What 'separates us from the animals'. as the saying goes?

Not a lot. We're all animals, of course, but among primates there is an easy-to-spot difference: Humans tend to walk in lateral sequences, a foot down and then a hand on the same side and then moved in the same sequence on the other side, while apes and other non-human 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. 

What does that mean? It means quadripedalism, such as among the five Turkish siblings profiled in the 2006 BBC2 documentary "The Family That Walks on All Fours", does not mean anyone is devolving or evolving backwards.

A comparison of footfall sequence in primate (baboon, above) and nonprimate (cat, below). Footfall sequence is depicted numerically, beginning with the right hind limb in each animal. The primate is walking in diagonal sequence (RH-LF-LH-RF), and the nonprimate is walking in lateral sequence (RH-RF-LH-LF), where R=right, L=left, H=hind limb, and F=forelimb. Images are from Muybridge E (1887) . Animal Locomotion: An Electro-photographic Investigation of Consecutive Phases of Animal Movements, 1872-1885: 112 Plates. Published under the auspices of the University of Pennsylvania.

According to a hypothesis developed by Uner Tan of Cukurova University in Turkey, people with Uner Tan Syndrome (UTS) do not walk in the diagonal pattern characteristic of non-human primates such as apes and monkeys. They are a human model for reverse evolution, or “devolution,” offering new insights into the human transition from four-legged to two-legged walking.

Other research 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 resembles that of nonhuman primates.

As part of a study, researchers analyzed 518 quadrupedal walking strides from several videos of people with various forms of UTS, including footage from the BBC2 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. 

"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," says 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 also shows that Tan and his colleagues 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 says. "The use of diagonal couplets in adult humans walking quadrupedally can thus be explained on the basis of biomechanical considerations, not reverse evolution."