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How our ape ancestors suddenly lost their tails 25 million years ago

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A wild chimpanzee walking through a forest

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Around 25 million years ago, our ancestors lost their tails. Now geneticists may have found the exact mutation that prevents apes like us growing tails – and if they are right, this loss happened suddenly rather than tails gradually shrinking.

“You lose the tail in one fell swoop,” says Itai Yanai at NYU Langone Health in New York.

His colleague Bo Xia says he used to wonder as a child why people didn’t have tails like other animals. “This question was in my head when I was a little kid,” he says. “I was asking, ‘Where is my tail?’.”

More recently, Xia’s coccyx – a small bit of bone at the base of the spine that is a vestige of mammalian tails – was injured in a car accident. “It was really painful,” he says. “It kept reminding me about the tail part of our body.”

That led Xia to investigate the genetic basis of tail loss. Any mutations involved in tail loss should be present in apes but not monkeys. He and his colleagues compared ape and monkey versions of 31 genes involved in tail development.

They found nothing in the protein-coding regions, so they looked in the bits of junk DNA found inside genes. If you think of proteins as flat-pack furniture, the genetic instruction booklets for making them come with lots of pages of gibberish that have to be removed before the instructions work. These extra bits, called introns, are cut from the mRNA copies of genes before proteins are made.

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What Xia found is that in the ancestor of apes, in a tail gene called TBXT, an Alu element landed smack bang in the middle of an intron. Alu elements are genetic parasites that copy and paste themselves all over the genome. “We have 1 million Alu elements littering our genome,” says Yanai.

Normally, an Alu in an intron would make no difference – it would get edited out with the intron. But in this case, there is another Alu element nearby, but it is in inverse order. Because the two sequences are complementary, Xia realised, they bind together, forming a loop in the mRNA.

That effectively glues several pages of the instruction booklet together, meaning that when the extra pages are cut out, some of the instructions are often lost too. This means the assembled furniture – the TBXT protein – often has a key piece missing.

The team did several experiments to demonstrate this. For instance, they showed that mice with this mutation produce a mixture of full-length and missing-bit TBXT proteins – like apes do – and that this usually results in complete tail loss.

“For something to be lost in one big burst is really significant, because you don’t then have to posit millions of years of successive tiny changes accumulating gradually,” says Carol Ward at the University of Missouri. “It may tell us why all of a sudden when we see the apes [emerge] they have no tails,” she says. While there’s no evidence of a slow reduction in tail length in the fossil record, says Ward, for now we have too few fossils to rule it out.

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What the finding cannot tell us is why our ancestors lost their tails; that is, why this mutation was selected for by evolution. Most proposed explanations involve tails being a disadvantage when early apes started moving in a different way, such as walking upright on branches. But fossils suggest the first tail-less apes still walked on all fours, says Ward.

Xia and Yanai think there must have been a strong advantage to losing tails because this mutation does also have a disadvantage. Some mice developed spinal abnormalities resembling spina bifida. They speculate that the relatively high rate of spina bifida in people is a lingering relic of the loss of our tails all those millions of years ago.

Reference: bioXriv, DOI: 10.1101/2021.09.14.460388

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