Why don’t humans have tails? | Top Vip News

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Humans have many wonderful qualities, but we lack something that is a common characteristic among most animals with backbones: a tail. Exactly why this is the case has been a mystery.

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

Now, scientists have attributed the loss of our tail to a short sequence of genetic code that is abundant in our genome but had been discarded for decades as junk DNA, a sequence that apparently serves no biological purpose. They identified the fragment, known as the 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 changing their location in the genome and triggering or undoing mutations.

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

Before this study, “there were many hypotheses about why hominoids evolved to be tailless,” the most common of which linked taillessness to upright posture and the evolution of bipedal gait, said the study’s lead author. . box xiaresearcher at the Genetic Regulation Observatory and principal investigator at the Broad Institute of MIT and Harvard University.

But as for pinpointing precisely how humans and great apes lost their tails, “nothing has (previously) been discovered or hypothesized,” Xia told CNN in an email. “Our discovery is the first time a genetic mechanism has been proposed,” she said.

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

A turning point for the researchers came when Xia was reviewing the TBXT region of the genome in an online database that is widely used by developmental biologists, the study’s co-author said. itai yanaiprofessor at the Institute of Systems Genetics, Biochemistry and Molecular Pharmacology at New York University Grossman School of Medicine.

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

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

“That happened in an instant. And then it took four years of working with mice to prove it,” Yanai said.

In their experiments, the researchers used CRISPR gene editing technology breed mice with the Alu insertion in their TBXT genes. They discovered that Alu caused the TBXT gene to produce two types of proteins. One of them led to shorter lines; 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 jumping gene families may not be “junk” after all, Yanai said.

“While we understand how they replicate in the genome, we are now forced to think about how they are also shaping very important aspects of physiology, morphology and development,” he said. “I think it’s amazing that an Alu element, such a small thing, can cause the loss of an entire appendage like the tail.”

The efficiency and simplicity of Alu’s mechanisms for affecting gene function have been underestimated for too long, Xia added.

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

Tailless and arboreal

Humans still have tails when we develop in the womb as embryos; This small appendage is inherited from the tailed ancestor of all vertebrates and includes 10 to 12 vertebrae. It is only visible between the fifth and sixth week of gestation, and by the eighth week of the fetus the tail usually has disappeared. 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, according to 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” remains 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 identify a genetic mechanism that might have been responsible for tail loss in hominoids, and this paper makes a valuable contribution in that regard,” Shapiro, who was not involved in the research, told CNN in an email. .

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

When ancient primates began to walk 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 although these ancient primates were tailless, they were tree-dwellers that walked on four limbs with a horizontal body posture like apes, Shapiro said.

“So first the tail was lost and then the locomotion that we associate with living apes evolved,” Shapiro said. “But it doesn’t help us understand why the tail was lost in the first place.”

The idea that upright walking and tail loss were functionally linked, and that tail muscles were repurposed as pelvic floor muscles, “is an old idea that is NOT consistent with the fossil record,” he added.

“Evolution works from what already exists, so I wouldn’t say that the loss of the tail helps us understand the evolution of human bipedalism directly. However, it helps us understand our simian ancestry,” he said.

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

He suggested that future research could investigate other consequences of the Alu element in TBXT, such as impacts on human embryonic development and behavior. Although the absence of a tail is the most visible result of the Alu insertion, the presence of the gene may also have triggered other developmental changes, as well as changes in locomotion and related behaviors in early hominoids, to adapt to the loss of tail.

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

“It is reasonable to think that during that time there were many more mutations related to the stabilization of tail loss,” Yanai said. And since this evolutionary change is complex, our tails have disappeared forever, she added. Even if the driver 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 modified to lose their tails, some developed neural tube deformities that resembled spina bifida in humans.

“Maybe the reason we have this condition in humans is because of this compensation 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 in the study of neurological defects.”

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

Correction: An earlier version of this story misstated Shapiro’s perspective on the type of locomotion that might have evolved to accommodate the loss of the tail.