By Ian Sample in London

GUARDIAN NEWS SERVICE: Scientists claim to have discovered a gene that helps humans walk upright,after studying families with a rare condition that causes some of their members to walk on all fours.

Only a handful of families worldwide are known to be affected by quadrapedal locomotion syndrome, a condition that gained widespread attention in 2006 when the BBC aired a documentary on the lives of five affected members of the Ulas family, who live in Turkey.

People with the syndrome do not walk upright but use the palms of their hands in what is described as a "bear crawl".

Prof Tayfun Ozcelik, a geneticist at Bilkent University, Turkey, tested four unrelated families affected by the condition, believed to be caused by faulty brain development. The disorder also impairs speech and mental ability.

All of the affected children tested by Ozcelik were the offspring of marriages between cousins. The scientist found that two of the families carried a rare mutation in a gene that governs levels of a protein important for healthy growth of the cerebellum area of the brain.

Ozcelik, who will discuss his findings today at a meeting of the European Society of Human Genetics in Barcelona, said: "We think this protein is critical for the proper development of the nervous system and our unique ability to balance and adopt a bipedal gait."

Prof Nicholas Humphrey, a psychologist at the London School of Economics, said more genes were likely to be involved in the disorder. Tests on families in Iraq and Brazil found different genes causing the syndrome, in each case. People affected have poor balance but can often walk upright with a walking frame.

According to Humphrey, the condition may could shed light on our evolutionary history and overturn the widely-held belief that our ancestors were knuckle walkers like modern-day apes. "What's intriguing is how easily these people seem to take to this alternative gait when they can't walk properly.

"It raises the question of whether this was how our ancestors walked."