Most other mammals walk in a lateral sequence, with the same-side limbs following each other: left foot, left hand, right foot, right hand. Human babies and adults asked to "bear crawl" on hands and feet typically walk in a lateral sequence, too, Shapiro said.
Shapiro said she became interested in studying the gait of people with Uner Tan Syndrome in 2006 after seeing the documentary on the Turkish family.
"It was all about whether or not it was evolutionary reversal, which kind of horrified me," she said. Immediately, though, she could see that the family was not using the primate diagonal gait.
Shapiro did not have access to good video of the family's walking patterns until recently, when one of her co-authors told her he had footage from the BBC. From that video, she and her colleagues were able to analyze more than 500 strides made by the five family members with the disorder.
About 99 percent of the strides were lateral, not diagonal — a blow against the notion that the family members had "rediscovered" an ancestral primate way of walking. Instead, they were walking like any typical adult would if asked to move on hands and feet.
A lateral gait is handy for long-limbed animals (such as humans) when walking on all fours, she said, because it helps keep the limbs from bumping into one another.
"They're doing what any human does in that situation where they can't stand up," Shapiro said.
Shapiro emphasized that even if the family had moved with a diagonal gait, the pattern would not prove anything about human evolution or the origins of bipedalism.
"Bipedalism requires a lot of changes, physical and anatomical changes in the body," she said. "Neurological changes. Motor changes. It's not just one thing."
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