Why don’t humans have tails? Ancient genetic clues discovered

NEW YORK — Scientists have made a genetic breakthrough that sheds light on why humans, unlike many other primates, do not have tails. A team from NYU Grossman School of Medicine say their discovery could help unravel one of the long-standing mysteries of human evolution.

At the heart of the study is a comparison between the DNA of humans and tailless apes, and that of tailed monkeys. The researchers identified a specific genetic alteration – an inserted piece of DNA – present in humans and apes but absent in monkeys. This insertion, located in a gene known as TBXT, is believed to play a critical role in the development, or rather the absence, of tails in humans.

To delve deeper into this genetic mystery, the team conducted experiments on mice, introducing the same DNA insertion into their genes. The results were varied, with some mice being born without tails, demonstrating the potential impact of this genetic change on tail development.

“Our study begins to explain how evolution removed our tails, a question that has intrigued me since I was young,” says lead study author Bo Xia, PhD, a graduate student at the time of the study in the labs of study senior co-authors Jef D. Boeke, PhD, and Itai Yanai, PhD, at NYU Grossman School of Medicine, in a university release.

This research builds on previous studies linking over 100 genes to tail development in vertebrates. However, the team’s discovery about the TBXT gene is particularly striking. Rather than a mutation in the gene itself, tail differences resulted from the insertion of a DNA snippet known as AluY into the gene’s regulatory sequence. This insertion affects how the gene’s instructions are processed and turned into proteins, the building blocks of the body’s structures and functions.

The study’s findings are significant not only for understanding tail loss in primates but also for decoding the complexity of the human genome. The genome contains vast amounts of non-coding DNA, often referred to as “dark matter,” which includes repetitive sequences and “jumping genes” that can move within the genome. The AluY insertion discovered by the researchers is an example of how these mobile elements can have a profound impact on genetic expression and physical traits, such as tail length.

“This finding is remarkable, because most human introns carry copies of repetitive, jumping DNAs without any effect on gene expression, but this particular AluY insertion did something as obvious as determine tail length,” says Dr. Boeke, the Sol and Judith Bergstein Director of the Institute for Systems Genetics and a professor in the Department of Biochemistry and Molecular Pharmacology at NYU Langone Health.

The absence of tails in humans and some apes, which likely occurred around 25 million years ago, remains a topic of debate among scientists. While the specific reasons for tail loss are unclear, scientists believe it may have granted some evolutionary advantages to early humans, possibly related to a shift from arboreal (tree-dwelling) to terrestrial (ground-dwelling) life. However, the genetic changes that facilitated this shift may also have had unintended consequences, such as an increased risk of birth defects impacting the spine.

The findings are published in the journal Nature.