Biologists at NYU Langone Health, a leading medical and research center in the United States, reported that a mutation in the TBXT gene played a key role in tail loss among human ancestors dating back about 25 million years. The findings were published in the journal Nature, highlighting a pivotal moment in human evolution where a diminutive change in the genome contributed to a major anatomical shift.
The TBXT gene governs tail length in animals that possess tails. During the investigation, researchers identified two “jumping” genes within the TBXT region. Jumping genes, or transposable elements, are DNA sequences that can move around the genome, occasionally creating new mutations or reversing existing ones. Their mobility can alter how genes are expressed and can even influence the overall size of the genome over time.
Scientists propose that the loss of tails in human ancestors arose from the accidental insertion of a small segment known as AluY into the TBXT DNA. This element is present in humans and certain primate lineages, but not in some other mammals. To explore this theory, researchers induced activity of these jumping elements in laboratory mice. The offspring of these mice showed a measurable decrease in tail length, and in some cases, complete absence of a tail in the next generation, suggesting a possible causal link between AluY integration and tail diminution.
In general, introns—the noncoding stretches within genes—often carry repetitive copies of jumping DNA without altering gene activity. The scientists noted that such elements are abundant in the human genome and typically exhibit a neutral or minimal effect on how genes are expressed. Yet, in this scenario, a specific insertion within TBXT appears to have steered a consequential developmental outcome, underscoring how tiny genomic edits can yield lasting anatomical changes.
There have also been rare medical observations in humans describing tail-like structures extending from the lower spine toward the hips and, in some cases, toward the rectum. Such phenomena are exceedingly uncommon and typically considered developmental anomalies rather than normal anatomy. They are subjects of careful clinical evaluation and documentation to understand their origins, implications, and the appropriate medical approaches for management. These rare cases, while not connected to the broader evolutionary narrative, remind researchers of the remarkable diversity that can occur in human development and how the genome can occasionally translate small changes into striking physical features.