Why don't humans have tails? Genetics is to blame, study finds

Like most vertebrates today, the common ancestors of humans and primates had a tail. Moreover, modern primates (monkeys, lemurs, etc.) still have one. However, this is not the case for humans and great apes (our close cousins, such as chimpanzees or bonobos). For a very long time, researchers wondered when this divergence occurred, why, and how it occurred.

Remember that when we are still embryos, we do indeed have a tail. After the first stages of embryonic development, around the eighth week, it disappears completely , ultimately leaving us only the coccyx. It is rare for babies to be born with a pseudo-tail made up of muscles, nerves, blood vessels and connective tissue, but without bones or cartilage. These pseudo-tails and the coccyx are, however, the only witnesses of our ancestry with ancient ancestors equipped with tails. But then why don't we have any?

According to a new study published on February 28 in the journal Nature, it was 20 or 25 million years ago that the tailless existence of hominids could have taken a turning point driven by genetic alteration. hinge.

No tail in humans: the work of genetics

When he was younger, Bo Xia, a doctoral geneticist at the Broad Institute of MIT and Harvard (United States), was surprised that almost all animals had a tail, but not him. However, it was only after a recent injury to his coccyx that he finally decided to carry out investigations to better understand this difference. With his team, he analyzed nearly 140 genes involved in tail development in animals . More specifically, the researchers carried out a comparative analysis of the genomes of six primates, including humans and fifteen species of monkeys, to try to identify key differences.

They were thus able to identify a 25 million year old mutation in Tbxt , a gene determining the formation of the tail. Indeed, by comparing the DNA sequences in baboons, rhesus macaques and other tailed monkeys with those of non-tailed primates (humans, chimpanzees, etc.), they noticed that the latter had a DNA fragment in more than was only found in the group without a tail: AluY . They therefore concluded that as minor as it was, this modification was at the origin of the anatomical changes observed.

An experiment to test this theory

However, they decided to carry out experiments to confirm their theory. To do this, they used the CRISPR-Cas9 technique, a genome editing tool which makes it possible to modify genetic material and mouse embryos by applying different modifications to the identified gene. Some mice are then born with shorter tails or no tails at all (see below). However, scientists are being cautious for the moment. Although the modifications did have an impact on the length of the mice's tails, they insist that it did not have the effect of an on-off button and therefore believe that other genes could have conditioned the absence of a tail .

Why did this mutation occur? In any case, its effects are not all positive…

Humans without tails, an evolutionary advantage?

Some scholars believe this may have provided an evolutionary advantage to tailless primates. However, this hypothesis encounters several limits. First of all, the tail also brings monkeys a whole bunch of advantages, whether in terms of locomotion, communication and more generally survival . Furthermore, the present study observed cases of neural tube defects, including spina bifida, in laboratory-modified mice . However, this is an anomaly that is also found in humans and which could therefore have this mutation as its origin. However, this would not be the first time that an evolutionarily advantageous mutation had a negative side result. For example, recent work has shown that genetic variants that help us fight pneumonia predispose us to Crohn's disease.

An essential step before walking on two feet?

Other scientists also believe that this was a first step prior to bipedalism, occurring even before our ancestors no longer lived in trees . This is particularly the opinion of Rick Potts, director of the Smithsonian Institution's Human Origins Project. In this respect, he recalls that certain primates without tails such as orangutans and gibbons still live in trees, but that they move in a very unique way compared to tailed monkeys, notably by swinging between the branches and clinging while remaining mostly vertical . However, as Itai Yanai, co-author of the study and a biologist at New York University, says, the only way to be sure “ would be to invent a time machine ”.

Additionally, as Gabrielle Russo of Stony Brook University in New York points out, research in the early 1900s linked tail loss to changes in human muscles that helped them stand vertically. However, changes in posture and learning to walk on two feet did not occur until several million years later . It is therefore difficult to imagine that this new study sheds real light on these traits in particular. According to her, future research should also look at other mammals with short tails or without tails (koala, bear, capybara, etc.) to see if they have undergone the same mutation.

Strong implications for research

In any case, this study ( read here ) not only enriches our understanding of human evolution, but also invites a reassessment of our approach to genetic diseases and anomalies, with possible implications for human health and research in this domain. Above all, it shows us to what extent human evolution, often presented as linear and progressive, is in reality a much more complex and dynamic process than we thought .