The Ulas family, in southern Türkiye, is like no other. For the last two decades, some of its members have been quietly challenging just about every theory we have about evolution and genetics – and all despite being unable to read, write, or even talk easily with outsiders.

Why? Because, unlike just about every human being on Earth, they aren’t bipedal. They walk on all fours – and scientists are kind of baffled as to why.

Why does the Ulas family walk on all fours?

From the moment the Ulas siblings first hit the headlines back in 2006, hypotheses have abounded as to what caused their peculiar gait. Today, though, after nearly 20 years of research, we’re still not completely sure what’s going on.

Out of 19 children in the family, it’s a group of only five siblings who are affected by this quadrupedalism – and while that’s certainly the symptom that tends to make the headlines, it’s definitely not the only difficulty they face. Brain scans have revealed that the siblings have remarkably small cerebella, the part of the brain that controls motor function; they also have severely impacted cognitive abilities, including language deficits to the point of being unable to communicate with those outside their immediate family.

For Üner Tan, the evolutionary biologist who first reported on the family, the answer to this puzzling collection of symptoms was simple: it was, he announced, an entirely new disorder – one he humbly named “Üner Tan Syndrome”. In a slightly bizarre paper detailing the supposed condition, he linked the siblings’ four-limbed gait to their impaired cognitive abilities, arguing that “to be able to stand up despite the very strong gravitational forces, one must first have a brain equipped with a resistive mind. This is the human mind.”

From there, it was just a short hop and a jump to “this genetic disorder suggests a backward stage in human evolution”- a direct quote from the paper’s conclusion – and thus on to attention-grabbing headlines claiming the family “shouldn’t exist” or “have undone three million years of evolution”. But was that the real story?

Backwards evolution?

As beguiling as this idea of a chance look back into our collective ancestry is, there are a few… let’s say, gaping holes in the theory. 

So, let’s start with the most obvious: chimps and other apes don’t actually walk like that.

“I was determined to publish this and set the record straight,” Liza J. Shapiro, a biological anthropologist in the Department of Anthropology at the University of Texas, told The Washington Post back in 2014. She and her colleagues had just put forward a paper detailing the exact mechanisms of the Ulas family’s walking pattern, and comparing it with those of nonhuman primates – something which, apparently, nobody had yet thought to do.

“[T]hese erroneous claims about the nature and cause of the quadrupedalism in these individuals have been published over and over again, without any actual analysis of the biomechanics of their gait,” Shapiro said, “and by researchers who are not experts in primate locomotion.”

And, after watching more than 500 pieces of footage of the family, the results were clear: they walked almost exclusively with a lateral-sequence gait – in other words, they moved the left hand after the left foot, and the right hand after the foot. Quadrupedal primates, on the other hand, have a diagonal-sequence gait: they move the left hand after the right foot, and vice versa.

Add to that the fact that the Ulas siblings walk on their palms – not, as our primate relatives universally prefer, their knuckles – and it all adds up to a not-very-apelike method of locomotion at all. In fact, there was only one type of primate the Ulas family’s particular method of walking resembled: “healthy adult humans,” wrote Shapiro and her colleagues, “asked to walk quadrupedally in an experimental setting.”

“We conclude that quadrupedalism in healthy adults or those with a physical disability can be explained using biomechanical principles rather than evolutionary assumptions,” they added.

The genetic factor

Further piercing Tan’s suggestion that his eponymous syndrome was the result of some genetic anomaly – perhaps a blip in whatever piece of DNA took us from Australopithecus to Homo – is the fact that, well, genetics doesn’t work that way, either.

“The interpretation that the members of this family are reflecting some sort of genetic throwback or some sort of ancestral situation – I don't agree with that interpretation whatsoever,” evolutionary developmental biologist Sean Carroll, a Professor in the Department of Biology at the University of Maryland, told PBS back in 2006. “They have a very serious defect in their cerebellum, and they are compensating for that balance problem by walking with the aid of their hands.”

In fact, the very notion of a single genetic mutation being responsible for a whole family of quadrupeds is “completely invalid,” he said. 

“There is no single gene for humans walking upright,” Carroll explained. “Walking upright is something that was enabled by the collective actions of lots of genes remodeling our skeleton and musculature.”

That’s certainly not to say there’s zero genetic component – clearly, something is going on with the family – but any mutation the family have is likely to be more generalist than specialist, Carroll explained. Defects in the cerebellum are linked not just to motor disorders, but also autism, problems with vision and hearing, problems with communication, cognition, emotional regulation, and executive function, seizure disorders, and even cardiac issues – so it’s likely that any genetic cause of the Ulases’ four-limbed walk will be part of a much bigger picture than simply some on/off button for bipedalism. 

“It's very common that these body-building genes affect multiple parts of the body,” Carroll said – adding that “you sort of miss the whole picture when you speak of a gene for bipedality. That's certainly going to be the case with things like speech and language as well.” 

“There's no single gene that if you have it you can talk, and if you don't have it you can't,” he said.

Falling through the net

Further complicating the idea that some genetic mutation, perhaps in the cerebellum, is responsible for the siblings’ inability to walk on two legs is a rather fundamental stumbling block: the fact that, if given the right support, they kind of can walk on two legs.

“With some assistance they can stand upright,” Carroll pointed out. “And you can see that other members of the family with the exact same anatomical defect and the exact same genetic lesion have walked upright to some degree.”

In fact, a cerebellum isn’t necessary for walking on two legs at all: it’s possible, though almost vanishingly rare, for that area of the brain to not develop at all – but those affected have been recorded walking normally. For the Ulas family, the simple addition of a walking frame was enough to revolutionize their day-to-day life: “within a few hours it was an astonishing transformation,” evolutionary psychologist Nicholas Humphrey, a professor at the London School of Economics, told 60 Minutes Australia in 2018.

“The children, who had never taken a step upright on two legs, were using this frame to walk across the room,” he said. “[They had] such delight on their faces, and a sense of achievement, and of having suddenly made a breakthrough into a world they never imagined they could enter.”

So, if not entirely genetic, what could make up the shortfall? What, in the end, is responsible for these siblings’ inability to walk on two feet?

The answer, it turns out, may simply come down to where the siblings live. Growing up in remote, rural Türkiye, the family lacked easy access to medicine and social programs – and the result was a group of children who got left behind when they failed to progress as is typical.

“We don’t know why nobody intervened,” Humphrey said. “Why they didn’t do the simplest things they might have done to help them […] as would have happened in Melbourne or in London or in New York.”

So, why do the Ulas family walk on all fours? It’s still an open question – but perhaps not the one that we ought to be asking right now.

“[It’s] a major phenomenon […] to explain from a scientific point of view,” Humphrey concluded, “but [it’s] also, of course, a major tragic human story.”