Ever thought why don’t humans have tails? It certainly is one of the marathon questions that scientists themselves have been racking their brains about for years. Whereas most of the other species share backbones and with it the tail, our own big family The great Apes and there lost the appendage 25 million years ago. As recently as a few decades ago, we knew very little about the mystery at the heart of the evolutionary puzzle, about how all of this had come about, but studies have recently begun unravelling some really riveting insights into our genetic history.
Most vertebrates use their tails to achieve balance, locomotion, communication, or in defense. Many of them swat at insects or steer when jumping from trees. But when our lineage diverged from the Old World monkeys, we lost ours. How that happened has long been controversial, according to paleoanthropologists; it’s only in the past few years that some tantalizing evidence has come in.
They found a tiny genetic feature an Alu element that might illuminate the way humans lost their tails. That would appear to have something to do with the so-called junk DNA, seemingly lacking any apparent function. One of the types of ‘jumping genes,’ the Alu element can move to some other place in the genome and cause mutations.
About 25 million years ago, an Alu element hopped into a gene in our ancestors. Scrutinizing DNA from various primates, the team observed that an Alu insertion in the gene occurs specifically in hominoids—that is, in the great apes and humans. When researchers inserted the mutation they identified in mice by using the gene editing tool CRISPR, the result was that they were left either with shorter or no tails at all. It was a breakthrough having been four years in the making, which helped change our tailed state. “For the very first time, we are able to look at a plausible genetic mechanism for an event that took place during evolution: the loss of our tail. It’s almost like you get this huge, very big important anatomical change, driving from this very small genetic change. Said co-author Itai Yanai of New York University, a systems biologist.
This very Alu element regulates the Tbox gene to produce two types of proteins. Among these two proteins, one produces a shorter tail. Discovering, this means that after all this Alu element is not a junk; it has a powerful contribution towards our development and physiology. According to Bo Xia, principal investigator of the study, “The more I study the genome, the more I realize how little we know about it”.
The most interesting part is the fact that all humans, at one point, went through a stage as embryos during which they had a tail. A small tail with 10 to 12 vertebrae grows on the very embryonic, fifth-to-sixth week of gestation, but it always disappears by the eighth week. Actually, some babies are born with tiny, boneless tails, although this is very rare.
Although the genetic “how” of our loss of tails is becoming pretty clear, the reasons “why” are still very much open to debate. One popular theory tied tail loss to our ancestors’ adoption of bipedalism, not useful to have an upright posture with a tail. However, even fossils of early tailless apes like Proconsul indicate such primates were tree-dwelling, four-legged creatures.
“First the loss of the tail occurred and then the changes in locomotion probably followed,” Liza Shapiro, a biological anthropologist at the University of Texas, said. “It helps us understand our ape ancestry.” Still, exactly what made the mutation stick. what provided an advantage in evolution or wasn’t harmful is uncertain.
Furthermore, the loss of the tail could have had a price of its own. The transgenic mice had moderately higher rates of severe birth defects— roughly the equivalent of spina bifida in humans that could be costs for losing the tail.
Other effects of the Alu element in TBXT can still be researched in future studies relating to human development and behavior. While such a single genetic change appears to have made a big difference, it is likely that another type of mutation also took part in the permanent loss of tails of our ancestors.
Evolution is a much more complex process that brings further enlightenment to our biological history. Though we may not get our tails back, the story of their loss is one of the great tales of genetic discovery and evolutionary adaptation.
