Professor Humphrey, from the London School of Economics (LSE), says that our own species' transition to walking on two feet must have been a more complex process that involved many changes to the skeleton and to the human genetic make-up.

However, a German group says a genetic abnormality does seem to be involved in the siblings' gait.

Coordination problem

Three of the sisters and one brother have only ever walked on two hands and two feet, but another sister alternates between a bipedal and quadrupedal gait. Another brother walks on two feet all the time, but only with difficulty.

The siblings live with their parents and five other brothers and sisters. They were born with what looks like a form of brain damage.

MRI scans seem to show that they have a form of cerebellar ataxia, which affects balance and coordination.

However, scientists are divided on what caused them to revert to quadrupedalism (walking on all fours).

The method of locomotion used by the Turkish children and by our closest relatives chimpanzees and gorillas, differs in a crucial way, said Professor Humphrey.

While gorillas and chimpanzees walk on their knuckles, the Turkish siblings put their weight on the wrists, lifting their fingers off the ground.

Tool use

"What's significant about that is that chimpanzees ruin their fingers walking like that," Professor Humphrey, an evolutionary psychologist, told the BBC News website.

"These kids have kept their fingers very agile, for example, the girls in the family can do crochet and embroidery."

He added that calluses pictured on the hands of one family member demonstrated that the behaviour was not a hoax.

Professor Humphrey said this could be the way that humankind's direct ancestors walked.

Hands which have kept the fingers dextrous would also have been able to manipulate tools, a key development which influenced the evolution of the human body and intelligence.

"I think it's possible that what we are seeing in this family is something that does correspond to a time when we didn't walk like chimpanzees but was an important step between coming down from the trees and becoming fully bipedal," the LSE researcher said.

'Infant walking'

Professor Humphrey thinks that the brain abnormality simply caused the siblings to rediscover a form of locomotion used by our ancestors.

"Because of the peculiar circumstances they were in, they kept walking as infants," he said.

But a team led by Stefan Mundlos of the Max Planck Institute in Berlin, Germany, thinks that the genetic abnormality which causes the children's unusual gait may have played a more fundamental role in evolution.

Professor Mundlos has located the gene on chromosome 17 and speculates that a gene important in the transition to bipedalism may have been knocked out in the children.

Producer on the documentary Jemima Harrison said the programme's producers were moved by the family's "tremendous warmth and humanity".