Why 5 Siblings Walk on All Fours and Can't Stand Up

Some people walk on all fours for fun, or for sport, but not for long stretches and certainly not for ease of mobility. Yet five siblings in a family of 19 in Hatay, Turkey, walk not just primarily but only on all fours, and lack the balance to do otherwise. Since the siblings were first discovered by scientists in 2005—and featured in a BBC documentary in 2006—one popular explanation of their unusual quadrupedal gait was "reverse evolution"—meaning they could be living proof that we can essentially devolve. But new research published this week suggests that theory is false, and that the condition is instead an adaptation to a rare disorder, reports the Washington Post.

While 14 of the Ulas family siblings are fine, five have Uner Tan Syndrome, named after the Turkish evolutionary biologist who first studied them; they suffer from reduced balance and cognition. (When Tan in his first study asked them the year, the five siblings replied: "80," "90," animals," "July," and "house.") Their resulting gait, which differs from other primates in that it is lateral instead of diagonal and involves putting weight on wrists instead of knuckles, is actually a sign of adaptation to help cope with a loss of balance, the new research suggests. The family is protective of the oft-ridiculed siblings, and the four sisters mostly stay indoors and crochet, while their brother is reported to be more adventurous. All move gracefully, even down stairs. (Click to read about a gruesome mystery disease that appears to affect just one family on the entire planet.)