US scientists have concluded that insertion of a self-replicating DNA region into one of the regions of the TVHT gene caused distant ancestors of humans and great apes to lose their tails.

Tail loss in the primate group consisting of gorillas, chimpanzees and humans is believed to have occurred approximately 25 million years ago. Later, the ancestors of humans and great apes diverged from the main branch of Old World monkeys. After this, fewer caudal vertebrae began to develop in great apes, eventually leading to the loss of the tail and the appearance of the coccyx. The reason for the loss of the tail is still unknown, but many researchers believe that it was facilitated by the fact that primates began to spend more time on the ground and not in trees.

The TVHT gene was discovered by geneticist Nadiya Dobrovolska-Zavadska in 1927 regarding the effect of radiation on the body of mice. The scientist learned that mutations in one of the two copies of this DNA region gave rise to short-tailed mice. Later, this extension of DNA was actually discovered and identified. Scientists then put forward the idea that mutations in the TVHT gene caused the loss of the tail, but no one knew exactly how this happened. Even after the discovery of TVHT, scientists discovered more than a hundred other genes that affect the shape and length of the mammalian tail. Therefore, the mystery remained unsolved.

Researchers from the University of New York Health (USA) examined how the structure of 140 genes possibly involved in tail formation in mammals differs in the genomes of humans, great apes and many other warm-blooded animals. The scientists’ conclusions were presented in the journal Nature.

Molecular biologists were able to identify nine mutations in these genes that are unique to primates and humans and significantly affect their functions. However, only one of these mutations affected the growth of the tail and did not cause malfunctions in other parts of the body. It was linked to a genetic insertion in the TVHT gene called Alu (one of the blocks of self-replicating DNA, which makes up about 10 percent of the total length of the primate genome) that caused information from the gene to be misread. As a result, a shortened version of the TVHT protein is produced and is unable to initiate the tail growth program.

Scientists have created genetically modified mice whose genome makes appropriate changes in the structure of TVHT. It turned out that various tail defects were observed in rodents with Alu adducts, and in some they were completely absent. Additionally, the mutation predisposed the mice to spina bifida, which is unusual for this mammal species but sometimes occurs in humans (about one in every 1,000 children). According to the scientists, this suggests that the mutations that deprived our ancestors of tails also made them vulnerable to this neural tube development defect in the embryo.