Why Humans Don’t Have Tails

A black spider monkey walks across and branch against a green backdrop
A mutation in a gene called TBXT be behind the sudden loss of great ape’s tails, according to researchers. Jabid Ishtiaque via Flickr under [CC BY-NC-ND 2.0]

Our primate ancestors used their tails for balance as they navigated treetops, but around 25 million years ago, tailless apes started appearing in the fossil record. How and why some primates like humans lost their tails is largely a mystery, but a new study suggests a single genetic mutation may be responsible for the sudden change.

“This question, ‘Where’s my tail?,’ has been in my head since I was a kid,” says study co-author Bo Xia, a graduate student NYU Grossman School of Medicine, to Carl Zimmer of the New York Times. Xia was further motivated to investigate the question after he injured his coccyx, the small triangular bone humans and some apes have at the base of their spine. “It took me a year to recover, and that really stimulated me to think about the tailbone,” he says.

To find out how and why humans lost their tails, Xia and his colleagues examined the early stages of embryonic development, during which certain genes are switched on and off. Those genes control the formation of different parts of a skeleton.

Scientists had already identified 30 different genes fundamental to tail development in other animals, reports Tibi Puiu for ZME Science, so the study authors suspected a genetic mutation or two might have erased humans’ tails. They compared the DNA of six species of tailless apes to nine species of tailed monkeys to find a mutation that apes and humans share, but monkeys lack. Eventually, their search led them to a gene called TBXT.

To see if the mutation could be linked to the loss of a tail, the team genetically tweaked mice to have the same TBXT mutation that humans have. When researchers made the genetic edit, many rodents didn’t grow tails, while others grew short ones.

Although it’s impossible to definitively prove that this single mutation is responsible for the disappearance of our tails, “it’s as close to a smoking gun as one could hope for,” says Cedric Feschotte, a geneticist at Cornell University who was not involved in the study, to the New York Times.

The discovery suggests our ancestors lost their tails suddenly, rather than gradually, which aligns with what scientists have found in the fossil record. The study authors posit that the mutation randomly might have cropped up in a single ape around 20 million years ago, and was passed on to offspring. Perhaps being tailless was a boon to the apes, and the genetic mutation spread like wildfire.

“For something to be lost in one big burst is really significant, because you don’t then have to posit millions of years of successive tiny changes accumulating gradually,” says Carol Ward, an anthropologist at the University of Missouri who wasn’t involved in the work, to the New Scientist’s Michael Le Page. “It may tell us why all of a sudden when we see the apes [emerge], they have no tails.”

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