Why don’t humans have tails?

Although our primate ancestors had them, the tail-less human is now something of a rarity among vertebrates. Almost all mammals, reptiles and birds have tails, and use them for everything from balance and communication to hunting and navigation. So why did humans lose our tails and how long ago did it happen?

A recent scientific breakthrough has helped answer these questions.

The reason we lost our tails has long been a great, unanswered question. We know it happened between 20 and 25 million years ago, when a group that came to be known as apes split off from other monkeys, but the exact reason is unknown. Most estimates focus on our descent from the trees to the ground, and our shift to bipedalism – walking upright on two legs.

The genetic factor behind the loss was even less known, until now. Earlier this year, scientists successfully traced our human loss to a short sequence of genetic code. It is called an Alu element, and though there is plenty of it in our DNA, it serves no obvious biological purpose. 

Published in February in the journal Nature, the study compared the DNA of six hominoid (Great Ape) species and 15 non-hominoid primates. The specific Alu genetic code was found to have jumped into the TBXT gene (responsible for making a protein that helps develop the embryonic notochord, a precursor of the spinal column) only in hominoid DNA. In all the non-hominoids, this jump hadn’t taken place. Even after noticing this, it took almost four years and experiments with genetically modified mice to prove the correlation, when altering the insertions of Alu into TBXT led to variations in the rodents’ tail lengths.

Bo Xia was the lead author of the study. He works as a research fellow at the Broad Institute of the Massachusetts Institute of Technology and Harvard University, and began exploring the topic of human tails back in 2019, after a coccyx injury led to nearly a year of discomfort. It motivated him to investigate the genetic differences between apes and tailed monkeys, hoping to work out why we have tailbones but no tails.

Xia was not the first to study jumping genes and Alu, but he saw something everyone else missed. “It must have been something that thousands of other geneticists looked at,” coauthor Itai Yanai told CNN. “That’s incredible, right? That everybody is looking at the same thing, and Bo noticed something they all didn’t.”

“[He noticed it] in a flash. And then it took four years of working with mice to actually test it,” Yanai continued

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“It’s a really nice example of how a strange evolutionary quirk can have interesting and important consequences. It gets to the basis of how this major change in primates occurred,” David Kimelman, a geneticist at the University of Washington’s Department of Genome Sciences, said to Scientific American.

We may never be certain what evolutionary pressures led to human tail loss. Breakthroughs like these help, however, as they are not only important in their own right, but could lead to a better understanding of why we descended from the trees or what caused our shift to bipedalism in the first place.