Contradicting earlier claims, “The Family That Walks on All Fours,” a group of quadrapedal humans made famous by a 2006 BBC documentary, have simply adapted to their inability to walk upright and in fact do not represent an example of backward evolution, according to new research conducted by Liza Shapiro, an anthropologist at The University of Texas at Austin.
Five siblings in a family, who live in a remote corner of Turkey, walk exclusively on their hands and feet. Since they were discovered back in 2005, scientists have debated over the nature of their disability, specifically the speculation that they represent a backward stage of evolution.
Shapiro’s study, published online this month in PLOS ONE, displays that contrary to previous claims, people with the condition, called Uner Tan Syndrome (UTS), do not walk in the diagonal pattern characteristic of nonhuman primates such as apes and monkeys.
According to a new theory developed by Uner Tan of Cukurova University in Turkey, people with UTS are a human model for reverse evolution, or “devolution,” offering new views of the human transition from four-legged to two-legged walking.
Previous research countering this view has proposed that the quadrupedalism associated with UTS is simply an adaptive response to the impaired ability to walk bipedally in individuals with a genetic mutation, but this is the first study that disproves claims that this form of walking is reminiscent of nonhuman primates.
Co-authors of the study are Jesse Young of Northeast Ohio Medical University; David Raichlen of the University of Arizona; and Whitney Cole, Scott Robinson and Karen Adolph of New York University.
The researchers analysed 518 quadrupedal walking strides from several videos of people with various forms of UTS, including footage from the BBC2 documentary of the five Turkish siblings. The team compared these walking strides to previous studies of the walking habits of healthy adults who were asked to move around a laboratory on all fours.
The study found that nearly all the human subjects (in 98% of the total strides) walked in lateral sequences, meaning they placed a foot down and then a hand on the same side and then moved in the same sequence on the other side. Nonhuman primates, however, walk in a diagonal sequence, meaning they put down a foot on one side and then a hand on the other side, continuing that pattern as they walk along.
“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 says. “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 also concluded that Tan and his colleagues appeared to have misidentified the walking patterns among people with UTS as primate-like by confusing the diagonal sequence with diagonal couplets. Sequence refers to the order in which the limbs touch the ground, while couplets (independent of sequence) indicate the timing of movement between pairs of limbs.
People with UTS more frequently use diagonal couplets rather than lateral couplets, but the sequence associated with the couplets is almost exclusively lateral.
“Each type of couplet has biomechanical advantages, with lateral couplets serving to avoid limb interference, and diagonal couplets providing stability,” Shapiro says. “The use of diagonal couplets in adult humans walking quadrupedally can thus be explained on the basis of biomechanical considerations, not reverse evolution.”
Contributing Source: University of Texas at Austin
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