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Assuming it’s human, the odds of it having a tail are not zero, but they are very, very low. There are only 40 known cases of babies being born with a tail. But in fact, we all had a tail, with 10 to 12 vertebrae — which we lost before we were born.
This is one of those universal questions that all children (and not only) ask at some point: in the end Why do we humans Don’t we have a tail?
Our dogs and cats have it; most of vertebrates too. Even some invertebrates have something that vaguely resembles a tail.
And if we want to play with the subject a little, we can say that even some pianos have grands.
And we, who still speak metaphorically of our “tail” or we complain about the pain caused by a fall on the coccyxwe never had one of these appendages — neither useful nor decorative. But why?
Technically, We’ve all had tailseven if for a brief moment.
“During the 5th and 6th week of intrauterine lifethe human embryo has a tail with 10 to 12 vertebrae”, said a scientist from 2012. “At around 8 weeks, the tail disappears.”
With very rare exceptions, we are born without a tail. To date, only 40 babies, and half of the cases reviewed, have been linked to a specific disease — spina bifida occulta.
Even in cases where a small extension appears in the lumbar area, it is almost always of a boneless and immobile structure — “an anomalous prolongation of the coccygeal vertebrae”.
Curious, isn’t it? Especially when we look around us: not animal worldthe tails are practically universalnote or .
The tails they help cats balance, they allow monkeys to climb and swing between branches, and function as a communication signal — from a dog’s enthusiastic wag to the vibrant warning of a rattlesnake’s rattle. And, in many cases, they can even be used as a distraction before a predator.
Then, Why did humans lose it?
The day the tail stopped being useful
The last time our ancestors showed a traditional tail was about 25 million years, before separation among the great apes and monkeys of the Old World.
Old World monkeys kept their “fifth member”; the great apes — us, gorillas, chimpanzeesbonobos and orangutans — no.
“None of us great apes have tails”, confirmed the zoologist David Youngauthor of The Discovery of Evolution and director of the Tiegs Museum, University of Melbourne. “And the small apes, such as gibbons, also do not — and that gives us a clue about the advantages of this loss.”
Gibbons, Young explained to , a magazine from the University of Melbourne, “use their long arms to swing from branch to branch in the tropical forests of Southeast Asia. In doing so, the body remains in an upright posturewith the torso and legs hanging below.”
Under these conditions, “a tail would just be a hindrance”.
This is why the absence of a tail in humans is often associated with bipedalism. A tail would not be particularly useful for an animal that moves on two legs, covering long distances on hunts. Thus, says the classical theory, we lost the tail because we stopped needing it.
But this explanation, although intuitive, is wrong.
“The truth is that the tail was lost first”, he explained in 2024 to the anthropologist Liza Shapirofrom the University of Texas at Austin. “The form of locomotion that we associate with modern apes emerged later.”
“Evolution works with what already exists“, he added. “The loss of the tail does not directly explain the evolution of bipedalism.”
The answer was in our genes
If the question of “why” is almost impossible to answer, the “how” is much more accessible — and intriguing. And the answer came from the results of a , published in 2024 in Naturewhich suggests that a “past” genetic change is the .
“I’ve been asking myself since I was a child: where is my tail?”, he told the biologist in 2021 Bo Xiathen a PhD student in biology at New York University (NYU), and first author of the study.
One coccyx injuryin 2019, had led him to seriously investigate the topic. “It took me a year to recover, and that made me think seriously about the tail bone.” Xia and his team realized that the answer didn’t lie in what humans no have, but than other animals he has.
During embryonic development, the genes Hox define the general plan of the body — including the spine and, in many species, the tail. From there, other genes come into play, determining whether the tail will be long, furryshort or even detachable.
Convinced that the answer would be in the DNAXia’s team compared the genomes of six species of tailless apes with those of nine species of tailed monkeys.
What they discovered was surprising.
“It was like lightning,” he recalled. Jeff Booksdirector of the Institute for Systems Genetics at NYU Langone Health, speaking to . “We found a non-coding DNA sequence, 100% conserved in all apes and 100% absent in all monkeys”.
And the most impressive? It was hidden in plain sight.
Genetic “junk” that changed everything
O gene responsible for the loss of our tail is tinya small fragment of DNA called Go elementand appears repeatedly in the human genome. For decades, it was considered “genetic junk”without any function.
But Alu elements belong to a special class of “jumping genes”capable of moving through the genome and causing mutations. At some remote moment, one of these elements installed itself in the gene TBXTresponsible for the development of the tail — and suddenly she disappeared.
“When the Alu element is present, the tail is lost all at once”, explained the biochemist Itai Yanai3 New Scientist.
And the team proved it: introduced the Alu element into the DNA of ratsand these were born without tails.
“It’s a brilliant example of how an evolutionary peculiarity can have profound consequences,” commented the geneticist David Kimelmanfrom the University of Washington, at .
More than that, he added, “it helps us understand how this major change occurred in primates” — suddenly, not gradually.
But the mutation had costs: mice with the Alu element had a higher risk of spinal cord defectssimilar to spina bifida — a congenital condition that still affects around one in every thousand human newborns today.
Despite this risk, the mutation persisted. “The evolutionary pressure to lose the tail must have been enormous,” explains Yanai. “So strong that it was worth it, even with the cost of a serious illness.”
The evolutionary exchange that shaped who we are
In short, we have no tail because of a random mutation caused by a Alu jumping gene. But the deeper question, “Why did we keep it?” remains unanswered.
“It’s fascinating to identify a plausible genetic mechanism for the loss of the tail in hominoids,” Shapiro told CNN. “But that doesn’t explain why it was lost.”
The mutation that took our tail has been discovered; the reason why we kept it, however, remains a mystery. Did the loss of tail really bring an advantage? Or was it simply not a serious enough disadvantage to eliminate us?
These are questions that, for now, We haven’t been able to respond yet. Anyway, maybe we should take advantage of the condition. After all, it’s been 25 million years since we lost our tails, and she’s unlikely to come back.
