Humans lost their tails roughly 25 million years ago, when a small piece of DNA inserted itself into a gene critical for tail development in the ancestor shared by all apes. That single genetic change altered how the body reads its own blueprint during embryonic growth, and every ape species since, including humans, has developed without an external tail. The story of how it happened is one of the most concrete examples of a major anatomical shift traced back to a specific mutation.
The Genetic Mutation Behind Tail Loss
A gene called TBXT (sometimes called Brachyury) plays a central role in building the tail during embryonic development. It’s one of several genes that guide the formation of the body’s midline and posterior structures. In all monkeys and most other mammals, this gene works uninterrupted. But in the lineage leading to apes and humans, something went wrong, in the most productive way possible.
A small, mobile piece of DNA called an Alu element jumped into the middle of the TBXT gene, landing inside a non-coding stretch between two important segments. On its own, this insertion wouldn’t have done much. But it landed near another, older Alu element that was already sitting in the gene, facing the opposite direction. During the process of reading the gene, these two elements fold toward each other and form a loop in the RNA molecule, trapping a critical section of the gene’s code (exon 6) inside that loop. The cell’s machinery then skips over that trapped section entirely, producing a shortened version of the TBXT protein.
A 2024 study published in Nature confirmed this mechanism by engineering mice to produce both the normal and shortened versions of the TBXT protein, mimicking what happens in apes. The results were striking: mice expressing both versions were born with dramatically shortened tails or no tails at all, depending on how much of each protein version was present. When the researchers deleted either of the two Alu elements, the skipping event nearly disappeared, and normal tail development resumed. This confirmed that both pieces of inserted DNA are necessary to produce the tailless outcome.
When Apes Lost Their Tails
The specific Alu insertion belongs to a subfamily that appeared in the common ancestor of apes and Old World monkeys. But the tail-loss effect only shows up in hominoids (the group that includes gibbons, orangutans, gorillas, chimpanzees, and humans), because only the hominoid lineage carries this particular insertion in the right location within the TBXT gene. That places the mutation’s origin at roughly 25 million years ago, before the various ape lineages diverged from one another.
Fossil evidence supports this timeline. Proconsul heseloni, an early ape species that lived between 17 and 18 million years ago in what is now Kenya, appears to have already lacked a tail. Researchers examined vertebral specimens and concluded that bones previously identified as possible tail vertebrae were actually deformed lumbar (lower back) vertebrae, and that a distal sacral vertebra from the same species could not have supported a tail. If confirmed across more specimens, this would push evidence of taillessness in apes back to the earliest phases of the group’s evolution.
Why Losing a Tail Was Worth It
Tails serve obvious purposes in other primates: balance during leaping, gripping branches, even communication. So losing one wasn’t trivially advantageous. The leading hypothesis ties tail loss to a shift in how early apes moved through their environment. Unlike monkeys, which typically run along the tops of branches, apes developed a more upright trunk posture suited to climbing, hanging, and swinging beneath branches. A tail offers little help for this kind of locomotion and could even get in the way.
Once the genetic mutation appeared and reduced or eliminated the tail, individuals who thrived without one likely had a slight edge in their particular ecological niche. Over millions of years, that advantage was enough for the trait to become fixed across all ape species. The shift toward an upright torso also set the stage, much later, for bipedal walking in the human lineage.
Every Human Embryo Briefly Has a Tail
During the fifth and sixth weeks of development in the womb, a human embryo has a visible tail containing 10 to 12 vertebrae. By the eighth week, it’s gone. The tail doesn’t fall off; instead, the body reabsorbs it through programmed cell death, a tightly controlled process where cells essentially self-destruct on schedule. The vertebrae that remain fuse together to become the coccyx, or tailbone.
This brief embryonic tail is one of the clearest examples of human development recapitulating evolutionary history. The genetic instructions for building a tail are still present in our DNA. They’re just interrupted by that Alu insertion, and the structures that do begin forming are dismantled before birth.
What the Tailbone Actually Does
The coccyx is small, typically made of three to five fused vertebrae (four being the most common, found in about 76% of people). Despite its size, it’s far from useless. It forms one leg of a three-point support system, along with the two sit bones, that bears your weight when you’re seated.
It also serves as an anchor point for a network of muscles and ligaments. The pelvic floor muscles attach to it, helping support your internal organs and contributing to voluntary bowel control. The gluteus maximus, the largest muscle in your body, partially inserts along its edges. Ligaments connecting the coccyx to the pelvis help stabilize the entire base of the spine. The coccyx also provides positional support for the anus. People who fracture or bruise their tailbone discover quickly how much everyday movement, from sitting to standing to bending, depends on this small structure.
Rare Cases of Babies Born With Tails
On very rare occasions, a baby is born with what appears to be a tail. These cases fall into two categories that look similar on the outside but differ significantly in structure. A true vestigial tail is made of fat, connective tissue, muscle, blood vessels, and nerves. It contains no bone or cartilage, can sometimes move, and is generally harmless, though it’s typically removed surgically for practical reasons.
A pseudo tail, on the other hand, may contain bone, cartilage, or even part of the spinal cord. It can be a sign of underlying spinal abnormalities like spina bifida. When a baby is born with a tail-like structure, doctors use MRI and ultrasound to determine which type it is and to check for any associated conditions. True vestigial tails are not a sign that the TBXT gene has somehow reverted to its ancestral form. They likely arise from incomplete programmed cell death during the eighth week of development, leaving behind tissue that would normally be reabsorbed.