The theory of devolution: Part one

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Researched by: Rachel John & Priyanka Gulati

First, tell me about this Turkish family…

The Ulas family: was first discovered in 2005 by biologist Uner Tan in a small village in southern Turkey. Of the 19 children, four have only walked on all fours—using both hands and feet. And one alternates between being bipedal and ‘quadrupedal’. They also suffer from cognitive problems and can barely speak. According to Tan, they have fewer than a hundred words in their vocabulary: 

“What is the year?” Tan said he asked one of the siblings. “Eighty,” one said. “Ninety,” another replied. “Animals,” said another. “July,” explained the fourth. “House,” the last said.

But they are able to move with ease and speed—without discomfort:

Still, their agility on all fours is impressive. “Their preferred form of locomotion, even when climbing or descending steps, is on all fours,” stated another study. “They move in this way fluently and effectively, and seemingly without discomfort. This contrasts markedly with normal adult humans who find such a gait — if and when they try it — tiring and uncomfortable even after practice.

You can see them in a clip of the BBC documentary below:

The Uner Tan Syndrome: Their condition—“loss of balance, impaired cognitive abilities and a habitual quadrupedal gait”—was named after the biologist who discovered them. Far more controversial was his diagnosis of the cause. Tan declared they were evidence of “a backward stage in human evolution.” He said:

In other words, the siblings were thought to be walking proof that our evolutionary advances could — poof — vanish, and we’d be back to walking on all fours. “The idea of reverse evolution was just a flash, an ‘aha’ experience,” Tan told NeuroQuantology. “I suddenly realised they were exhibiting the walking style of our ape-like ancestors. … I was the scientist who first suggested the existence of reverse evolution in human beings.”

Really, backward evolution? Umm, maybe not. In 2005, two British scientists took a closer look at the family—accompanied by a BBC documentary crew (watch the near 40-minute doc here). They concluded that the children suffered from a genetic condition called congenital cerebellar ataxia—damage to the cerebellum that causes balance and coordination problems. Because of this disorder, they never graduated to walking on two legs—and perfected being quadrupedal instead.

Nope, not like primates: Over the years, other researchers have added to the burden of evidence disproving Tan’s theory. One study showed that the siblings do not walk like primates (apes, monkeys etc). They put all their weight on their wrists not their knuckles. Another 2014 paper confirmed the conclusion:

Primates walk in a diagonal sequence, in which they put a hand on one side and a foot on the other, repeating this pattern as they progress forward. These humans, meanwhile, walk laterally — similar to other quadrupedals.

It’s easier to understand when you see the comparison below: 

Their condition instead was likely caused by a genetic mutation of a key chromosome. But to this day, scientists have not been able to pinpoint the exact underlying cause. 

So, basically, devolution is impossible…

Not exactly. The debate is muddied by the fact that we no longer agree on a single theory of evolution. 

Evolution 101: It all started with the Greek philosopher Anaximander of Miletus, a Greek philosopher who in the 500s BCE speculated that we may have descended from fish—and that all life began at sea. Both of which proved astonishingly correct. But he didn’t have any scientific proof for his bold claims. 

The modern version: we all study in school dates back to good old Charles Darwin and his seminal book ‘On the Origin of Species’—where he laid out the idea of ‘natural selection’. It claimed that all creatures produce more offspring than can survive. Those with traits best equipped to survive pass on their genes—which determines how that species evolves. But if you alter the environment, then the traits required to survive also change or evolve. Here’s a simple example:

If a creature with poor eyesight happens to produce offspring with slightly better eyesight, thanks to random mutations, then that tiny bit more vision gives them more chance of survival. The longer they survive, the more chance they have to reproduce and pass on the genes that equipped them with slightly better eyesight. Some of their offspring might, in turn, have better eyesight than their parents, making it likelier that they, too, will reproduce.

But, but, but: Darwin had plenty to say about bats and baboons—but offered very little of the origins of homo sapiens. And there is now a mountain of evidence to show that life on earth did not evolve in a single file—with a “drumbeat march of progress toward a predestined goal.” A lot of what we think is established fact has now been proven wrong.

Extinction is normal: For starters, the notion that we evolve in response to our environment has proved to be false: 

Chief among these misconceptions is that species evolve or change because they need to change to adapt to shifting environmental demands; biologists refer to this fallacy as teleology. In fact, more than 99% of all species that ever lived are extinct, so clearly there is no requirement that species always adapt successfully. As the fossil record demonstrates, extinction is a perfectly natural—and indeed quite common—response to changing environmental conditions. 

In other words, species do not always evolve in response to a changing environment. 

What natural selection? Darwin’s core principle has suffered some of the greatest blows. We now know that some species evolve out of pure luck. Sometimes, it’s because some members of its population happen to possess traits that allow them to survive in a changing environment. Then there is the ‘genetic drift’ theory of evolution that argues some organisms—purely by chance—produce more offspring—

and therefore pass their genes onto the next generation. 

The biggest shocker: human molecules are mutating at great speed—but for no discernible reason whatsoever:

The genes were changing – that is, evolving – but natural selection wasn’t playing a part. Some genetic changes were being preserved for no reason apart from pure chance. Natural selection seemed to be asleep at the wheel.

Complexity ≠ ‘progress’: The other big mistake we make is that we confuse more complex features with evolutionary advantage. When it comes to Mother Nature, less is often more:

For example, the lower jaw in vertebrates shows decreasing complexity, as measured by the numbers of bones, from fish to reptiles to mammals. (Evolution adapted the extra jaw bones into ear bones.) Likewise, ancestral horses had several toes on each foot; modern horses have a single toe with a hoof.

So can we actually evolve backwards? This is getting confusing…

Yes it is! And the answer isn’t straight-forward. In part two, we look at whether humans are moving forwards or backwards—and whether our technological tinkering will replace ‘natural selection’.

Reading list

The Washington Post (splainer gift link) has the best reporting on the Ulas family. The Scientific American has two good reads: A deep dive into the “staggering complexity” of the theories of evolution; a short but insightful take on what we get wrong about evolution. Also worth your time: This 2022 Guardian long read on the bitter battle over evolutionary theory—and whether it is time to junk Charles Darwin.