Changes in diet are thought to be the main cause. The switch to softer, farmed foods means that jawbones, teeth, skulls and muscles do not need to be as strong as in the past.

The shrinkage has been blamed for a surge in dental problems caused by crooked or overlapping teeth.

“Over the past 10,000 years there has been a trend toward rounder skulls with smaller faces and jaws,” said Clark Spencer Larsen, professor of anthropology at Ohio State University.

“This began with the rise in farming and the increasing use of cooking, which began around 10,000 years ago.”

His conclusions are based on measurements from thousands of teeth, jawbones, skulls and other bones collected from prehistoric sites around the world.

Skulls from the site of a 9,000-year-old city in Turkey — thought to be the world’s oldest — show that the faces of city-dwellers had already begun to shrink compared with contemporaries who had not settled down.

Details will be reported at a forthcoming conference on the global history of health. Larsen will suggest that a typical human of 10,000 years ago would have had a much heavier build overall because of the hard work needed to gather food and stay alive.

He said: “Many men then would have had the shape of Arnold Schwarzenegger’s head while women might have looked more like Camilla [the Duchess of Cornwall]. By contrast, Tony Blair and George Bush are good examples of the more delicate modern form.”

Other studies are confirming Larsen’s findings. George Armelagos, professor of anthropology at Emory University in Atlanta, Georgia, has made extensive measurements on people from Nubia in modern Egypt and Sudan to see how their appearance has changed.

He found that the top of the head, or cranial vault, had grown higher and more rounded, a pattern also seen in human remains found at sites in other parts of the world.

Charles Loring Brace, professor of anthropology at the University of Michigan, said: “Human faces are shrinking by 1%-2% every 1,000 years.

“What’s more, we are growing less teeth. Ten thousand years ago everyone grew wisdom teeth but now only half of us get them, and other teeth like the lateral incisors have become much smaller. This is evolution in action.”

Softer food may not be the only cause. Some scientists blame sexual selection — the preference of prehistoric people for partners with smaller faces.

Dr Simon Hillson, of the Institute of Archaeology at University College London, has studied humans living from 26,000 years ago to about 8,000 years ago. He measured 15,000 prehistoric teeth, jaws and skulls collected by museums around the world and found the same pattern of shrinking faces.

He said: “The presumption is that people must have chosen mates with smaller, shorter faces — but quite why this would be is less clear.”

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