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Humans do not have external tails because our hominoid ancestors lost them roughly 25 million years ago. The change occurred long before modern humans evolved. We still retain three to five reduced caudal vertebrae as the coccyx, and human embryos briefly form a tail-like posterior structure before it regresses. A 2024 Nature study proposes a leading genetic explanation involving altered splicing of the TBXT gene, but the evolutionary reason natural selection favored taillessness remains unknown.
What happened to the ancestral tail?
Humans belong to the hominoid group, which includes apes rather than tailed Old World monkeys. Comparative evidence places the loss of an external tail in the hominoid lineage at about 25 million years ago, around the period when hominoids diverged from ancient Old World monkeys.
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Tail loss did not remove the entire posterior structure. Most hominoids retain three to five small caudal vertebrae at the base of the spine. These fused or reduced bones form the coccyx, commonly called the tailbone. The coccyx is not a useless leftover: ligaments, muscles and parts of the pelvic floor attach to it, and it contributes to pelvic stability and sitting mechanics.
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What a tail does in tailed primates
In many monkeys and other mammals, a tail helps with balance while moving through branches. Some primates also use it to grasp or control movement. Those functions create an obvious question: why would an ape lineage give up a structure that can be valuable in trees?
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The leading posture hypothesis
One possibility is that a tailless body suited the more upright, or orthograde, climbing style that evolved in apes and later supported parts of human locomotion. A shorter rear appendage could have interacted differently with the spine, pelvis and balance demands of upright movement.
That idea is not established fact. Gibbons and orangutans are tailless but remain highly arboreal, so tail loss cannot simply be equated with a move from trees to the ground. The primary evolutionary study says the selective pressures are still unclear. As study co-author Itai Yanai told the Associated Press, the only way to know the reason with certainty “would be to invent a time machine.”
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What the TBXT discovery adds
An ape-specific DNA insertion
In 2024, Bo Xia and colleagues reported a plausible molecular mechanism in Nature. They compared genomes from six ape species and 15 tailed monkey species and focused on an insertion called AluY inside an intron of the TBXT gene. The human genome also contains a related AluSx1 sequence elsewhere in the gene.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchBecause the two Alu sequences can pair as an inverted repeat, they can change how the cell processes TBXT RNA. The result is skipping of exon 6 and production of a hominoid-specific TBXT isoform. TBXT is involved in early body-axis and tail development, making the altered transcript a biologically plausible link to shortened or absent tails.
Cell and mouse tests
The researchers tested the mechanism in human embryonic stem cells. Removing either AluY or its AluSx1 partner eliminated the alternative TBXT transcript, supporting the claim that the two elements work together to alter splicing.
They also expressed the exon-6-skipped Tbxt isoform in mouse embryos alongside the full-length form. The edited mice developed shorter or absent tails, demonstrating a developmental effect in a living mammal. Homozygous edited mice also had severe developmental defects, so the experiment does not establish that the same genetic configuration would be harmless or sufficient in humans.
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These results make the AluY–AluSx1 interaction a leading, experimentally supported explanation. They do not prove that one mutation alone explains every step of hominoid evolution. Other genetic variants, developmental changes and selection pressures may also have contributed. As Xia summarized to the Associated Press: “We found a single mutation in a very important gene.”
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Why human embryos briefly have a tail
During normal development, a human embryo forms a posterior structure containing somites—the segmented building blocks that help organize the developing trunk. This structure looks tail-like for a time, then is reduced as development proceeds. The temporary feature reflects shared developmental ancestry; it does not mean a fetus is turning into a monkey or that humans are expected to grow visible adult tails.
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As the embryonic tail region regresses, the remaining caudal vertebrae become the coccyx. Developmental anatomy therefore connects the embryo’s transient structure, the adult tailbone and the ancestral primate tail without implying that modern humans once had fully functioning external tails as individuals.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Did humans ever have tails?
At the species-lineage level, yes: the hominoid ancestor was part of a primate group whose earlier relatives had external tails, and the external structure disappeared from the hominoid lineage about 25 million years ago. An individual human, however, does not lose a tail during childhood or adulthood. The evolutionary change happened across ancestral populations over deep time.
Can a person be born with a tail?
Rare congenital caudal appendages are medically recognized. They vary in composition: some contain skin and soft tissue, while others may include cartilage, vertebral elements or other developmental tissues. These appendages are anomalies, not normal adult primate tails and not evidence that humans are “supposed” to have visible tails.
Because their anatomy and causes differ, a newborn with a caudal appendage requires clinical assessment rather than comparison with a monkey’s functional tail. The presence of such an anomaly does not overturn the evolutionary history of hominoid tail loss.
What is known—and what is still open
| Question | Best-supported answer | Confidence and limits |
|---|---|---|
| When was the external tail lost? | About 25 million years ago in the hominoid lineage. | An inferred evolutionary date based on comparative evidence; it is not a timestamp from a single fossil. |
| What remains today? | Usually three to five reduced caudal vertebrae forming the coccyx. | The coccyx still provides structural and muscle-attachment functions. |
| What genetic mechanism is implicated? | An AluY insertion pairing with AluSx1 to alter TBXT splicing and promote exon-6 skipping. |
Supported by human-cell experiments and mouse editing, but not proof that one mutation explains the entire evolutionary event. |
| Why was tail loss favored? | Possibly related to upright posture and locomotion. | The selective advantage remains unresolved; bipedalism is not a demonstrated cause. |
The bottom line on human tails
Humans are tailless because an ancient hominoid lineage lost the external tail and retained only the coccyx. Embryos briefly recreate a tail-like developmental structure, and modern genetic evidence points to an Alu-driven change in TBXT splicing as one plausible mechanism. The experiments explain how tail development could be altered; they do not yet explain why natural selection favored the change or prove that the TBXT event acted alone.
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