Why Don’t Humans Have Tails? An Old Genetic Mutation Could Explain Why Monkeys, but Not Apes, Have the Extra Appendage

Scientists have pinpointed a genetic change that might have led the ancestors of humans to lose their tails

A black spider monkey walks across and branch against a green backdrop
A mutation in a gene called TBXT may be behind the loss of great apes' tails, according to a new study. 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 monkeys kept the appendage, while the ancestors of humans lost their tails, is largely a mystery. But now, a new study suggests a 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,” study lead author Bo Xia, a geneticist at the Broad Institute, told Carl Zimmer of the New York Times in 2021, when the study was first posted online as a preprint. Xia was further motivated to investigate after he injured his coccyx, the small triangular bone humans and some apes have at the base of the spine. “It took me a year to recover, and that really stimulated me to think about the tailbone.”

Now, Xia and others have proposed an answer to his enduring question. Humans and apes carry a particular gene with extra letters inserted into it—a mutation that monkeys and other primates with tails do not have, according to their paper published last week in the journal Nature.

“It’s a really nice example of how a strange evolutionary quirk can have interesting and important consequences,” says David Kimelman, a geneticist at the University of Washington who was not involved in the study, to Scientific American’s Sara Novak. “It gets to the basis of how this major change in primates occurred.”

Zeroing in on a gene

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 several genes fundamental to tail development in other animals, so the study authors suspected a genetic mutation, or a combination of them, might have erased tails in our ancestors. They compared the DNA of six species of tailless apes, including humans, to 15 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.

In TBXT, an extra sequence of letters, called an insertion, was present in humans and apes. The researchers say it comes from a “jumping gene,” or a sequence of DNA that can transfer to a new area of the genome. This type of change is common in humans, but this particular shift appeared to have major consequences.

To see if the mutation could be linked to the loss of a tail, the team used CRISPR gene-editing technology to tweak the DNA of mice, mimicking the TBXT mutation that humans and apes have. When researchers made the genetic edit, many rodents didn’t grow tails, while others grew short ones.

The researchers “clearly show that this change contributes to tail loss,” Malte Spielmann, a human geneticist at Kiel University in Germany who peer-reviewed the paper, tells Nature News’ Ewen Callaway. “But it’s not the only one.” Notably, the study examined 140 genes involved in tail development for vertebrates.

Although it hasn’t been definitively proven that this single mutation alone caused the disappearance of apes’ tails, “it’s as close to a smoking gun as one could hope for,” Cedric Feschotte, a geneticist at Cornell University who was not involved in the study, told the New York Times.

A potential evolutionary advantage

The new 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 the mutation might have cropped up randomly in a single ape around 20 million years ago and gotten passed on to its offspring. Perhaps being tailless was a boon to the apes, and the genetic mutation spread like wildfire.

For instance, when apes lost their tails, this change in anatomy might have paved the way for new lifestyles and adaptations. Scientists have long suspected that apes evolved their upright posture and bipedal movement in part because they no longer had tails, per the new study. Walking upright, in turn, could have enabled apes to dwell on the ground, rather than in trees. Even the apes that remained arboreal use an upright position as they swing from branches, rather than walking along the trees’ limbs on all fours.

two bonobos swing from a tree branch
Apes, which do not have tails, now walk or swing from tree branches in an upright position. Anup Shah via Getty Images

But Gabrielle Russo, a biological anthropologist at Stony Brook University who was not involved in the study, tells Nature News that this potential connection might not be as simple as the study implies. The fossil record shows early, tailless apes walked on four legs for millions of years before becoming bipedal, Russo tells the publication.

Additionally, the findings revealed a potential disadvantage of the mutation: Some tailless mice in the study developed defects resembling spina bifida in humans, a condition that impacts spine development. This suggests spina bifida could stem from the same mutation that eliminated tails.

Still, for the mutation to have persisted so successfully, it must have some kind of advantage, Xia told New Scientist’s Michael Le Page in 2021.

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