Why don't humans have tails?

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Humans have many wonderful traits, but we lack one thing most animals with a backbone have in common: a tail. Exactly why this was 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 goodbye to their tails about 25 million years ago, when the group split off from Old World monkeys. Loss has long been linked to our transition to bipedalism, but little was known about the genetic factors that led to primate taillessness.

Now, scientists have traced the loss of our tail to a short sequence of genetic code that is abundant in our genome, but dismissed for decades as junk DNA, a sequence that appears to serve no biological purpose. They identified the snippet, known as the Alu element, in the regulatory code of a tail length-related gene called TBXT. Alu is also part of a class known as transposable genes, which are genetic sequences capable of switching their location in the genome and triggering or reversing mutations.

At some point in our distant past, the AluAluY element jumped into the TBXT gene in the ancestor of hominins (great apes and humans). When scientists compared the DNA of six hominin species and 15 non-hominin primates, they found AluY only in the hominin genomes, the scientists reported Feb. 28 in the journal. nature. In experiments with genetically modified mice—a process that took nearly four years—manipulating Alu insertions in the rodents' TBXT genes resulted in altered tail lengths.

Before this study, “there were many hypotheses about why hominins evolved to be tailless,” the most common being linking taillessness to upright posture and the evolution of bipedalism, said the study's lead author. Bo Xiaa research fellow at the Gene Regulation Observatory and a principal investigator at the Broad Institute of MIT and Harvard University.

But in terms of pinpointing how humans and great apes lost their tails, “there was (previously) nothing that had been discovered or hypothesized,” Shea told CNN in an email. “Our discovery is the first time we have suggested a genetic mechanism,” he said.

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

The study's co-author said the researchers' breakthrough moment came when Shea was reviewing the TBXT region of the genome in an online database widely used by developmental biologists. Itai Yanaiprofessor in the Institute of Genetics, Biochemistry, and Molecular Pharmacology at NYU Grossman School of Medicine.

“It must have been something that thousands of other geneticists looked at,” Yanai told CNN. “That's unbelievable, isn't it? That everyone is looking at the same thing, and Po noticed something that they all didn't notice.”

Alu elements are abundant in human DNA; The insertion in TBXT is “literally one in a million that exists in our genome,” Yanai said. But while most researchers dismissed the Alu insertion in TBXT as junk DNA, Shea noted its proximity to the neighboring Alu element. It was suspected that if they coupled, they might disrupt the process of protein production in the TBXT gene.

“It happened in the blink of an eye. Then it took four years of working with mice to actually test it,” Yanai said.

The researchers used it in their experiments CRISPR gene editing technology To breed mice with an Alu insertion in their TBXT genes. They found that Alu made the TBXT gene produce two types of proteins. One of those resulted in shorter tails; The more genes produce this protein, the shorter the tails.

The 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.

“As we understand how they replicate in the genome, we are now forced to consider how they also shape very important aspects of physiology, morphology, and development,” he said. “I think it's amazing that a single item of alo — something so small — can lead to the loss of an entire appendage like a tail.”

The efficiency and simplicity of Alu's mechanisms in influencing gene function have not been appreciated for a very long time, Shea added.

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

Tailless and tree-dwelling

Humans still have tails when we develop in the womb as fetuses; This small appendage is the caudal progenitor of all vertebrates and includes 10 to 12 vertebrae. It is only visible from the fifth to sixth week of pregnancy, and by the eighth week of the fetus's life, its tail has usually disappeared. Some babies retain embryonic tail remnants, but this is extremely rare, and such tails usually lack bone and cartilage and are not part of the spinal cord, another team of researchers reported. mentioned 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, the biological anthropologist said. Lisa Shapiroprofessor in the Department of Anthropology at the University of Texas at Austin.

“I think it's really interesting to determine what genetic mechanism may have been responsible for tail loss in hominins, and this paper makes a valuable contribution in 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 raises the question of whether or not the mutation was maintained because it was functionally advantageous (an evolutionary adaptation), or just not a liability,” Shapiro said. , which studies how primates move and the role of the spine in primate movement.

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 back to 21 million years ago and 18 million years ago, respectively). Shapiro said the fossils show that although these ancient primates were tailless, they lived in trees and walked on four limbs with a horizontal body position like apes.

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

Bipedalism may have evolved to accommodate the loss of the tail, which would have made it difficult for primates to balance on branches, “but it doesn't help us understand why the tail was lost in the first place,” Shapiro said. She added that the idea that upright walking and the loss of the tail are functionally related, with tail muscles being repurposed as pelvic floor muscles, “is an old idea that is not consistent with the fossil record.”

“Evolution works from what's already there, so I can't say that losing the tail helps us understand the evolution of human bipedalism in any direct way. It helps us understand our ape ancestors,” she said.

For modern humans, tails are a distant genetic memory. The story of our tails is far from over, Shea said, and there is still a lot for scientists to explore about tail loss.

He suggested that future research should look into other consequences of the Alu component in TBXT, such as effects on the growth and behavior of the human fetus. Although the absence of the tail is the most obvious consequence of the introduction of Alu, it is likely that the presence of the gene also led to other developmental transformations—as well as changes in locomotion and related behaviors in early hominins—to accommodate the loss of the tail.

Additional genes may have played a role in the tail loss as well. While Alu's role “seems to be very important,” Shea said, other genetic factors likely contributed to the permanent disappearance of the tails of our primate ancestors.

“It is reasonable to believe that during that period, there were many mutations associated with fixation of tail loss,” Yanai said. Such evolutionary change is complex, he added, and our tails are gone forever. Even if the driving mutation identified in the study could be undone, “it would not be able to bring back the tail.”

The new findings may also shed light on a type of neural tube defect in fetuses known as spina bifida. In their experiments, the researchers found that when mice were genetically engineered to lose the tail, some of the mice developed neural tube defects similar to spina bifida in humans.

“Maybe the reason 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've made this connection to this particular genetic element and this particularly important gene, it could open doors to study neurological defects.”

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