A quadrupedal family made famous by the 2006 BBC documentary The Family That Walks on All Fours have simply adapted to their inability to walk upright and do not represent an example of backward evolultion, as previously suggested.
Five siblings in the family, who live in a remote corner of Turkey, have a condition called Uner Tan Syndrome which causes them to walk exclusively on their hands and feet. Four sisters of the Ulas family, Safiye, Hacer, Senem and Emine, and their brother, Hüseyin, walk with the unusual gait.
Since they were discovered in 2005, scientists have debated the nature of their disability, with speculation they represent a backward stage of evolution.
According to a theory developed by Uner Tan of Cukurova University in Turkey, people with UTS are a human model for reverse evolution or "devolution", offering new insights into the human transition from four-legged to two-legged walking.
Now, new research by Liza Shapiro, an anthropologist at The University of Texas at Austin, reveals that people with the condition do not walk in the diagonal pattern characteristic of nonhuman primates such as apes and monkeys.
For the study, 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.
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, nearly all 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.
Yet apes and other nonhuman primates walk in a diagonal sequence, in which they put down a foot on one side and then a hand on the other side, continuing that pattern as they move 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 said, as quoted by Science Daily.
"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 shows that Tan's theory appeared to have misidentified the walking patterns among people with UTS as primate-like by confusing 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 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 told Science 2.0.
"The use of diagonal couplets in adult humans walking quadrupedally can thus be explained on the basis of biomechanical considerations, not reverse evolution."
The study has been published in PLOS One.