The work suggests that wearing shoes promotes more delicate small toes. When people walk barefoot, the four smaller toes on each foot flex to allow better traction. This promotes growth of sturdy toe bones.
By contrast, sandals, sneakers, and other supportive footwear lessens the load on the four small toes, thus weakening them.
Trinkaus compared the toe anatomy of western Eurasian human skeletons from the Middle and Upper Paleolithic periods (about 100,000 to 40,000 years ago and 40,000 to 10,000 years ago, respectively).
The anatomy of the skeletons' feet began to change around 26,000 to 30,000 years ago, becoming more delicate in later skeletons, the anthropologist found.
"I discovered that the bones of the little toes of humans from that time frame were much less strongly built than those of their ancestors, while their leg bones remained large and strong," Trinkaus said. "The most logical cause would be the introduction of supportive footwear."
"[These] people were routinely using semi-rigid- to rigid-soled shoes, boots, and sandals to protect the foot," he said.
The findings are detailed in a recent edition of the Journal of Archaeological Science.
Support and Skepticism
Mike O'Brien directs the Museum of Anthropology at the University of Missouri-Columbia.
He said it's frustrating that the earliest direct evidence of footwear comes from North America, when "almost assuredly Upper Paleolithic peoples living in Europe and elsewhere were wearing footwear before anyone entered the Americas, 13,000 years ago."
"Looking at the anatomical evidence of footwear is a very novel and interesting approach," he said. "And the evidence strongly suggests that there was a significant increase in the use of footwear between Middle Paleolithic and middle Upper Paleolithic humans."
"Did Upper Paleolithic people always wear shoes? Apparently not always, but certainly routinely," O'Brien said.
Cameron Kippen, a podiatrist and shoe historian with Curtin University of Technology in Perth, Australia, counters that "the idea that shoes influenced toe function seems unlikely. But it is an interesting theory."
Kippen argues that shoes have made little genetic impact on foot anatomy in the last 9,000 years. He also notes that majority of the population would still have been barefoot even after shoes were adopted.
"There are still more people unshod today on Earth than wear shoes," he said.
Kippen believes that shoes first appeared as a decorative garment worn by only a few important people in a tribeperhaps witch doctors and chieftainson special occasions.
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