What if humans had tails?

Wiggle, wiggle! A dog wags its tail. A monkey swings by its tail.

People don't have tails. What if you did?

Lots of animals have tails: dogs wag them, cats twitch them, monkeys swing from them. But look in the mirror and you won't find one. People and apes are tailless.

So what if humans had tails? What would you use yours for? Would it help you balance, or hold things, or tell your friends how you feel? In this lab we follow three clues: tails for balance, monkeys that grab with their tails, and the little tailbone you already have.

Tails are widespread among vertebrates and serve many functions: balance, communication and, in some primates, grasping. Humans and other apes are unusual among primates in lacking one.

This thought experiment gives humans a tail and asks what changes. It follows three threads: tails as balancing organs, the prehensile tail of spider monkeys, and the human coccyx, along with recent genetic evidence about how our ancestors lost their tails. Facts come from NYU Langone Health, Smithsonian Magazine, National Geographic and the Smithsonian's National Zoo; what a human tail would do is reasoned from other animals.

Ring-tailed lemurs hold their stripy tails up high, like flags. That helps the group stay together.

Some monkeys can hang from a branch by their tail!

Tails do all sorts of jobs. Ring-tailed lemurs, the Smithsonian's National Zoo explains, hold their striped tails up in the air like flags when the group travels, so everyone can stay together. Each tail has 13 black and white bands.

Spider monkeys go further. National Geographic describes their tail, usually longer than their body, as a fifth limb. It can hold the monkey's whole weight, so a spider monkey can hang from a branch by its tail and keep its hands free. Smithsonian Magazine notes that our own primate ancestors used tails for balance as they moved through the treetops.

Tails serve varied roles in primates. The Smithsonian's National Zoo describes ring-tailed lemur troops traveling with tails raised like flags to keep the group together; the tail bears 13 alternating black and white bands and can reach 2 feet (61 cm).

Spider monkeys (genus Ateles) have prehensile tails generally longer than the body, which National Geographic describes as a fifth limb: the tail can support the animal's full body weight, letting it hang from branches with its hands free, and its thumbs are extremely short. Smithsonian Magazine notes that early primate ancestors used tails for balance while navigating treetops.

Now pretend you have a tail. You could hold on to the monkey bars with it!

Maybe your tail would help you balance on a wall, too.

Imagine waking up with a long, strong tail. If it worked like a spider monkey's, you might hang from the monkey bars with it, or pick up a pencil while your hands are busy. If it worked like a lemur's, you could hold it up high on a school trip so your class could find you!

You'd also need new clothes with tail holes, and chairs you could sit on comfortably. Would a tail help us balance when we walk on two legs? Nobody knows. All of these are fun ideas, not facts.

What a human tail would do depends on what kind of tail it was. A prehensile tail like a spider monkey's might serve as an extra grasping limb; a long, flexible tail might act as a counterbalance in climbing, as tails did for our treetop ancestors; a lemur-style tail might signal to others.

Whether any of these would help an upright, two-legged walker is an open question. Our posture, furniture and clothing are built around tailless bodies. Every consequence here is an idea drawn from other primates, not a prediction about human biology.

Long, long ago, our ape relatives had tails. Over a very long time, their tails got smaller and disappeared.

Now we have just a tiny tailbone.

Scientists think tailless apes first appeared about 25 million years ago, when the group that includes gorillas, chimpanzees and humans branched away from monkeys. Over many generations, their tails shrank, leaving a small bone at the base of the spine: the tailbone, or coccyx.

In 2024, a team led by researchers at NYU Grossman School of Medicine found a clue. Humans and apes share a small extra piece of DNA inside a gene called TBXT, which helps control tail growth. Monkeys don't have it. When the scientists added this piece to mice, some mice were born with shorter tails or no tail at all.

According to NYU Langone Health, tail loss in the primate group including gorillas, chimpanzees and humans is believed to have occurred about 25 million years ago, when it evolved away from Old World monkeys; this group then evolved fewer tail vertebrae, giving rise to the coccyx.

In the 2024 Nature study led by Bo Xia, then at NYU Grossman School of Medicine, the team found an AluY insertion, a 'jumping' DNA element, in an intron of the TBXT gene, shared by apes and humans but absent in monkeys. It alters splicing to make two forms of TBXT RNA. Mice engineered with the insertion showed a range of tail effects, including absent tails. A reviewer quoted by Smithsonian Magazine notes it contributes to tail loss but is not the only cause.

Feel the bottom of your back. There is a small bone there. It is your tailbone!

Scientists are still learning why our ancestors lost their tails.

You can't see it, but you have a little leftover tail: the coccyx, a small triangle-shaped bone at the very bottom of your spine. Some apes have one too.

Why did our ancestors lose their tails? Scientists aren't sure. Some think life on the ground, rather than in the trees, may have suited tailless apes better. Others point out that the earliest tailless apes still walked on four legs for millions of years before any walked upright. It's a mystery scientists are still working on.

The coccyx, a small triangular bone at the base of the spine in humans and some apes, is the remnant of the ancestral tail. Why tails were lost is unresolved. NYU's authors note that some experts propose tail loss better suited life on the ground than in the trees, and Smithsonian Magazine reports a long-standing suggestion linking tail loss to upright posture.

Biological anthropologist Gabrielle Russo of Stony Brook University cautioned, as reported by Smithsonian Magazine, that the fossil record shows early tailless apes walked on four legs for millions of years before becoming bipedal. And the 2024 study examined about 140 tail-related genes; TBXT is one piece of a larger story.