LONDON (AFP) - The discovery of a Turkish family that walks on all fours could aid research into the evolution of humans.
Researchers believe the five brothers and sisters, who can walk naturally only on all fours, may provide new information on how humans evolved from four-legged hominids to walk upright.
Nicholas Humphrey, evolutionary psychologist at the London School of Economics, told The Times the discovery opened "an extraordinary window on our past".
"I do not think they were designed to be quadrupeds by their genes, but their unique genetic make-up allowed them to be," he said.
"It has produced an extraordinary window on our past. It is physically possible, which noone would have guessed from the [modern] human skeleton."
The siblings, the subject of a new BBC documentary to be aired on March 17, suffer from a genetic abnormality that may prevent them from walking upright.
Instead, they use their palms like heels with their fingers sticking up from the ground.
The BBC said the documentary would contribute to fierce scientific debate and raised profound questions about what it is to be human.
Humphrey, who has contributed to the documentary, believes the style of walking may be a throwback to a form of behaviour abandoned by humans more than three million years ago.
Two sisters and one son have only ever walked on two hands and two feet, while another daughter and son occasionally walk on two feet.
All five are mentally retarded and have problems with language as a result of a form of underdevelopment of the brain known as cerebellar ataxia.
However Humphrey told the Times their behaviour may be partly the result of their parents tolerating the behaviour in childhood.
They are aged between 18 and 34 and live in southern Turkey, athough the makers of the documentary have not disclosed their exact location.
"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," Jemima Harrison of Passionate Productions told the Times.
Does the fact that this family, transformed into a remnant of the human past and discovered in remote Turkey, is made to tell an uplifting story about "warmth and humanity" -- by a documentary outfit named Passioante Productions, no less -- bother anyone else?