Why Don’t You Have A Tail?

Of course, technically, humans do have tails. Only for a brief period, mind: “During the 5th to 6th week of intrauterine life, the human embryo has a tail with 10-12 vertebrae,” points out one 2012 report. “By 8 weeks, the human tail disappears.”

With vanishingly few exceptions, then, humans are born tailless – and even when we do come out caudally blessed, it’s almost never a true tail: usually empty of bones and unable to be moved by its owner, it’s little more than an “anomalous prolongation of the coccygeal vertebrae,” the report notes.

But that’s strange, right? After all, tails are ubiquitous in the animal kingdom. They aid cats with balance, and they help monkeys climb and swing from trees. Tails are useful for communication – witness the wagging behind of a dog or the warning flick of a rattlesnake’s rattle. If nothing else, they can be ditched and left as a distraction for any predators that might be after you. 

So why would we want to get rid of them?

The tale of the tail

The last time our species habitually had tails was about 25 million years ago, before the great apes split from Old World monkeys. The latter group would go on to keep their fifth limbs; we – along with fellow great apes like gorillas, chimps, orangutans, and bonobos – would not.

“None of us [great apes] have tails,” confirmed zoologist David Young, author of The Discovery of Evolution and Director of the University of Melbourne’s Tiegs Museum, in 2016. But we’re not alone in that distinction: “The lesser apes like gibbons don’t have tails either,” Young pointed out, “and they give us a clue as to how not having a tail can be an advantage.”

“Gibbons are able to use their long arms to swing from branch to branch in the treetops of south-east Asian forests,” Young told the University of Melbourne’s Pursuit Magazine. “As they swing along, the trunk and legs hang below giving the body an upright posture.” 

“A tail would just get in the way and be a nuisance to this type of locomotion.”

It’s for this rather obvious reason that our lack of tails is often linked with our bipedalism. Having a tail just wouldn’t be all that helpful to an upright, two-legged creature that specialized in long-distance hunting across the ground – and so, the idea goes, we ditched them. 

It’s certainly a neat solution, and it’s hard to deny that it feels right, jibing as it does with a few million years’ worth of experience with upright gaits. But unfortunately, it’s completely backwards. 

“[T]he tail was lost first,” biological anthropologist Liza Shapiro, a professor in the department of anthropology at the University of Texas at Austin, told CNN in 2024. “The locomotion we associate with living apes evolved subsequently.”

“Evolution works from what is already there,” she pointed out. “I wouldn’t say that loss of the tail helps us understand the evolution of human bipedalism in any direct way.”

Fitting into our genes

The question of why we lost our tails is, then, something of a non-starter – evolution just doesn’t work that way. Perhaps more achievable is this: how did it happen?

“This question – where’s my tail? – has been in my head since I was a kid,” said Bo Xia, then a graduate student in stem cell biology at NYU Grossman School of Medicine, in 2021. It wasn’t until a fateful coccyx injury in 2019, though, that he would be spurred to change the world of tailology: “It took me a year to recover,” he told the New York Times, “and that really stimulated me to think about the tailbone.”

As it turned out, the key to the conundrum wasn’t to consider what humans don’t have, but what other animals do. For other species, the stage is set for the tail early in fetal development: the eventual body plan of an animal is governed by the so-called Hox genes, and they kick in when the organism is only just complex enough to have an inside and an outside.

Part of the job of these genes is, of course, figuring out the design of the spine – and, for many animals, that includes the tail. After this initial push from the Hox genes, a whole gamut of other genes come into play: some code for long and thin; others for bushy and furry; still more for “in an emergency, fall off”. Frankly, we’re still a ways off from figuring out precisely which gene has exactly what effect – but surely, Xia reasoned, it must be here that the answer for our own missing appendage would be found.

So, he and his colleagues compared the DNA of six species of tailless apes with nine species of tailed monkeys – and found something nobody was expecting. It “was like a lightning bolt,” Jef Boeke, director of the Institute for Systems Genetics at NYU Langone Health and a senior author of that study, told Live Science at the time. 

“It was noncoding DNA,” he explained – sequences “that [were] 100 percent conserved in all the apes and 100 percent absent in all the monkeys.”

But here’s the real kicker: it had been right under our noses the whole time.

Junk in the trunk

The genes responsible for our tail loss are small – a short snippet of DNA known as an Alu element – and it’s easily found throughout our genome. Until about five years ago, however, it was assumed to be useless – “junk” genetic material that our species has long outgrown the need for. 

But Alu elements are also part of a class known as “jumping” genes: they can move around in the genome, triggering mutations as they do so. Evidently, at some point in the distant past, one Alu element found itself in the tail-length governing TBXT gene – and, once there, it started to misbehave.

When the Alu element is present, “you lose the tail in one fell swoop,” Itai Yanai, a professor in NYU Langone Health’s Department of Biochemistry and Molecular Pharmacology and coauthor of the study, told New Scientist in 2021. 

And how do we know? Because they proved it – the team inserted the Alu element into the tail genes of mice, and boom: no tail. “It’s a really nice example of how a strange evolutionary quirk can have interesting and important consequences,” David Kimelman, a geneticist at the University of Washington’s department of genome sciences, told Scientific American at the time. 

More than that, Kimelman said, it “gets to the basis of how this major change in primates occurred” – which was, to the surprise of many, all at once.

Not for apes was the slow, incremental loss of this once-major limb – no, this change must have happened almost overnight. And it wasn’t without cost: Xia’s team noticed that mice spliced with the Alu element didn’t just lose their tails, but also had a much higher risk of developing a spinal cord defect similar to spina bifida – a lifelong congenital problem that affects around one in 1,000 human newborns. 

Nevertheless, we kept the mutation.

“There must have been such huge evolutionary pressure to lose the tail,” Yanai told Scientific American. “[Enough] that even if it came with the price tag of this horrible disease, it was worth it.”

The trade-off

So, why don’t humans have tails? Well, the most direct answer is simple: because of a random mutation caused by a jumping Alu gene. But the deeper question – the one you were really asking all those years ago in your childlike curiosity – is far more difficult.

“I think it’s really interesting to pinpoint a genetic mechanism that might have been responsible for loss of the tail in hominoids, and [Xia’s] paper makes a valuable contribution that way,” Shapiro told CNN. “But it does not help us understand why the tail was lost in the first place.”

The mutation that took our tails may have been found, but the fact that we kept it is still a puzzle. Was it because the loss of a tail was genuinely helpful? Or was it just not enough of a drawback to kill us off? These are questions that, sadly, we have yet to resolve.

Still, we should probably make the most of it. After all, it’s been 25 million years since we lost our tails, and those babies are probably not coming back: “It’s reasonable to think that during that time, there were many more mutations related to stabilizing the loss of the tail,” Yanai told CNN. 

That means that, even if we could undo the Alu mutation, “it still wouldn’t bring back the tail,” he said.