Scientists may finally have an answer for why members of a family in a remote region of Turkey use both their hands and feet to walk.
The world first learned of the clan eight years ago following the popularity of the 2006 BBC documentary "The Family That Walks On All Fours." Just check out the video above.
At the time, researchers were puzzled by why these five siblings exhibited the behavior while their parents did not. Backward evolution became the primary explanation for the family's quadrupedalism -- but scientists may have had it all wrong. Now, new research suggests adaption to a genetic mutation -- not "devolution" -- is responsible.
In fact, Dr. Liza Shapiro, an anthropologist at The University of Texas at Austin, discredited earlier claims that the quadrupedal humans represent a stage of reverse evolution. Instead, she counters, members of the family adopted the walking style as a result of Uner Tan Syndrome, a human quadrupedalism condition that also includes impaired intelligence and rudimentary speech.
"Although it’s unusual that humans with UTS habitually walk on four limbs, this form of quadrupedalism resembles that of healthy adults and is thus not at all unexpected," Shapiro said in a written statement. "As we have shown, quadrupedalism in healthy adults or those with a physical disability can be explained using biomechanical principles rather than evolutionary assumptions."
Uner Tan, a Turkish neuroscientist and evolutionary biologist, first described the syndrome following his observations of the Kurdish family in 2005. He later encountered more instances of human quadrupedalism in other small villages in the region and interpreted the habitual motor behavior in terms of evolution -- the affected individuals were walking like our ape-like ancestors.
But Shapiro noticed one key difference: the gait.
Shapiro and the team analyzed videos of 518 quadrupedal walking strides and compared the gaits to the walking sequences of healthy adults, who were asked to move on all fours around the laboratory.
What they found is, unlike nonhuman primates, patients with UTS walk in a lateral sequence, placing a foot and hand down on one side, before switching to the other. Apes, on the other hand, walk in a diagonal sequence, meaning they put a foot and hand down on opposite sides when they stride.
So, though the family walks on all fours, they walk like any normal adult would (if that person were asked to walk on their hands and feet). Based on the finding, Shapiro and the team ruled out backward evolution entirely.
"I was determined to publish this and set the record straight, because these erroneous claims about the nature and cause of the quadrupedalism in these individuals have been published over and over again, without any actual analysis of the biomechanics of their gait, and by researchers who are experts in primitive locomotion," Shapiro told The Washington Post.
Shapiro's research was published online on July 16 in PLOS One.