People Are Sometimes Born With a Tail — A Rare Genetic Change May Explain Why Humans Lost Them

Key Takeaways on People Born With A Tail

• People born with a tail either have true tails or pseudo-tails. A true tail is a vestigial body part that serves no function, while a pseudo-tail is the result of a medical condition.
• People born with a tail often have it surgically removed when they are children, and they typically have no adverse effects.
• While our primate relatives have tails, humans don’t — likely due to an element that became inserted into our genes.

As human embryos develop, a little tail is present during weeks four through six. Around weeks six to seven, the tail regresses through tissue remodeling, and around 30 to 32 weeks later, the baby is born tailless.

In rare instances, the tail doesn’t regress, and the child is born with a small appendage on their lower back. This “human tail” is medically rare and has long fascinated scientists.

Medical professionals are now able to use imaging to visualize the cause of the tail and safely remove it. Scientists are increasingly understanding why these tails form. They are also asking why these tails don’t form.

Read More: 773,000-Year-Old Fossils Capture a Moment Before Human Lineages Split

What Are The Different Types of Human Tail?

Clinicians typically define human tails as true tails or pseudo-tails. A true tail is vestigial, meaning it is an evolutionary remnant of a body part that was needed by our ancestors but serves no important function today. Pseudo-tails may result from other conditions, such as lesions or neural tube defects.

A true tail has fatty and connective tissues as well as blood vessels, muscles, and nerves, according to a study in Advances in Orthopedics. It appears under the skin, but it does not have bones, cartilage, or any part of the spinal cord. A pseudo-tail can be the result of several deformities. It can be an extension of the tailbone, the result of a tumor, or even the remains of a twin that stopped developing and became attached to the surviving twin.

Both types of human tails are extremely rare, and a 2024 study in Surgical Neurology International found that only 60 cases had been documented in the medical literature as of 2020.

In a 2023 article in The Journal of Pediatric Surgery Case Reports, for example, researchers described a six-month-old girl who had an almost three-inch-long pseudo-tail stemming from her lower back. They determined the tail was due to spina bifida and were able to remove it surgically. At the time the article was submitted, the patient was 3 years old and had no complications from the tail removal.

Why Don’t Humans Have Tails?

Scientists have an understanding of why the two tail types form, but why don’t humans have tails like other primates?

A 2024 study in Nature presented evidence that tail loss in human evolution may be due to an insertion of an Alu element into an intron of the TBXT gene.

“It’s a very fascinating gene. It was originally called ‘Brachyury,’ which derives from Greek and stands for ‘short tail,’” says Bo Xia, the lead author on the study and an assistant professor of pathology at Harvard Medical School and Massachusetts General Hospital, as well as an associate member of the Broad Institute of MIT and Harvard.

Within the TBXT gene, the research team found a type of protein mutation that may be responsible for humans not growing tails. The research team is currently testing their hypothesis with mice that have been genetically manipulated to be able to express the protein. So far, the mice with this mutation do not grow tails.

“Every experiment is very clear and consistent with our hypothesis,” Xia says.

Future Research On Human Tails

Future research may help scientists better understand which genes contribute to the development of conditions like spina bifida, which in extremely rare instances can be responsible for the development of a pseudo-tail.

However, Xia cautions that scientists might find that many genes are responsible, not just one. With other diseases, researchers have found that dozens or even hundreds of genes are involved.

“For example, with schizophrenia, there can be over 100 genes contributing to that disease,” Xia says.

This article is not offering medical advice and should be used for informational purposes only.

Read More: A 2-Million-Year-Old Skeleton Shows Early Humans Were Still Built for the Trees

Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article: