Human brains have shrunk over the past 30,000 years, puzzling scientists who argue it is not a sign we are growing dumber but that evolution is making the key motor leaner and more efficient.

The average size of modern humans – the Homo sapiens – has decreased about 10 per cent during that period – from 1,500 to 1,359 cubic centimetres, the size of a tennis ball. Women’s brains, which are smaller on average than those of men, have experienced an equivalent drop in size.

These measurements were taken using skulls found in Europe, the Middle East and Asia.

“I’d called that a major downsizing in an evolutionary eye blink,” John Hawks of the University of Michigan told Discover magazine.

But other anthropologists note that brain shrinkage is not very surprising because the stronger and larger we are, the more gray matter we need to control this larger mass.

The Neanderthal, a cousin of the modern human who disappeared about 30 millennia ago for still-unknown reasons, was far more massive and had a larger brain.

The Cro-Magnons who left cave paintings of large animals in the monumental Lascaux cave over 17,000 years ago were the Homo sapiens with the biggest brain. They were also stronger than their modern descendants.

Psychology professor David Geary of the University of Missouri said these traits were necessary to survive in a hostile environment.

He has studied the evolution of skull sizes 1.9 million to 10,000 years old as our ancestors and cousins lived in an increasingly complex social environment.

Dr Geary and his colleagues used population density as a measure of social complexity, with the hypothesis that the more humans are living close together, the greater the exchanges between group, the division of labour and the rich and varied interactions between people.

They found that brain size decreased as population density increased.

“As complex societies emerged, the brain became smaller because people did not have to be as smart to stay alive,” said Dr Geary.