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Debunked: Researchers Say Walking on All Fours Isn't Backward Evolution

Jul 17, 2014 04:50 AM EDT

New study finds that walking on all four limbs isn't backward evolution, but is an adaptation to a medical condition.

Back in 2006, BBC aired a documentary called "The Family That Walks on All Fours." Five siblings from Turkey were filmed walking on all four limbs. At that time, several researchers believed that the Turkish siblings' case supports the idea of backward evolution. Other researchers said that the case could provide a missing link in human evolution.

The condition of the Turkey siblings is called as Uner Tan Syndrome (UTS). It is named after Uner Tan of Cukurova University in Turkey, who had developed a theory to explain the siblings' condition.

Now, researchers at University of Texas at Austin and colleagues show that the walking pattern of the family is just an adaptation to a medical condition and is not an example of backward evolution. According to the team, people with UTS don't walk in the diagonal pattern, but walk in lateral sequence.

Apes and other monkeys walk in diagonal pattern whereas humans walk using lateral pattern, using hand and leg of the same side.

For the study, researchers looked at 518 quadrupedal walking strides of people with different forms of UTS, including the footage from BBC documentary. The team then compared the walking strides of healthy participants who were asked to walk on all fours in the laboratory.

Researchers found that 98 percent of healthy humans without UTS walk using a lateral pattern by placing one hand and one leg to make a stride, then switching to the other hand and leg to make a second stride and so forth. Apes use diagonal pattern and use limbs on the opposite sides to make a stride.

"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," said Liza Shapiro, an anthropologist at The University of Texas at Austin. "As we have shown, quadrupedalism in healthy adults or those with a physical disability can be explained using biomechanical principles rather than evolutionary assumptions."

The study shows that Tan and colleagues might have confused diagonal sequence with diagonal couplets. Sequence refers to the order in which limbs touch the ground while couplet refers to the timing of the movement between the limbs, according to a news release.

"Each type of couplet has biomechanical advantages, with lateral couplets serving to avoid limb interference, and diagonal couplets providing stability," Shapiro said in a news release. "The use of diagonal couplets in adult humans walking quadrupedally can thus be explained on the basis of biomechanical considerations, not reverse evolution."

Drugmaker giant Roche had, in 2010, announced that its researchers had identified the genetic mutation responsible for UTS.

The study is published in the journal PLOS One.

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