Five brothers and sisters who can only walk naturally on all fours are being hailed as a unique insight into human evolution after being found in a remote corner of rural Turkey.

Scientists believe that the family may provide invaluable information on how man evolved from a four-legged hominid to develop the ability to walk on two feet more than three million years ago.

A genetic abnormality, which may prevent the siblings, aged 18 to 34, from walking upright, has been identified.

The discovery of the Kurdish family in southern Turkey last July has triggered a fierce debate.

Two daughters and a son have only ever walked on two palms and two feet, with their extended legs, while another daughter and son occasionally manage a form of two-footed walking. The five can stand up, but only for a short time, with both knees and head flexed.

Some researchers claim that genetic faults have caused the siblings to regress in a form of "backward evolution." Other scientists argue more strongly that their genes have triggered brain damage that has allowed them to develop the unique form of movement.

But all agree that the family's walk, described as a "bear crawl," may offer invaluable information on how our ape-like ancestors moved.

Rather than walking on their knuckles like gorillas and chimpanzees, the family are "wrist walkers," using their palms like heels with their fingers angled up from the ground.

Scientists believe this may be the way hominids moved, allowing them to protect their fingers for the more delicate and dextrous maneuvers so critical in the evolution of man.

Nicholas Humphrey, evolutionary psychologist at the London School of Economics, who has visited the family, said that the siblings appeared to have reverted to an instinctive form of behavior encoded deep in the brain, but abandoned in the course of evolution.

"I do not think they were destined to be quadrupeds by their genes, but their unique genetic makeup allowed them to be," Humphrey said. "It is physically possible, which no one would have guessed from the [modern] human skeleton."

Humphrey, who has been contributing to an upcoming BBC program, "The Family that Walks on All Fours," to be broadcast in Britain on March 17, said that weeks of study, and factors such as their hands' shape and callouses, showed that this was a long-term pattern of behavior and not a hoax.

"However they arrived at this point, we have adult human beings walking like ancestors several million years ago," he said.

The siblings, who live with their parents and 13 other brothers and sisters, are mentally retarded, as a result of a form of cerebellar ataxia — an underdevelopment of the brain similar to that in cystic fibrosis.

Their mother and father, who are themselves closely related, are believed to have passed down a unique combination of genes resulting in the behavior.

While Humphrey said that cultural influences in their upbringing may have played a crucial role, with parental tolerance allowing the children to keep to quadrupedal walking, others believe that the cause is more purely genetic.

Uner Tan, a professor of physiology at Cukurova University in Adana, Turkey, who first brought the family to the attention of scientists, argues that the gene mutations have made them regress to a "missing link" primate state, also explaining their severe problems with language.

A team of German geneticists believes that the family holds the key to a breakthrough gene for bipedality.

Researchers said that while the women affected, Safiye, 34, Senem, 22, and Amosh, 18, tended to spend their time sitting outside the family's basic rural home, one brother, Huseyin, 28, went into the local village on all fours, where he could engage in basic interactions.

Jemima Harrison, of Passionate Productions, which is producing the documentary, said: "They walk like animals and that's very disturbing at first. But we were also very moved by this family's tremendous warmth and humanity."