Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news about fascinating discoveries, scientific advancements and more.
Humans have many wonderful features, but we lack something that most animals with a backbone have in common: a tail. The exact reason for this was something mysterious.
Tails are useful for balance, propulsion, communication, and defense against biting insects. But 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. This 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 was abundant in our genome but was ignored for decades as junk DNA, a sequence that apparently serves no biological purpose. They identified the particle, 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 jumping genes, which are genetic sequences that can change their position in the genome and trigger or reverse 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 they compared DNA from six hominoid species and 15 non-hominoid primates, they found AluY only in hominoid genomes, the scientists reported Feb. 28 in the journal Nature. In experiments with genetically modified mice (a process that took about four years), changes made by inserting Alu into the rodents’ TBXT genes resulted in variable tail lengths.
Before this study, “there were many hypotheses about why hominoids evolved tailless,” said lead study author Bo Xia, a research associate at the Gene Regulation Observatory, with the most common linking taillessness to upright posture and the evolution of bipedal walking. Principal investigator at MIT Broad Institute and Harvard University.
But when it comes to pinpointing how humans and great apes lost their tails, “there was nothing (previously) discovered or hypothesized,” Xia told CNN in an email. “Our discovery is the first to suggest a genetic mechanism,” he said.
Since tails are an extension of the spine, the findings may also help understand malformations in the neural tube that can occur during human fetal development, according to the study.
‘One in a million’
A turning point for the researchers came when Xia examined the TBXT region of the genome in an online database widely used by developmental biologists, said study co-author Itai Yanai, a professor at New York’s Institute of Systems Genetics and Biochemistry and Molecular Pharmacology. York University Grossman School of Medicine.
“It must be something that thousands of geneticists have studied,” Yanai told CNN. “This is incredible, isn’t it? “Everyone was looking at the same thing and Bo noticed something they all didn’t.”
Alu elements are abundant in human DNA; The insertion into TBXT is “literally one in a million that we have in our genome,” Yanai said. But while most researchers dismissed TBXT’s Alu insertion as junk DNA, Xia recognized its proximity to a neighboring Alu element. He suspected that if they matched, it might trigger a process that disrupted protein production in the TBXT gene.
“It just happened. Then we had to study mice for four years to really test it,” Yanai said.
In their experiments, the researchers used CRISPR gene editing technology to breed mice with Alu inserted into their TBXT genes. They found that Alu enables the TBXT gene to produce two types of proteins. One of these led to shorter queues; The more proteins the genes produce, the shorter the tails.
This discovery adds to growing evidence that Alu elements and other jumping gene families may not be “junk” after all, Yanai said.
“As we understand how they replicate in the genome, we are also forced to consider how they shape very important aspects of physiology, morphology and development,” he said. “I think it’s surprising that a single Alu element – something small, small – can cause the complete loss of an appendage such as a tail.”
Xia added that the efficiency and simplicity of Alu mechanisms affecting gene function have been underappreciated for too long.
“The more I study the genome, the more I realize how little we know about it,” Xia said.
Tailless and tree-dwelling
While developing as embryos in the womb, humans still have tails; This small appendage is a remnant from the tailed ancestor of all vertebrates and contains 10 to 12 vertebrae. It can only be seen between the fifth and sixth weeks of pregnancy, and by the eighth week the fetus’s tail usually disappears. Some babies are left with a remnant of an embryonic tail, but this is extremely rare and such tails usually lack bone and cartilage and are not part of the spinal cord, another research team reported in 2012.
Although 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. Austin
“I think it’s really interesting to identify a genetic mechanism that might be responsible for tail loss in hominoids, and this paper makes a valuable contribution in that regard,” Shapiro, who was not involved in the research, said in an email to CNN. .
“However, if this is a mutation that led to random tail loss in our monkey ancestors, this raises the question of whether the mutation was maintained because it was functionally beneficial (an evolutionary adaptation) or simply because it was not a hindrance. It is important to understand how primates move and how the spine is important in primate movement.” Exploring his role, Shapiro said:
When ancient primates started walking on two legs, they had already lost their tails. The oldest members of the hominid lineage are the early monkeys Proconsul and Ekembo (found in Kenya and dated to 21 million years ago and 18 million years ago, respectively). Shapiro said the fossils show that although these ancient primates were tailless, they were tree dwellers that walked on four limbs with a horizontal body posture like monkeys.
“So the tail disappeared first, and then the mobility we associate with living apes later evolved,” he said.
Shapiro said bipedal walking may have evolved to accommodate tail loss, which would have made it harder for primates to balance on branches, but that doesn’t help us understand why the tail was lost. He added that the idea that upright walking and tail loss were functionally linked and that tail muscles were repurposed as pelvic floor muscles was “an old idea that is NOT consistent with the fossil record.”
“Evolution works from what already exists; Therefore, I cannot say that the loss of the tail directly helps us understand the evolution of human bipedalism. “Still, it helps us understand our monkey ancestors,” he said.
A tail as old as time
For modern humans, tails are a distant genetic memory. But the story of our tails is far from over, and scientists still have a lot to discover about tail loss, Xia said.
He suggested that future research could investigate other consequences of the Alu element in TBXT, such as effects on human embryonic development and behavior. Although the absence of a tail is the most visible consequence of Alu insertion, it is possible that the presence of the gene triggered changes in locomotion and related behaviors in early hominoids, as well as other developmental changes to adapt to tail loss.
Additional genes likely also played a role in tail loss. Although 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 there were many more mutations related to compensation for tail loss at that time,” Yanai said. And because such evolutionary change is complex, he added, our tails disappeared completely. Even if the repulsive mutation identified in the study could be reversed, “that still wouldn’t bring the tail back.”
The new findings may also shed light on a type of neural tube defect in embryos known as spina bifida. In their experiments, researchers found that when mice were genetically modified for tail loss, some developed neural tube deformities that resemble spina bifida in humans.
“Perhaps 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 have established this connection with this particular genetic element and this particularly important gene, it may open the door to 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.
For more CNN news and newsletters, create an account at CNN.com