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An extraordinary family who walk on all fours are being hailed as the breakthrough discovery which could shed light on the moment Man first stood upright.
Scientists believe that the five brothers and sisters found in Turkey could hold unique insights into human evolution.
The Kurdish siblings, aged between 18 and 34 and from the rural south, 'bear crawl' on their feet and palms.
Study of the five has shown the astonishing behaviour is not a hoax and they are largely unable to walk otherwise.
Researchers have found a genetic condition which accounts for their extraordinary movement.
And it could provide invaluable information on how humans evolved from a four-legged hominid into a creature walking on two feet.
Two of the daughters and a son have only ever walked on two palms and two feet, but another son and daughter sometimes manage to walk upright.
The five can stand upright, but only for a short time, with both knees and head flexed.
Their remarkable story is told in a television documentary, to be screened next week, which shows scientists studying their movement, but also their struggle to fit in with modern society.
Professor Nicholas Humphrey, evolutionary psychologist at the London School of Economics, visited the family twice. He said: "It's amazing as an example of a strange, strange aberration of human development. But their interest is how they can live in the modern world."
The five are all mentally retarded. Their mother and father, who are closely related are believed to have handed down a unique combination of genes which result in the behaviour.
Some researchers argue the genetic fault has caused the brothers and sisters to regress to a form of 'backward evolution'. Others believe it has led to brain damage which has allowed them to develop the walk.
Rather than walking on their knuckles, like gorillas or chimpanzees, they walk on the palms of their hands, with their fingers spread upwards.
Scientists believe this may be the way hominids moved to protect their fingers for more delicate movements.
Prof Humphrey said he thought the family had reverted to an instinctive form of behaviour encoded deep in the brain but abandoned during evolution.
He said: "I do not think they were destined to be quadrupeds by their genes, but their unique genetic make-up allowed them to be.
'It has produced an extraordinary window on our past. It is physically possible, which no one would have guessed from the modern human skeleton."
Study of their hands has shown they are heavily callused and have been walking like this for years.
Prof Humphrey said: "However they arrived at this point, we have adult human beings walking like ancestors several million years ago."
The five siblings spend most of their time sitting outside the family's basic rural home.
However, one brother travels to the local village where he engages in basic interactions with people.
The documentary, to be shown on BBC2 on Friday, March 17, is called The Family That Walks On All Fours.
