All families have their own quirks and habits, but one group of relatives has such a unique trait that scientists have branded them a total anomaly of the human species.

The Ulas family has been the subject of evolutionary fascination for years after they were discovered in a remote village in Turkey walking on all fours.

Back in the early 2000s, a scientific paper was published on five of the Ulas siblings and their strange bear crawl-style of movement, with experts divided over the cause of the abnormality.

In the years following the paper’s publication, evolutionary psychologist Professor Nicholas Humphrey, of the London School of Economics (LSE), travelled to Turkey to meet with the extraordinary family.

The Ulas mother and father had a staggering 18 children, however, of these, only six were born with quadrupedalism (walking on all fours), which has never been seen before in modern adult humans.

“I never expected that even under the most extraordinary scientific fantasy that modern human beings could return to an animal state,” Humphrey told 60 Minutes Australia, which made a documentary about the family back in 2018.

“The thing which marks us off from the rest of the animal world is the fact that we’re the species which walks on two legs and holds our heads high in the air,” he added.

“Of course, it’s language and all other sorts of things too, but it’s terribly important to our sense of ourselves as being different from others in the animal kingdom. These people cross that boundary.”

The documentary describes the Ulas as “the missing link between man and ape” and suggests that they “shouldn’t exist” at all.

And yet, no one has yet figured out the precise cause of the strange walking style.

Whilst some experts have suggested that it’s caused by a genetic problem which has “undone the last three million years of evolution”, others have rejected the idea that there’s a specific “gene” for upright walking and suggested something else is at play.

Humphrey pointed out that the affected siblings – five of whom are still alive and aged between 22 and 38 – all suffer from a particular form of brain damage.

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He also stressed that the Ulas’ form of quadrupedalism differs from that seen in our closest animal relatives – chimpanzees and gorillas – in one key way.

Whilst these primates walk on their knuckles, the Turkish children’s use the palms of their hands – putting their weight on their wrists while lifting their fingers off the ground.

“What’s significant about that is that chimpanzees ruin their fingers walking like that,” Humphrey told the BBC News website back in 2006 when the BBC broadcast its own documentary about the family.

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

Humphrey has hypothesised that this could indeed be the way our direct ancestors walked.

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The LSE researcher also suggested that there are more basic explanations for the Ulas children’s quadrupedalism: they were simply not encouraged to walk on two feet.

In the Turkish village where they grew up, there was no local health service to help the disabled kids make the transition from crawling as babies (on hands and knees) to walking fully upright.