When Turkish evolutionary biologist Uner Tan introduced the world to a Turkish family with some members who could walk only on all fours, in a "bear crawl," he and other scientists speculated this odd gait was the resurgence of a trait lost during human evolution.
Not so, a new study finds.
The family and other people with Uner Tan syndrome do not represent "a backward stage in human evolution," as Tan wrote in a 2006 paper in the International Journal of Neuroscience, said Liza Shapiro, a professor of anthropology at the University of Texas at Austin. In new research, Shapiro and her colleagues compared videos of the family's gait with the gaits of nonhuman primates, such as chimpanzees or gorillas. They found the gait patterns did not match. Instead of recreating ape walks, people with Uner Tan are simply adapting to their disorder, Shapiro and her colleagues reported July 16 in the journal PLOS ONE.
On all fours
Tan first noticed the syndrome that now bears his name in a family of 19 living in rural southern Turkey. Five of the family members walk using their feet and hands, and also have cognitive disabilities. The family was the subject of the 2006 BBC2 documentary, "The Family That Walks on All Fours."
Research has since revealed that the disorder is caused by a genetic mutation on chromosome 17, which affects the cerebellum, part of the brain responsible for movement and balance. From the beginning, Tan's statements about the evolutionary nature of the affected family's walking patterns were controversial. The affected children never had physical therapy or adaptive technology such as wheelchairs, making their gait a necessity. [See a video of walking on all fours]
But no one ever challenged the primary claim: that the affected children walked like nonhuman primates. Primates that walk on all fours do so differently than most other mammals, Shapiro told Live Science. Primates walk in a diagonal sequence, putting down a hind limb and then the opposite front limb: left foot, right hand, right foot, left hand.
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.
Adapting, not devolving
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."