A new study challenges one of the most established beliefs in human science: that our brains gradually increased in size over two million years because "smarter means better."

According to the study, published on July 7 in the journal Nature Communications, the major increase in brain volume within the genus Homo is attributed much more to random genetic changes and prolonged periods of evolutionary stagnation than to continuous pressure from natural selection favoring high intelligence.

What the scientists did

Paleoanthropologist Katerina Harvati from the University of Tübingen (Germany) and her colleagues analyzed 3D data from 87 fossil skulls, spanning nearly the entire history of the genus Homo — from early species to Neanderthals and modern humans.

The researchers tested six different mathematical models of evolution to see which one best matched the actual paleontological record. Special attention was paid to two main trends: the enlargement of the braincase and the reduction of the facial region.

The main conclusion

Directional natural selection, which assumed that smarter hominins gradually replaced less clever ones, was virtually unsupported. The observed changes are best described by neutral evolutionary processes — random genetic drift and long periods of evolutionary stasis (stagnation).

"We see the same overall trends — the brain got bigger, the face got smaller. But the variations within the genus Homo are explained much better by neutral processes and long phases of stability than by continuous selection for intelligence," noted study co-author Mark Hubbe.

What this changes

The study does not deny the actual fact of brain enlargement and facial reduction — these trends remain obvious. What is questioned is the mechanism behind these changes. Instead of the idea that "the brain grew because intelligence provided a constant advantage," scientists offer a different picture: the brain and face evolved within certain biological constraints that occasionally loosened, allowing random changes to accumulate.

This was especially evident in the structure of the face, which demonstrated very strong stasis, remaining virtually unchanged for hundreds of thousands of years.

A new perspective on human evolution

"Our results shift the focus," says Katerina Harvati. "Instead of asking 'why did humans continuously evolve toward a larger brain and a smaller face,' it is now more accurate to ask: what specific constraints were periodically lifted, allowing these changes to happen."