Contradicting earlier claims of “backward evolution,” Shapiro and her team of researchers found the group of siblings made famous by a 2006 BBC documentary, “The Family That Walks on All Fours,” have simply adapted to their inability to walk upright.
Shapiro’s study, published in PLOS One, shows that people with the family members’ condition, called Uner Tan Syndrome (UTS), do not walk in the diagonal pattern characteristic of nonhuman primates such as apes and monkeys. The condition is named after Turkish researcher Uner Tan, who theorized that the quadrupedal family represented a human model for reverse evolution.
The researchers analyzed 518 quadrupedal walking strides from several videos of people with various forms of UTS, including footage from the documentary. They compared these walking strides to previous studies of the walking patterns of healthy adults who were asked to move around a laboratory on all fours.
According to the findings, published in PLOS One, nearly all human subjects (in 98 percent of the total strides) walked in lateral sequences, placing a foot down and then a hand on one side and then repeating the sequence on the other side. Apes and other nonhuman primates, however, walk in a diagonal sequence, placing a foot down on one side and then a hand on the other side.
“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.”