Humans cannot grow a tail after birth, but every human actually had one during early development. For a brief window in the womb, you possessed a tail with 10 to 12 vertebrae. In rare cases, roughly 40 documented in medical literature, babies are born with a true vestigial tail that failed to disappear on schedule. Growing one from scratch as an adult, though, is biologically impossible given our current genetics and healing mechanisms.
Every Human Starts With a Tail
Between the fifth and sixth week of embryonic development, the human embryo has a clearly visible tail containing multiple vertebrae. By the eighth week, that tail is gone. The cells are reabsorbed through a process called apoptosis, or programmed cell death, and the leftover vertebrae fuse into the coccyx, your tailbone. The adult coccyx is a small curved bone made of three to five fused vertebrae (four is most common). Far from useless, it anchors your gluteal muscles, pelvic floor muscles, and several tendons and ligaments. It also bears weight and helps you balance when sitting.
Why Humans Lost Their Tails
The genetic story behind human taillessness was pinned down in a 2024 study published in Nature. About 25 million years ago, a small piece of DNA called an Alu element inserted itself into a gene called TBXT, which plays a central role in tail development across vertebrates. This inserted segment paired with another Alu element already sitting nearby in the reverse orientation, and together they caused the gene to produce an altered, shortened version of its protein.
Researchers confirmed this by engineering mice to produce both the normal and the shortened version of the same protein, mimicking what happens in human cells. Depending on the ratio of normal to shortened protein, the mice were born with either no tail at all or a significantly shortened one. This single genetic insertion, shared by all apes and humans but absent in monkeys, appears to be the event that eliminated tails from our entire evolutionary lineage.
Rare Cases of Babies Born With Tails
It happens, but it’s extraordinarily uncommon. Medical literature documents only about 40 cases of true human tails, with a broader review covering 59 cases of tail-like appendages reported between 1960 and 1997. A true tail emerges from the lowest part of the spine and contains fat, connective tissue, muscle, nerves, and skin. It can sometimes move or contract, though it lacks bone.
A pseudotail, by contrast, is a tail-shaped growth caused by an underlying structural issue like a fatty tumor or a cyst. Both types are considered markers for possible spinal abnormalities. In one review of 33 historical cases, spina bifida was the most frequently coexisting condition. A separate analysis found that 50% of people with tail-like appendages also had either a meningocele (a protrusion of the spinal membrane) or spina bifida occulta. For this reason, any infant born with a tail-like structure gets spinal imaging to check for conditions like tethered cord syndrome, split cord malformations, or other developmental issues beneath the surface.
When a true tail is present without serious underlying complications, surgical removal is straightforward. The tail is excised from the surface and dissected away from the tissue beneath it. When spinal abnormalities are also involved, the surgery becomes more complex and may include untethering the spinal cord, with nerve monitoring throughout the procedure.
Why You Can’t Regrow One
Some animals pull off impressive feats of regrowth. Lizards can regenerate a lost tail through a process that starts when specialized cells gather at the wound site and form a structure called a blastema, essentially a mass of progenitor cells that can become cartilage, muscle, or other tissues. A regenerating spinal cord grows into this cell mass and sends chemical signals that direct the new tissue to organize into a functional tail. The whole process depends on the wound staying open, the right cell populations being recruited, and precise signaling between the spinal cord and the new growth.
Mammals can’t do any of this. When a mammalian limb or appendage is amputated, the body doesn’t form a blastema. Instead, it defaults to fibrotic scar formation, rapidly sealing the wound with dense, stiff tissue that blocks any regenerative process from starting. This scarring response is likely an evolutionary trade-off: closing wounds quickly protects against infection, which matters more for survival than regrowing a limb over weeks or months with an open wound.
The problem goes deeper than scarring. Experiments with frogs illustrate the issue clearly. Tadpoles can regenerate limbs, but after metamorphosis into adult frogs, that ability largely vanishes. Researchers grafted tadpole limb buds onto adult frogs and found they could still regenerate. But grafting adult frog tissue onto tadpole stumps did not produce regeneration, even though the tadpole environment was otherwise supportive. The cells themselves had changed at a fundamental level, losing the ability to revert to a flexible developmental state and respond to patterning signals. Mammalian cells face the same wall. The epigenetic locks on our adult cells prevent the kind of dedifferentiation that tail regeneration would require.
What It Would Actually Take
For a human to grow a tail, several biological barriers would need to be overcome simultaneously. The TBXT gene would need to produce its full-length protein at the right time and place. Adult cells near the tailbone would need to revert to an embryonic-like state capable of forming new vertebrae, muscles, nerves, and skin. The body’s scarring response would need to be suppressed in favor of blastema formation. And chemical signaling networks that haven’t been active since the eighth week of fetal development would need to reactivate in a coordinated sequence.
No current technology can accomplish any one of these steps reliably in humans, let alone all of them together. The genetic blueprint for a tail isn’t completely erased from our DNA. It’s still there in altered form, as the embryonic tail proves every pregnancy. But the distance between having remnants of the genetic instructions and actually executing them in a grown human body remains enormous.