strongly favored by selection for cognitive advantages. The study suggests that brain enlargement emerged as an indirect consequence of selection on other developmental and reproductive traits .

Meanwhile, a separate 2024 study in PNAS disentangled brain size changes that occur within species from those occurring across species. It found that increases in brain size primarily occurred within individual species' lineages rather than across species, and revealed a trend of accelerating brain size growth in more recent lineages .

What about the shrinking face?

Facial reduction and retraction are defining characteristics of Homo sapiens, distinguishing us from archaic Homo . The modern human face is the result of a combination of biomechanical, physiological, and social influences . A 2025 preprint found that humans show the highest evolutionary rate across almost all craniofacial regions compared with other apes, indicating strong human-specific selective pressures in both neurocranial and facial evolution .

Other studies have linked facial reduction to changes in diet — the switch to softer, cooked foods reduced the need for large jawbones and muscles — and to developmental changes: in Homo sapiens, facial growth slows during childhood and stops at puberty, unlike in Neanderthals and chimpanzees where growth continues longer .

A key caveat

No single Nature Communications study jointly addresses both brain enlargement and facial reduction together. The most relevant challenge to traditional natural selection — that brain expansion was an indirect byproduct rather than a direct selective target — comes from a Nature Human Behaviour study , not Nature Communications.

What this means for the big picture

The emerging view is more complex than the old "selection for big brains" story. Brain enlargement appears to have been an indirect outcome of developmental and genetic constraints — a byproduct rather than a direct adaptive target. Facial reduction, on the other hand, seems driven by a separate mix of dietary shifts, developmental changes, and social factors. Together, these findings suggest that human evolution was more contingent and less teleological than previously assumed.