The tail has been a very useful tool since the first animals appeared, more than 500 million years ago. Fish have used it to propel themselves through the water, dinosaurs balanced with them and scorpions use them as a weapon. Closer to our species, 25 million years ago, ancestral primates used them as another limb, to cling to the branches of the trees where they lived, just as new world monkeys do now. But something happened then that made that useful appendage 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 month of gestation. For a few weeks, you can see the tail, as a reminder of the lineage we share with the countless beings who walked and walk with that extension attached to their asses. Then, the genetic programming does its work and the tail is concentrated in the three to five fused vertebrae that form the tailbone. This change has been 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 had been proposed to explain such a relevant change.
Magazine Nature today publishes a work led by researchers from 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 apes, looking for genetic variants that the latter share and those that do not have. Thus, they identified the TBXT gene, which is essential in embryonic development and which, in many primates, regulates tail formation. “The change in the gene that we observed is that, a short gene jumper —a fragment of DNA known as an Alu sequence—landed in a non-coding part of a 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 usually makes the tail grow.
To test their theory, which they had previously presented in an unreviewed publication, the New York team, led by Bo Xia, engineered mice to express different forms of the TBXT gene. When they produced the variant of the protein that in humans, gorillas or chimpanzees is generated by the effect of the jumping gene, the mice lost their tails or developed a short tail. “It is surprising that such a large anatomical change can be caused by such a small genetic change,” says Yanai.
In addition to the loss of the tail, the scientists observed that mice that expressed that protein were more likely to suffer from developmental defects such as spina bifida. This group of malformations, known as neural tube defects, occur in one in every thousand births. “This suggests that the evolutionary pressure to lose our tail was so great that, despite creating the possibility of these diseases, we still lost our tail,” says the person responsible for the study, who speculates that, since having a tail is something so basic to vertebrates, “eliminating it with a single mutation may have caused the observed defects.”
Although the results offer an explanation for such a characteristic trait of humans and their closest relatives, the authors recognize that other genetic changes may have served to stabilize this aspect. Furthermore, the loss of the tail or the reduction in its size has happened several times in the evolution of primates, as attested by the lorises, baboons or Gibraltar monkeys. The availability of genetic sequences from more primate species may help understand these processes of convergent evolution in which the same trait appeared at different times among animals with different ecological pressures and, perhaps, by different genetic mechanisms.
In a commentary on the study also published in Nature, Miriam Konkel, from Clemson University, and Emily Casanova, from Loyola University, both in the United States, recall that, although some researchers interpret that the loss of the tail may offer evolutionary advantages, such as greater ease in walking upright, there are indications that “having a tail does not prevent bipedal locomotion and may even favor it.” As an example, the researchers remember capuchin monkeys, which use their tail to maintain balance when carrying stone tools with their hands. “Although humans typically carry loads by standing upright, robotics research suggests that a tail mounted around the waist can increase stability,” they say. That would make it possible that a tail might offer adaptive advantages for modern humans, and would keep the loss of the appendage that our animal family suffered 25 million years ago a mystery.
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