Why don’t humans have tails?

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Humans have many wonderful qualities, but we lack something that’s a common feature among most animals with backbones: a tail. Exactly why that is has been something of a mystery.

Tails are useful for balance, propulsion, communication and defense against biting insects. However, humans and our closest primate relatives — the great apes — said farewell to tails about 25 million years ago, when the group split from Old World monkeys. The loss has long been associated with our transition to bipedalism, but little was known about the genetic factors that triggered primate taillessness.

Now, scientists have traced our tail loss to a short sequence of genetic code that is abundant in our genome but had been dismissed for decades as junk DNA, a sequence that seemingly serves no biological purpose. They identified the snippet, known as an Alu element, in the regulatory code of a gene associated with tail length called TBXT. Alu is also part of a class known as jumping genes, which are genetic sequences capable of switching their location in the genome and triggering or undoing mutations.

At some point in our distant past, the Alu element AluY jumped into the TBXT gene in the ancestor of hominoids (great apes and humans). When scientists compared the DNA of six hominoid species and 15 non-hominoid primates, they found AluY only in hominoid genomes, the scientists reported February 28 in the journal Nature. And in experiments with genetically modified mice — a process that took roughly four years — tinkering with Alu insertions in the rodents’ TBXT genes resulted in variable tail lengths.

Prior to this study “there were many hypotheses about why hominoids evolved to be tailless,” the most common of which connected taillessness to upright posture and the evolution of bipedal walking, said lead study author Bo Xia, a research fellow in the Gene Regulation Observatory and principal investigator at the Broad Institute of MIT and Harvard University.

But as for identifying precisely how humans and great apes lost their tails, “there was (previously) nothing discovered or hypothesized,” Xia told CNN in an email. “Our discovery is the first time to propose a genetic mechanism,” he said.

And because tails are an extension of the spine, the findings could also have implications for understanding malformations of the neural tube that can occur during human fetal development, according to the study.

A breakthrough moment for the researchers came when Xia was reviewing the TBXT region of the genome in an online database that’s widely used by developmental biologists, said study coauthor Itai Yanai, a professor with the Institute for Systems Genetics and Biochemistry and Molecular Pharmacology at the New York University Grossman School of Medicine.

“It must have been something that thousands of other geneticists looked at,” Yanai told CNN. “That’s incredible, right? That everybody is looking at the same thing, and Bo noticed something they all didn’t.”

Alu elements are abundant in human DNA; the insertion in TBXT is “literally one out of a million that we have in our genome,” Yanai said. But while most researchers had dismissed TBXT’s Alu insertion as junk DNA, Xia noticed its proximity to a neighboring Alu element. He suspected that if they paired up, it could trigger a process disrupting protein production in the TBXT gene.

“That happened in a flash. And then it took four years of working with mice to actually test it,” Yanai said.

In their experiments, the researchers used CRISPR gene-editing technology to breed mice with the Alu insertion in their TBXT genes. They found that Alu made the TBXT gene produce two kinds of proteins. One of those led to shorter tails; the more of that protein the genes produced, the shorter the tails.

This discovery adds to a growing body of evidence that Alu elements and other families of jumping genes may not be “junk” after all, Yanai said.

“While we understand how they replicate in the genome, we now are forced to think about how they’re also shaping very important aspects of physiology, of morphology, of development,” he said. “I think it’s astounding that one Alu element — one small, little thing — can lead to the loss of a whole appendage like the tail.”

The efficiency and simplicity of Alu mechanisms for affecting gene function have been underappreciated for far too long, Xia added.

“The more I study the genome, the more I realize how little we know about it,” Xia said.

Tailless and tree-dwelling

Humans still have tails when we’re developing in the womb as embryos; this wee appendage is a hand-me-down from the tailed ancestor of all vertebrates and includes 10 to 12 vertebrae. It’s only visible from the fifth to sixth week of gestation, and by the fetus’ eighth week its tail is usually gone. Some babies retain an embryonic remnant of a tail, but this is extremely rare and such tails typically lack bone and cartilage and are not part of the spinal cord, another team of researchers reported in 2012.

But while the new study explains the “how” of tail loss in humans and great apes, the “why” of it is still an open question, said biological anthropologist Liza Shapiro, a professor in the department of anthropology at the University of Texas at Austin.

“I think it’s really interesting to pinpoint a genetic mechanism that might have been responsible for loss of the tail in hominoids, and this paper makes a valuable contribution that way,” Shapiro, who was not involved in the research, told CNN in an email.

“However, if this was a mutation that randomly led to tail loss in our ape ancestors, it still begs the question as to whether or not the mutation was maintained because it was functionally beneficial (an evolutionary adaptation), or just not a hindrance,” said Shapiro, who investigates how primates move and the role of the spine in primate locomotion.

By the time ancient primates began walking on two legs, they had already lost their tails. The oldest members of the hominid lineage are the early apes Proconsul and Ekembo (found in Kenya and dating to 21 million years ago and 18 million years ago, respectively). Fossils show that though these ancient primates were tailless, they were tree-dwellers that walked on four limbs with a horizontal body posture like monkeys, Shapiro said.

“So the tail was lost first, and then the locomotion we associate with living apes evolved subsequently,” she said.

Two-legged walking may have evolved to accommodate tail loss, which would have made it more difficult for primates to balance on branches, “but it does not help us understand why the tail was lost in the first place,” Shapiro said. The notion that upright walking and tail loss were functionally linked, with tail muscles being repurposed as pelvic floor muscles, “is an old idea that is NOT consistent with the fossil record,” she added.

“Evolution works from what is already there, so I wouldn’t say that loss of the tail helps us understand the evolution of human bipedalism in any direct way. It helps us understand our ape ancestry, though,” she said.

For modern humans, tails are a distant genetic memory. But the tale of our tails is far from over, and there is still much about tail loss for scientists to explore, Xia said.

Future research could investigate other consequences of the Alu element in TBXT, such as impacts on human embryonic development and behavior, he suggested. Though the absence of a tail is the most visible result of the Alu insertion, it’s possible that the gene’s presence also triggered other developmental shifts — as well as changes to locomotion and related behaviors in early hominoids — to accommodate tail loss.

Additional genes probably played a part in tail loss, too. While Alu’s role “seems to be a very important one,” other genetic factors likely contributed to the permanent disappearance of our primate ancestors’ tails,” Xia said.

“It’s reasonable to think that during that time, there were many more mutations related to stabilizing the loss of the tail,” Yanai said. And because such evolutionary change is complex, our tails are gone for good, he added. Even if the driving mutation identified in the study could be undone, “it still wouldn’t bring back the tail.”

The new findings may also shed light on a type of neural tube defect in embryos known as spina bifida. In their experiments, the researchers found that when mice were genetically engineered for tail loss, some developed neural tube deformities that resembled spina bifida in humans.

“Maybe the reason why we have this condition in humans is because of this trade-off that our ancestors made 25 million years ago to lose their tails,” Yanai said. “Now that we made this connection to this particular genetic element and this particularly important gene, it could open up doors in studying neurological defects.”

Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine.