One of the main anatomical changes in the evolution of the human being was the loss of the tail . This morphological reprogramming of hominids has always been related to our way of moving on two legs. But the reality is that it is still one of the most unknown characteristics of the human being . Thanks to scientific advances in genetic studies, we can formulate more accurate answers to this mystery. This is how researchers consider that we lost our tail 25 million years ago and, instead of a process throughout our evolution, it could have been a sudden transformation.
Genetics guides the way
The tail appeared in the first living beings, more than 500 million years ago. This appendix has had and has multiple uses . Fish propel themselves with it in the water, some reptiles can lose it and regenerate it again, scorpions use it as a weapon, rattlesnakes to scare their predators, birds to direct their flight and, although cats play at hunting its own tail, utility for most mammals has to do with balance.
But in the evolution of mammals, we apes lost our tails. Chimpanzees, gorillas, and humans don’t have this appendage hanging off our backs. The coccyx is all we have left as a memory of when we had a tail. Why would we lose our tail and how did it benefit us?
Bo Xia , of New York University Grossman School of Medicine, had been asking this very question since childhood, and after injuring his coccyx while traveling, Xia remembered his childhood curiosity and went to work trying to find out. Why do humans have no tail?
The effort of Xia and his colleagues was rewarded and, in an article published in bioRxiv, they launched a hypothesis as an answer to this mystery. They studied 31 genes that influence tail development and found that the reason why this appendage stopped growing is because two ‘Alu sequences’ were inserted into an intron of a specific tail gene. In Spanish: a mutation made the protein in charge of making the tail grow useless .
They tested mice and found that subjects with this genetic mutation were born with shorter tails or directly lost it completely. This scientific evidence led us to think that the disappearance of the tail was not gradual, over millions of years, but that we lost the tail “in one fell swoop” , in the words of Itai Yanai, from the Institute of Computational Medicine of the University from New York.
What came before bipedalism or the absence of a tail?
The absence of a tail in humans has always been related to bipedalism. Our head, although heavy, sits on our spine. Animals that walk on all fours need a counterweight to balance. When walking upright, humans not only do not need the tail, but it would bother us to move.
These explanations propose that we lost our tails because it was awkward to walk upright, but the issue is more complex because, if you look at the fossil record, the earliest tailless apes still walked on all fours . It is a case of doubt between cause and effect. It was before chicken or the egg? Did we lose our tails because we walk upright? Or can we walk upright because we lost our tail?
We have the how, we lack the why
In any case, the research led by Bo Xian points to how we lost our tails, but we do not know exactly why our ancestors stopped having them, nor the causes that allowed this mutation to survive in time . Well, once again, a change in the evolution of the human being could also have negative consequences. Researchers speculate that the loss of our tails has led to abnormalities such as spina bifida.
It is clear that we should have taken more advantage of the absence of a queue to have come this far without it. Mutations occur constantly in all species . If the host benefits from it, it is possible that it will transfer its change to the next generation. But if the mutation is negative, that is, it is harmful to the individual, it is normal for it to have fewer options to reproduce, so the mutation does not survive. If a white rabbit is born from a herd of brown rabbits living in the field, it will be seen first by a hawk.
Therefore, not all mutations represent progress for the species, they are just a change. The ability to adapt to the environment and reproduce is what measures the success of a species, which can be favored or harmed by mutations.
Scientists don’t think it’s likely that our tails will grow out again, but if it did, we’d already be able to shoo away flies without taking our hands off our mobile.
References:
Vicente, A. et al. 2018. In search of the lost origin. Paid.
Xia, B. et all. 2021. The genetic basis of tail-loss evolution in humans and apes. BioRxiv. DOI: 10.1101/2021.09.14.460388.
