Sturdy shoes first came into widespread use between 40,000 and 26,000 years ago, according to a US scientist. Humans' small toes became weaker during this time, says physical anthropologist Erik Trinkaus, who has studied scores of early human foot bones. He attributes this anatomical change to the invention of rugged shoes, that reduced our need for strong, flexible toes to grip and balance.

While early humans living in cold northern climates may have begun covering up their feet to insulate them as early as 500,000 years ago, protective footwear comparable to modern-day shoes is thought to be a much later innovation.

It has been difficult for archaeologists to determine exactly when humans stopped going barefoot, however, because the plant and animal materials used to make prehistoric shoes is highly perishable.

"The oldest shoes in the world are about 9,000 years old, and they're from California," said Professor Trinkaus, of Washington University in St Louis, US.

But by examining the foot bones of early modern humans (Homo sapiens) and Neaderthals (Homo neanderthalensis) dating from 10,000 to 100,000 years ago, Professer Trinkaus says he has determined the period in which footwear became the norm.

He found Neanderthals and early moderns living in Middle Palaeolithic times (100,000 to 40,000 years ago) had thicker, and therefore stronger, lesser toes than those of Upper Palaeolithic people living 26,000 years ago.