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Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Humans do not have external tails because our ancestors in the hominoid lineage lost them roughly 25 million years ago. The change happened long before modern humans appeared. We retain three to five reduced caudal (tail) vertebrae as the coccyx, and human embryos briefly form a tail-like posterior structure that is later absorbed during development.
A 2024 Nature study identified the strongest genetic explanation yet: a hominoid-specific DNA insertion appears to alter splicing of the TBXT gene, a key regulator of embryonic body development. The experiment supports a mechanism for shortening or eliminating tails, but it does not establish exactly why natural selection favored taillessness or prove that one mutation alone completed the evolutionary change.
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When did humans lose their tails?
The external tail disappeared on the lineage leading to hominoids—the ape group that includes humans, gibbons, orangutans, gorillas, chimpanzees and bonobos—about 25 million years ago. Hominoids diverged from ancient Old World monkeys around that time. Monkeys generally retained external tails, while hominoids kept only a shortened series of caudal vertebrae inside the body.
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This was not a recent transformation in human history. Anatomically modern humans inherited the tailless body plan from earlier apes; there was no population of historical humans that gradually shed a visible tail within recorded history.
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| Evidence | What it shows | How strong the inference is |
|---|---|---|
| Comparative anatomy | Humans and other hominoids have a coccyx rather than an external tail. | Direct observation of living anatomy |
| Fossil timing | Tail loss is placed near the hominoid divergence roughly 25 million years ago. | An evolutionary estimate, not a directly observed event |
| Embryology | Human embryos briefly develop a tail-like posterior region that regresses. | Direct observation of development |
| Molecular genetics | An AluY insertion can change TBXT RNA splicing. | Experimentally supported developmental mechanism |
Is the coccyx a leftover tail?
Yes, in evolutionary terms. The coccyx is formed from the remaining caudal vertebrae that once belonged to a longer tail region. It is not a useless bone: ligaments, muscles and parts of the pelvic floor attach to it, and it contributes to pelvic stability and seated posture.
The coccyx cannot move like a monkey’s tail because it is short, fused or tightly joined, and enclosed at the base of the spine. Calling it a “tail remnant” describes its ancestry, not a claim that it should function as an external appendage.
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Why do human embryos have tails?
Early human development temporarily builds a posterior structure containing somites, the segmented tissue that contributes to the vertebral column and surrounding muscles. As development proceeds, that tail-like region is reduced and incorporated into the lower spine. The process is a normal part of human embryology, not a fetus turning into a monkey.
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What gene made apes lose their tails?
The leading candidate is TBXT (also called brachyury), a gene involved in forming the embryonic body axis and tail structures. The 2024 study found a hominoid-specific AluY insertion inside a TBXT intron. That sequence can pair with an older AluSx1 sequence in the opposite orientation, creating an inverted repeat that changes how the cell processes the gene’s RNA.
What the 2024 study tested
- Genome comparison: Researchers examined genomes from six ape species and 15 tailed monkey species, looking for changes shared by apes but absent from tailed monkeys.
- Human stem-cell experiments: Deleting either the AluY insertion or its AluSx1 partner eliminated the alternative TBXT transcript, showing that the two elements work together to alter splicing.
- Mouse editing: CRISPR-edited mouse embryos were engineered to express an exon-6-skipped Tbxt isoform alongside full-length Tbxt. The resulting mice showed shortened or absent tails.
The proposed mechanism is that the AluY–AluSx1 pairing promotes skipping of exon 6, producing a hominoid-associated form of TBXT. Bo Xia, a Broad Institute geneticist and study co-author, described it as “a single mutation in a very important gene.”
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What the experiment does—and does not—prove
The results demonstrate a plausible causal developmental effect: changing TBXT splicing can produce a shorter or missing tail in mice, and the same regulatory arrangement changes transcripts in human cells. Homozygous edited mice also developed severe defects, so the health consequences of particular TBXT combinations require caution and further study.
The findings do not prove that one DNA change alone explains every step of hominoid tail loss. Evolutionary changes can involve additional variants, gene regulation and population-level selection. A mouse phenotype is functional evidence, but it is not a complete reconstruction of the ancient hominoid event.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why would natural selection favor losing a tail?
No definitive answer is known. In tailed primates, tails can help with balance, grasping and movement through trees. One hypothesis is that a shorter, more upright body—often called an orthograde posture—made a tail less useful as hominoids climbed and later adopted more ground-based movement. A tailless trunk may also have fit changes in the spine, pelvis and limb coordination.
That explanation remains a hypothesis, not an established cause. Gibbons and orangutans are tailless yet spend much of their lives in trees, so tail loss cannot simply mean a switch from arboreal to terrestrial living. As Itai Yanai, a New York University biologist and study co-author, noted, the only certain way to know the original selective pressure “would be to invent a time machine.”
Can people be born with tails?
Rare newborns have congenital caudal appendages, sometimes called human tails. These structures vary widely: some contain skin and fat, while others include connective tissue, cartilage or vertebral elements. They are medical anomalies, not normal adult primate tails and not proof that humans are “supposed” to have visible tails.
Doctors evaluate an appendage to determine its tissue composition and whether it is associated with spinal or other congenital abnormalities. The appropriate description is a congenital caudal appendage, with treatment based on the individual anatomy—not a claim that a full evolutionary tail has reappeared.
Quick Recap
What the evidence says in one view
- About 25 million years ago: the hominoid lineage lost the external tail while retaining three to five caudal vertebrae.
- During human development: a temporary tail-like structure forms and then regresses.
- In the 2024 study: an AluY insertion paired with AluSx1 altered TBXT splicing; edited mice developed short or absent tails.
- Still unresolved: the exact environmental or functional pressure that made taillessness advantageous, and whether other genetic changes were required.
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