Why Don’t Humans Have Tails — And Would We Have Survived If We Did?

ACROSS AMERICA — A Harvard researcher with an injured tailbone and a lifelong fascination about why he and other humans don’t have tails has unlocked a genetic mutation that occurred 25 million years ago that may finally answer his question.

A study published last week in the journal Nature sheds more light on how we became tailless —one of the most profound changes along the evolutionary lineage from “anthropomorphous apes” and eventually to modern humans.

The lead author of the study, Bo Xia, formerly a graduate student at New York University and now the principal investigator at the Broad Institute of MIT and Harvard, injured himself when he sat on a seat belt buckle as he shifted to make room for another passenger.

His tailbone bothered him for a year, reigniting his childhood fascination.

“This question — where’s my tail? — has been in my head since I was a kid,” Xia told The New York Times in 2021, when he was investigating the evolutionary change as a graduate student studying stem cell biology at NYU’s Grossman School of Medicine.

Our fish ancestors formed two tails, a fleshy one and a more flexible fin used to propel themselves through water. They retained the fleshy tail when they evolved many years later into primates, needing them for balance as they scrambled from branch to branch in the jungle, but shed the fin.

The crucial change when the great apes — which include bonobos, chimpanzees, gorillas, orangutans and humans — evolved from Old World monkeys. They formed a coccyx, a tiny set of vertebrae nestled at the base of the spine that Charles Darwin, the father of modern evolutionary studies, concluded was “a rudimentary tail.”

But why that happened has been a riddle for scientists from Darwin’s time on. It’s scientifically known that tail development is tied to the genome known as the TBXT gene, and also that mutations within the gene contribute to stumpy or short tails, as seen in the Manx cat, for example.

Xia and his team began the research using a genome browser, which is akin to an internet search engine, to explore the genes related to tail loss in an investigation in the difference between six species of apes, including humans and early human ancestors, and 15 species of tailed monkeys.

They discovered what’s commonly called a “jumping gene” that can disrupt or enhance a gene’s function. When the primate-specific gene was inserted in the embryos of mice, their offspring were born without tails.

“We found a single mutation in a very important gene,” Xia told The Associated Press.

Xia cautioned there may be other reasons that tails were shed.

Still unanswered is whether being tailless allowed apes and humans to walk upright. Or, it could be that apes and humans thrived for other reasons.

“It could be random chance, but it could have brought a big evolutionary advantage,” Miriam Konkel, an evolutionary geneticist at Clemson University, who was not involved in the study, told The AP.

Rick Potts, who directs the Smithsonian Institution’s Human Origins Project and was not involved in the research, told The AP that being tailless may have been a first step toward some apes adopting a vertical body posture, even before they left the trees.

Not all apes live on the ground today. Gibbons, classified as lesser apes due to their smaller size, scamper along the tops of branches, using their tails for balance. Tailless orangutans, the largest arboreal animal on the planet, travel from branch to branch by climbing, clambering and brachiating, remaining largely upright.

Questions still remain about why great apes are tailless, and if shedding them allowed them to walk bipedally and, thus, survive long enough to continue their species.

Some studies over the past decade have cast doubt on whether primates shed their tails so they could walk upright. A 2016 study found that some upright-walking monkeys, such as capuchins, rely on their tails for balance. In 2019, researchers in another study found that adding an artificial tail improved humans’ balance.

New York University biologist Itai Yanai, a co-author of the study, told The AP that losing our tails was clearly a major transition.

But the only way to certainly know the reason “would be to invent a time machine,” he said.

The Associated Press contributed reporting.