The DNA sequence that explains why humans and great apes don't have tails

A common feature of most of the animal kingdom is that the tail is present in all mammals – including… The wise one– At some stage of embryonic development. In the case of humans, gorillas or orangutans, it disappears at the end of pregnancy, although certain marks remain in internal parts such as the lower part of the spine (specifically between the last part of the sacrum and the coccyx, an area usually called the rump).

Tail loss has long been identified as a distinctive feature of apes, and some scientists believe it could play an important role in the evolution of bipedalism. It has been present in the primate lineage since its inception, more than 65 million years ago. Its loss should have occurred after the separation of the branch that gave rise to humans, chimpanzees and gorillas, about 25 million years ago.

Magazine this Wednesday nature Its cover is dedicated to research conducted by scientists at the Grossman School of Medicine in New York that reveals how The introduction of a mobile gene sequence would be associated with its loss.

Previous work had linked more than 100 genes to tail development in different vertebrate species, and the authors of this work explored the hypothesis that their absence was due to a modification in the DNA of one or more of them. They focused in particular on a gene called TBXT, whose mutations have been linked to particularly short tails in many animals, including Manx cats.

But the study authors discovered that it does not depend on TBXT mutations, but rather on the insertion of a piece of DNA called AluY into the gene's regulatory code. Specifically, the research refers to the process by which genetic instructions regulate proteins, molecules that make up an organism's structures and signals. The DNA is read and converted into RNA and mature RNA (mRNA), which produces those proteins.

A key step in this process occurs when certain sections, called 'spacers', direct the way certain regions are sewn together (known as RNA splicing), before they are removed. Thanks to the combination of different spacers, the same gene can encode many proteins. The authors believe that AluY, a specific type of spacer (sometimes called a jumping gene), is what affects TBXT reading and causes the tail to not develop.

In fact, when the research team conducted a series of experiments on mice to examine whether modifying this motor gene affected their tails, Discover a variety of monuments, “More than some mice that were born without them.” This finding is noteworthy because most human introns [porción de ADN transcrita en ARN] They carry repetitive, mobile copies of DNA, with no effect on gene expression. However, this particular AluY insertion does something as obvious as specify tail length,” says Jeff Boeke, one of the study's authors.

Evolutionary advantage or risk

Through experiments in mice (made possible by the use of CRISPR genetic scissors), the researchers also observed that the appearance of AluY increases the risk of defects in the embryonic structure that later forms the brain and spinal cord, known as the neural tube. This leads them to suggest that tail loss in our ancestors would have had a greater risk of developing defects of this type.

“Future experiments will test this theory that, in ancient evolutionary exchange, Loss of the tail in humans has contributed to neural tube birth defects. “Such as those related to spina bifida, which today is observed in one in every thousand newborns,” says Itai Yanai, one of the study's authors.

Previous research has suggested that the loss of the tail could have occurred to give an evolutionary advantage to our ancestors who moved from a life that evolved primarily in trees to spending more time on the ground, such as Ardipithecus ramidus. But the article is in nature He offers an alternative explanation: 25 million years ago, major geographic changes (caused by tectonic movements) isolated a population in which genetic drift, a random mutation, could have contributed to this change.