Anyone who has ever watched “The Jungle Book”, the Walt Disney classic, remembers the facetiousness with which the monkeys move, from trees to stones, using their tails.
Like their feet which resemble hands, this attribute symbolizes in our eyes the skill of these animals.
This forgets that not all monkeys have such an organ.
Thus, chimpanzees, gorillas, orangutans and gibbons, united within the family of great apes or hominoids, do not have it.
No more than their close cousin, the primate that is most familiar to us: Man!
How did our ancestors lose this appendix?
Researchers from several American universities announce today that they have discovered what is believed to be at the origin of this transformation: the insertion of a tiny segment of DNA into a gene, TBXT, certainly known to be involved in the development of tail.
“We were surprised and excited!
It is surprising that such a large anatomical change can be caused by such a small genetic change,” says Itai Yanai, a researcher at NYU Langone Health University Medical Center in New York.
The disruptive element, so to speak, is specific to hominoids, as clarified in the study published this Wednesday in Nature and of which he is the main author.
The dating of its intrusion into the genome coincides, according to the researchers, with the moment when hominoids lost their tails: around 25 million years ago.
A step towards bipedalism?
To ensure they had identified the correct piece of DNA among the 140 genes examined, the scientists created mice with the same type of mutation.
To do this, they used the famous CRISPR-Cas9 technology, “genetic scissors” which make it possible to edit DNA, in other words to remove or add segments.
The result is clear: certain mutant mice were born without tails or with significantly shortened appendages.
“It is inspiring to be able to study the genome to discover how our ancestors may have evolved to lose their tails, and therefore adapt to life on the ground, developing large brains and using tools,” rejoices Itai Yanai.
“A tail can be useful when you live in trees.
As soon as you go on the ground, it can become a handicap, continues Itai Yanai.
Lack of a tail may have had an evolutionary advantage in that its loss facilitated the evolution of bipedal locomotion.
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The theory is still debated.
In another article published in Nature, two researchers, Miriam K. Konkel and Emily L. Casanova, indicate that in certain species, the tail has on the contrary favored bipedalism.
They put forward another hypothesis: brutal climatic changes which would have isolated the populations of great apes, 25 million years ago, and would have led to “random genetic drift” due to a lack of sufficient mixing.
A legacy disease
Whatever the advantage of no longer having a tail, this development would have had a cost.
The rear appendage, which is found in most vertebrates, is located in the extension of the column.
However, Itai Yanai's team suggests that its loss could have led to an increased probability of malformations at this location in humans, encountered in one in 1,000 newborns. Some mutant mice suffering from these "neural tube defects », this disease would be, according to the study, a legacy of the missing limb.
A heritage that we all share, however, is the coccyx.
Three to five vertebrae fused together and discreetly located above the buttocks.
What about the rare cases of children who are born with a growth in the lower back?
“These are not tails containing vertebrae,” notes Itai Yanai, who therefore believes that there should be no link with the genetic mutation he identified.
But it is difficult for him to say with certainty: “We tried to obtain the DNA of people with tails
,
but we did not succeed.
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