Tails are a common feature across the animal kingdom. Nearly every class of vertebrate has them. Reptiles use them for self-defense and attacking prey. Tails help with balance and communication in dogs and cats and aid navigation in birds. Humans’ primate ancestors had them, too. Our forebears used them to grasp branches while swinging through the trees—until their tails vanished from the fossil record about 25 million to 20 million years ago in one of the most important evolutionary changes in human ancestry.
Researchers aren’t sure what evolutionary pressures led to the loss of our tails. It could have been related to our descent from the trees, for example, or to changes required for walking upright. But now, for the first time, scientists have identified the genetic mechanism behind this dramatic change. The authors of a new study, published on Wednesday in Nature, say their finding provides a genetic explanation of tail loss in hominoids and could also provide a better understanding of the evolutionary pressures that led to human bipedalism.
Lead study author Bo Xia of the Broad Institute of the Massachusetts Institute of Technology and Harvard University began exploring the topic back in 2019, when a coccyx injury led to nearly a year of discomfort. Sitting was a constant reminder of this vulnerable portion of the vertebral column that we commonly call the tailbone. “It caused me to really think about this part of the body,” Xia says. So using a genome browser—something a little like a “search engine” for genomes—he began exploring all the genes related to tail loss in an investigation of differences between apes (including our human ancestors) and tailed monkeys.
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Past research on mice has linked 100 or so genes to tail loss, and Xia surmised that a mutation in a human version of one of them caused the change. His search turned up the AluY element, a type of “jumping gene.” Such sequences of DNA are so named for their ability to bounce around the genome. When AluY jumped into a gene called TBXT, the insertion seemed to result in tail loss in apes—including human ancestors. (TBXT is responsible for making a protein that is important to the development of the embryonic notochord, a precursor of the spinal column.)
Xia says finding the element was the result of hard work but also serendipity. “It was like finding a needle in a haystack,” agrees Itai Yanai, a co-author of the study and founding director of the Institute for Computational Medicine at the NYU Grossman School of Medicine.
AluY was an unexpected piece of the puzzle because millions of such elements are present in our cells—and for a long time they were referred to as “junk DNA” because researchers believed they were littering the human genome at random and seemingly with no purpose. “It shows that these elements aren’t just cluttering the genome, Yanai says. “They’re doing something important.”
The AluY element is part of a larger family of jumping genes called Alu elements. And after jumping into the TBXT gene, the AluY element happened to be positioned one exon, or protein-encoding segment, over from another Alu element. Since the Alu elements were pointing in opposite directions, they paired in the RNA produced by the gene. This formed a loop, preventing the exon from being included in the production of the protein needed for tail development.
“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,” says David Kimelman, a geneticist at the University of Washington’s department of genome sciences, who was not involved in the study.
Identifying the insertion was a start, but to prove their findings, Xia and Yanai had to test their hypothesis in mice. Using CRISPR gene-editing technology, the researchers simultaneously inserted both Alu elements into the TBXT gene of a mouse embryo. At first, the resulting mice still retained their tail. But when the researchers added larger amounts of the same elements, the mice had a shorter tail or none at all.
This result suggests that although AluY is a major contributor to tail loss, it’s probably not the whole story, Kimelman says. At first there was likely variability in our ancestors’ tail length as there was in the mouse models, but over time, additional mutations made our tail loss more distinct—and eventually permanent. (If AluY was removed from the TBXT gene in humans, we wouldn’t just start growing tails.)
The new study also found that some of the mice who lost their tail developed neural tubes that didn’t completely close, resulting in a spinal cord defect similar to a condition called spina bifida—which currently affects one in 1,000 human newborns. “There must have been such huge evolutionary pressure to lose the tail that even if it came with the price tag of this horrible disease, it was worth it,” Yanai says.
We may never know for sure what evolutionary pressures led to tail loss, and it’s very likely that the reasons could have been different in every lineage of mammals that lack a tail today, says Gabrielle A. Russo, a biological anthropologist at Stony Brook University, who was not involved in the study.
Russo says the fossil record shows that the events of tail loss, descending from the trees, upright posture and bipedalism all happened millions of years apart. To say these events were all somehow linked, as the study implies, is too simplistic, she adds, noting that “these events are really far apart in the fossil record.” Yanai says that while the paper mentions bipedalism, it’s really only about the genetic basis for tail loss, as is suggested by its title (“On the Genetic Basis of Tail-Loss Evolution in Humans and Apes”).
Russo says that knowing the genetic mechanism is very helpful for shedding light on these important paleoanthropological changes, even if there are still more questions than answers. For example, she says, “one of the things that’s so interesting about tail loss is that it appeared to have occurred in the context of us still being in the trees.”