FIVE brothers and sisters who can walk naturally only 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 the family may provide information on how man evolved from afour-legged hominid to develop the ability to walk on two feet more than three million years ago.
A genetic abnormality that may prevent the siblings, aged from 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 legs extended, 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 apelike 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 manoeuvres so critical in the evolution of man.
Nicholas Humphrey, evolutionary psychologist at the London School of Economics, who has visited the family, said the siblings appeared to have reverted to an instinctive form of behaviour encoded deep in the brain, but abandoned in the course of evolution.
"It has produced an extraordinary window on our past," he said. "It is physically possible, which no one would have guessed from the (modern) human skeleton."
Professor Humphrey, who has been contributing to a BBC program, The Family that Walks on All Fours, to be broadcast on March 17, said that weeks of study, and factors such as the shape of their hands and the callouses on them, showed this was a long-term pattern of behaviour and not a hoax.
The siblings, who live with their parents and 13 other brothers and sisters, are mentally [color=red]*[/color][color=red]*[/color][color=red]*[/color][color=red]*[/color][color=red]*[/color][color=red]*[/color][color=red]*[/color][color=red]*[/color] 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 behaviour.
Professor Humphrey said cultural influences in their upbringing might have played a crucial role, with parental tolerance allowing the children to keep to quadrupedal walking.
But others believe 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.
Researchers said that while the women affected - Safiye, 34, Senem, 22, and Amosh, 18 - tended to spend their time sitting outside the family's very basic rural home, one brother, Huseyin, 28, went into the local village on all fours, where he could engage in the most basic interactions.
Jemima Harrison, of Passionate Productions, which produced the documentary, said the family's identity and location were not being disclosed.
"They walk like animals and that's very disturbing at first," she said. "But we were also very moved by this family's tremendous warmth and humanity."