by Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news on exciting discoveries, scientific advances and more.
Humans have many wonderful qualities, but we lack something that is common 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 protection against biting insects. However, humans and our closest primate relatives – the great apes – said goodbye to tails around 25 million years ago, when the group split from the Old World monkeys. The loss has long been associated with our transition to dimorphism, but little was known about the genetic factors that drove the primate tail.
Now, scientists have traced our tail loss to a short sequence of genetic code that is abundant in our genome but dismissed for years as junk DNA, a sequence that apparently has no biological purpose. They identified the fragment, called 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 jumper genes, which are genetic sequences that can change their position in the genome and induce or suppress mutations.
At some point in our distant history, the Alu AluY element jumped into the TBXT gene in the ancestors of the hominoids (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 about four years – tinkering with Alu insertions in the rodents’ TBXT genes resulted in variable tail lengths.
Prior to this study “there were many hypotheses as to why hominoids evolved without a tail,” the most common of which involved an upright posture and the evolution of bipedal walking, said the lead study author Bo Xia, a research fellow at the Gene Regulation Observatory. and principal investigator at the Broad Institute of MIT and Harvard University.
But when it comes to identifying exactly 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 to suggest a genetic mechanism,” he said.
And because tails are an extension of the spine, the findings may also have implications for understanding neural tube malformations that may occur during human fetal development, according to the study.
‘One in a million’
A breakthrough moment for the researchers came when Xia was reviewing the TBXT region of the genome in an online database widely used by developmental biologists, said study coauthor Itai Yanai, a professor at the Institute of Systems Genetics and Biochemistry and Pharmacology. Molecular at the New York. York University Grossman School of Medicine.
“It must be something that thousands of other geneticists have looked at,” Yanai told CNN. “That’s incredible, right? That everyone is looking at the same thing, and Bo noticed something they all didn’t.”
Alu elements are abundant in human DNA; the disruption in TBXT is “literally one in a million that we have in our genome,” Yanai said. But while most researchers dismissed the Alu TBXT insertion as junk DNA, Xia noted its proximity to a neighboring Alu element. He suspected that if they were paired, it could start a process that would affect the production of a protein in the TBXT gene.
“That happened in a flash. And then it took four years of working with mice to actually test it,” said Yanai.
In their experiments, the researchers used CRISPR gene editing technology to breed mice with Alu insertions in their TBXT genes. They found that Alu caused the TBXT gene to produce two types of proteins. One of these resulted in shorter tails; the more of that protein the genes produce, 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 are now forced to think about how they are shaping very important aspects of physiology, morphology and development,” he said. “I think it’s surprising that an entire appendage like the tail of one Alu element would be lost – a small, small thing.”
The efficiency and simplicity of Alu’s mechanisms to influence gene function has long been underappreciated, Xia said.
“The more I study the genome, the more I realize how little we know about it,” Xia said.
No tail and tree place
Humans still have tails when we are developing in the womb as embryos; This small appendage is an arm above me from the caudal ancestor of all vertebrae and includes between 10 and 12 vertebrae. It is only visible from the fifth to the sixth week of gestation, and by the eighth week of the fetus its tail is usually gone. Some babies retain 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 team of researchers reported in 2012.
But while the new study explains the “how” of tail loss in humans and great apes, the “why” 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 very interesting to find a genetic mechanism that could be responsible for hominoid tail loss, and this paper is a big step in that direction,” Shapiro, who was not involved in the research, told CNN in an email. .
“However, if this is a mutation that arose randomly from the loss of our bird ancestors, the question remains whether or not the mutation was maintained because it was functionally beneficial (evolutionary adaptation), or it wasn’t a hindrance,” said Shapiro, who investigates how primates move and the role of the spine in primate locomotion.
By the time ancient primates started 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 although these ancient primates were sedentary, they were tree dwellers who walked on four limbs with a horizontal body posture similar to monkeys, Shapiro said.
“So the tail was lost first, and then the locomotion that we associate with living apes evolved later,” Shapiro said. “But it doesn’t help us understand why the tail was lost in the first place.”
The idea that straight walking was functionally related to tail loss, with tail muscles repositioning as pelvic floor muscles, is an old idea that does NOT match the fossil record,” she said.
“Evolution works off of what’s already there, so I wouldn’t say that the loss of the tail helps us understand the evolution of human bisexuality in any direct way. It helps us understand our sinful ancestors, though,” she said.
A tail as old as time
To modern humans, tails are a distant genetic memory. But the story of our tails is far from over, and there is still a lot about tail loss for scientists to explore, Xia said.
Future research could investigate other consequences of the Alu element in TBXT, such as effects on human embryonic development and behavior, he suggested. Although the absence of a tail is the most visible result of the Alu insertion, the presence of the gene may also trigger other developmental changes — as well as changes in related movements and behavior in early hominids — to cope with tail loss.
It is likely that additional genes were involved in tail loss as well. Although the role of Alu appears to be “very important,” other genetic factors likely contributed to the permanent loss of tails in our ancestors,” Xia said.
“It’s reasonable to think that there were many more mutations during that time that stabilized the loss of the tail,” Yanai said. And because such evolutionary change is complex, our tails have evolved, he said. Even if the driver mutation identified in the study could be reversed, “it still wouldn’t bring the tail back.”
The new findings may also shed light on a type of neural tube defect in embryos called spina bifida. In their experiments, the researchers found that when mice were genetically engineered for tail loss, they developed some neural tube deformities that resembled spina bifida in humans.
“Maybe the reason we have this condition in humans is because of this trade-off our ancestors made 25 million years ago to lose their tails,” Yanai said. “Now that we have made this connection with this particular genetic element and this very 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 magazine.
For more CNN news and newsletters create an account at CNN.com
