Scientists regard scepticism as their professional duty; not one who has visited the Ulas family believes they are perpetrating a hoax. “The best dramatist could not have invented this family,” says Professor Nicholas Humphrey, an evolutionary psychologist at the London School of Economics whose visits were filmed for the BBC documentary.

The family has captivated the world’s media because of the possibility that they are “living fossils”, flesh-and-blood genetic throwbacks to an ancestral creature that, four million years ago, roamed the Earth on all fours, later pulled itself upright and eventually evolved into us. The medical literature contains very rare and controversial reports of babies born with small tails at the base of their spines, and children with excessively hairy faces — such reports suggest that genetic defects can cause the most recent adaptations in our DNA to be peeled away, allowing an ancient gene to resurface and remind us, spectacularly, of our animal origins. A trait that reappears after a long absence is called an atavism.

The family first came to scientific attention in 2004, when Dr Osman Demirhan, a geneticist at Cukurova University in southern Turkey, read a newspaper report about them and hunted them down. Demirhan was researching rare genetic defects that arise through inbreeding. He alerted a university colleague, Professor Uner Tan. His opinion was that the retarded siblings spoke their own “primitive language” and had limited manual dexterity. This, Tan suggested, showed the siblings were “genetic throwbacks”. Tan named the condition after himself (UnerTan syndrome). The family, devout Muslims who do not believe in evolution, find Tan’s theory upsetting (other scientists are dismissive, pointing out that the siblings can speak limited Kurdish and show dexterity).

Professor Humphrey believes that both nature and nurture are at work: “My view is that their genetic defect exposed an atavism and they have rediscovered or reinvented an ancient way of walking.” This is why the Ulas family are described as “dynamite”: although evolutionary scientists believe human ancestors walked on their hands, nobody knew how. People assume that they were knuckle-walkers, but it would be hard to develop manual dexterity if our hands were dragged on the ground. Wrist-walking, as practised by the Ulas family, leaves palms calloused but fingers undamaged, allowing them to perform such nimble tasks as knot-tying, needlework and holding a cigarette.

All five are brain-damaged because of a mutation in a gene 17p, located on chromosome 17, which occurred because mother and father are closely related: Hatice’s maternal grandmother was the sister of Resit’s father (which makes them one-sixteenth related). The result is cerebellar ataxia, an underdevelopment of the brain, which has left the five children mentally retarded. The brain damage has been confirmed by MRI scans. Another brother, Gulin, 34, shows the same genetic mutation, brain damage and retardation, but, crucially, can walk normally.

A team in Berlin, led by Professor Stefan Mundlos, recently published a paper in the Journal of Medical Genetics claiming that gene 17p must hold the secret to bipedality. Humphrey says: “Mundlos says he can’t believe that something in the local environment could keep them on four legs. My feeling is that families matter more than we could ever believe.”

Gulin is Professor Humphrey’s most potent weapon against the German team’s genetic theory of bipedality — how can Gulin show the same genetic mutation and brain damage as his quadrupedal siblings and yet not be condemned to life on all fours? His view, instead, is that the unique environment into which the siblings were born provided the perfect circumstances for this ancestral gait to resurface. Nature and nature conspired, he says, to produce “an extraordinary conjunction of circumstances” that led to quadrupedality.

Professor Humphrey says that a combination of factors — the genetic defect, the huge family, limited parental attention, sibling imitation, skeletal anomalies, the lack of intervention from the outside world — led to this remarkable outcome.

“If there was just a one in a thousand chance of each of these things happening, this soon adds up to a one in a billion chance of it happening in the same family. It’s not surprising we’ve never seen it before. We may never see it again.”