What happened to the human's tail?

"Where is my tail?" Geneticist Xia Bo often wondered about this question when he was a child. Several years ago, while pursuing his doctoral degree at New York University (NYU), his coccyx was accidentally injured. During the recovery process, this question resurfaced in his mind. Now, after years of being troubled by this question, Xia Bo and his colleagues have finally found an answer. They discovered that approximately 25 million years ago, a gene mutation occurred in humans and other ape-like primates, which may have resulted in the loss of their ancestors' tails. On February 28th, the related research was published in Nature. Unlike most monkeys, apes, including humans, and their extinct relatives do not have tails. Their coccyx is a remnant of the vertebral bones that make up the tails of other animals. Originally, Xia Bo did not intend to focus his doctoral research on finding the genetic basis for this feature, but the coccyx injury sparked his curiosity once again. Based on intuition, Xia Bo decided to study a gene that plays a role in tail development. In 1927, Ukrainian scientist Nadine Dobrovolskaya-Zavadskaya described a short-tailed laboratory mouse that carried a gene mutation called T, which is similar to the human TBXT gene. Humans and other ape-like primates carry a DNA insertion in TBXT, while other tailed primates such as monkeys do not. As early as September 2021, Xia Bo and his colleagues published this research on the preprint platform bioRxiv. The study showed that this DNA insertion in primates can shorten the protein encoded by TBXT. Moreover, the shortening occurs after gene transcription into messenger RNA, which is the splicing of multiple protein-coding segments of the gene. Gene-edited mice carrying a copy of the mouse version of TBXT exhibit a range of tail defects - in some cases, the tail becomes shorter or completely absent, while in other cases, it becomes twisted or longer. Malte Spielmann, a human geneticist at the University of Kiel in Germany, said that this discovery has prompted numerous news reports, but the preprint study did not indicate whether the insertion of primate genes into the mouse version of TBXT leads to tail loss. They have not yet conducted the main experiments. Itai Yanai, a systems biologist at New York University, co-led this research. He said that when the paper was submitted to Nature, these experiments were still ongoing. They ultimately demonstrated that when the gene is inserted into the mouse genome, it does not produce a highly shortened version of the protein, resulting in mice with normal tails. The researchers also inserted different TBXT genes into mice. Unexpectedly, this caused the gene to be spliced incorrectly in the same way as in humans, resulting in mice born with tails that are very short or completely absent. It turns out that this is a more convincing paper. Spielmann added that they clearly showed that this change leads to tail loss, but it is not the only genetic change. The researchers analyzed 140 genes related to tail development and identified thousands of primate-specific genetic changes that may be associated with tail loss. Gabrielle Russo, a biological anthropologist at Stony Brook University in the United States, said that she is really glad to see people studying the genetic mechanisms behind the disappearance and shortening of tails. Xia Bo's research group suggests that the loss of tails may have facilitated upright walking in primates and reduced the time they spent in trees, but Russo is uncertain about this. Fossils indicate that early primates walked on all fours like arboreal monkeys, and it took millions of years for them to evolve into bipedal walkers. Apes are not the only primates without tails; some macaques and a nocturnal animal called an aye-aye also lack tails, indicating that this feature has evolved multiple times. Xia Bo said that during the process of evolution, there may have been multiple ways to lose the tail, and our ancestors chose this one.GeneticistXia Bogene mutationhominoidTBXT genetail lossprimate-specific genetic changesgenetic mechanism.