The tail has been a very useful tool since the first animals appeared, more than 500 million years ago. Fish swam through it, dinosaurs balanced on it, and scorpions use it as a weapon. Closer to our species, 25 million years ago, proto-primates used them as an extra limb to hold on to the branches of the trees where they lived, just as New World monkeys do now. But then something happened that caused this useful accessory to disappear from our evolutionary branch. Neither chimpanzees, nor orangutans, nor monogamous gibbons have tails. Neither do humans, at least as adults.
Humans have tails, but only for a brief moment, between the first and second months of pregnancy. For several weeks you can see the tail as a reminder of the line we share with countless beings who have walked and walked with this extension attached to their butt. Then the genetic programming does its job and the tail is concentrated in the three to five fused vertebrae that form the coccyx. This change was associated with greater ease in walking upright, leaving trees, freeing our hands, and beginning to create the technology that characterizes our species. However, until recently, no plausible genetic mechanism was proposed to explain such a relevant change.
Magazine Nature today publishes work led by scientists at New York University, in which the authors identify a genetic change that would explain the disappearance of the tail. To find this modification, the researchers compared the DNA of several species of tailed monkeys with that of other great apes, looking for genetic variants that these monkeys share and those that they do not. They thus identified the TBXT gene, which is essential in embryonic development and which regulates the formation of the tail in many primates. “The change in the gene we observed is a short gene. jumper — a DNA fragment known as an Alu sequence — landed in the non-coding part of the gene,” explains Itai Yanai, lead author of the study. There, its proximity to another Alu element changed the activity of the TBXT gene, which began to produce a different protein than the one that normally causes the tail to grow.
To test their theory, which they previously presented in a non-peer-reviewed publication, the New York team led by Bo Xia engineered mice to express different forms of the TBXT gene. When they created a variant of a protein that is generated by the action of a jump gene in humans, gorillas or chimpanzees, the mice lost their tails or developed a short tail. “It’s surprising that such a large anatomical change can be caused by such a small genetic change,” says Yanai.
In addition to tail loss, the researchers observed that mice that expressed this protein were more likely to suffer from developmental defects such as spina bifida. This group of malformations, known as neural tube defects, occurs in one in a thousand births. “This suggests that the evolutionary pressure to lose the tail was so great that despite creating the possibility of these diseases, we still lost the tail,” says the person responsible for the study, which speculates that having a tail is something so fundamental to vertebrates, “removing it is the only mutations could have caused the observed defects’.
Although the results offer an explanation for such a characteristic of humans and their closest relatives, the authors acknowledge that other genetic changes may have served to stabilize this aspect. In addition, loss of the tail or reduction in its size has occurred several times in the evolution of primates, as evidenced by outlaws, baboons or Gibraltar monkeys. The availability of genetic sequences from multiple primate species may help to understand these processes of convergent evolution, in which the same trait appeared at different times among animals with different environmental pressures and possibly different genetic mechanisms.
In a commentary on the study also published in NatureMiriam Konkel of Clemson University and Emily Casanova of Loyola University, both in the United States, point out that although some scientists interpret that the loss of a tail may offer evolutionary advantages, such as easier upright walking, there are indications that “having a tail does not prevent bipedal locomotion and may even support it’. As an example, scientists remember capuchin monkeys, which use their tails to balance stone tools with their hands. “Although humans typically carry loads upright, research on robots suggests that a tail attached around the waist can increase stability,” they say. This would allow the tail to offer adaptive advantages to modern humans, and the loss of the appendage that our animal family suffered 25 million years ago would remain a mystery.
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