- Daniel Gonzalez Cappa
- BBC world news
Have you ever looked at the back of your body and wondered where your tail is?
It sounds like a joke or the kind of question a child would innocently ask. But for scientists, this is serious business.
After all, if humans are so biologically similar to apes, why do they have tails and we don’t?
“That’s a really good point,” agrees Bo Xia, a graduate student in stem cell biology at New York University’s Grossman School of Medicine.
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Tails can have a number of benefits in the animal world.
Since their appearance in the first living things more than 500 million years ago, they have taken on many roles.
In fish, it facilitates propulsion in the water. In birds, in flight. And in mammals, in balance.
It can also be a defensive weapon, like scorpions. Or a warning sign, as with rattlesnakes.
In primates, the tail is adapted to a variety of environments. American howler monkeys, for example, have a long, prehensile tail that helps the animal grab or hold objects in trees.
But hominids, the primate family that includes humans and great apes like orangutans, chimpanzees and gorillas, lack this.
The why and how are questions that have intrigued scientists for decades.
The answer appears to lie in a recently discovered genetic mutation that affected the genes that shaped hominid tails around 25 million years ago.
Furthermore, the mutation survives over time and is passed on from generation to generation, altering hominid locomotion, which could be related to humans walking on two legs.
“Everything seems to be related to each other and happened at the same time of evolution. But we didn’t know the genetics at work in this development process and, of course, in evolution”, adds Xia.
“As you can imagine, this is one of the most crucial points in evolution, what makes us human.
And to test this, Xia applies the same mutation to mice.
He observed that mice had different shaped tails. Some had shorter tails, while others didn’t.
A puzzle with a tail and legs
Charles Darwin had already said this. Homo sapiens sapiens (or, in other words, the current human species) was related to the tailed apes.
The English naturalist published ‘The Origin of Man’ in 1871, a book in which he explains that the theory of evolution is entirely applicable to the human species.
A true revelation for the time. After all, humans have always marked the distance between the animal world and modern society: we live in houses, our skin is different, and we use our brains to solve complex dilemmas.
Darwin had already upset the foundations of science at the time with the publication of ‘The Origin of Species’. His explanation of the origin of humanity was a revolution, as until then most Western scientists shared the idea that God had designed every creature on the planet.
However, humans and chimpanzees, with whom we have common ancestors, share over 98% of our DNA.
Even the first hominids that appeared around 20 million years ago lacked a tail.
So if the tail is linked to the evolution of apes and humans and has influenced locomotion and gait, the question arises, which came first, the tail or the legs?
“It’s like the chicken and the egg,” says Xia. “And as you can imagine, that’s not an easy question to answer.
In short, it’s virtually impossible to know exactly what early events made our ancestors walk on two legs, and whether this was related to the fact that they didn’t have a tail.
Or vice versa, if we don’t have a tail because we walk upright and it’s easier to balance on our legs.
“We would need a time machine to know all this. We could go back in time, observe and analyze the initial events. But since we don’t have one, I can say that we don’t know, and that will be the end of the discussion. So someone might ask why that we’re talking about all this.”
“The real answer is that these two processes are always discussed together or interfere with each other.”
In other words, we cannot talk about human evolution without referring to tails or bipedal locomotion, regardless of which came first.
The answer lies in genetics
Ever since he injured his tailbone on an Uber ride two years ago, Xia has been turning the subject of tails and humans all over the place.
The coccyx is the last piece of the spine formed by four fused vertebrae and represents the remnant of what was a tail millions of years ago.
If you look at images of human embryos, you can see a tail, which is absorbed by the embryo after a few weeks to form the spine.
This coccyx, which supports the buttocks, is located where other animals have a tail.
“We raise all these questions because humans are interested in science and we’re looking for answers. And in science, in the last 100 years, we’ve made great strides in genetics,” said Itai Yanai, researcher and director of the Institute for Computational Medicine at New York University.
“You really need to know a lot of concepts about development, alternative splicing, comparative genomics. And Bo showed that if you understand those concepts, you can look at the genome, understand it, and see what’s in it.”
The mutation identified by Xia consists of 300 genetic letters in the middle of a gene known as TBXT, a stretch of DNA that is largely the same in humans and apes.
To test the link between this mutation and the tail, Xia genetically modifies mice with the same mutation.
Eureka! Xia and her colleagues note that the mice’s tails do not grow as normal.
But this discovery is the first in a long series that could help understand the role of mutations in our ancestors. Scientists say there are more than 30 genes involved in tail formation in animals, and the New York researchers are only talking about one.
As Xia says, all humans have very similar tail bones to each other, but in the case of the mice in the experiment, the tails were either different sizes or completely absent.
Xia’s conclusion is that there were a number of mutations, not just one, that affected different genes in hominids 25 million years ago and altered our evolution.
“It could be a crucial mutation, but we don’t think it’s solely responsible,” he said.
Scientists already know how the ancestor of man lost its tail millions of years ago.
But the real reasons why this mutation survived over time are still unclear.
And for Xia and Yanai, that’s a question that won’t be answered, at least for now.
“Mutations happen all the time,” says Yanai.
Some mutations can be positive and others negative, depending on the environment, as Xia points out.
Normally, if a mutation is negative, it can harm the host, making it sick or killing it. Therefore, this mutation does not survive over time.
But if a mutation brings benefits, then it is absorbed by the most adaptive individuals, causing it to be passed on from generation to generation.
What Xia means is that the loss of the tail may have provided hominids with important evolutionary advantages that explain their duration over time.
Maybe not balance in trees, but better bipedal locomotion, or using hands to manipulate objects.
This is not to say that losing the tail only brought us good things.
Xia and his team observed that the mice that participated in the experiment had spinal malformations very similar to the neural tube defects that affect one in every thousand human newborns.
These defects are linked to spina bifida, which occurs when the fetal spine does not close completely, leading to nerve damage and eventual paralysis.
“So I wouldn’t say mutations are good or bad. It just happens,” says Xia.
“I think it’s very important,” he said. “You just have to look at the genome. And that’s why I hope it’s a lasting contribution.”
In turn, Mr. Yanai indicates that this work may contribute to the genome-wide understanding of other cases of events that occurred in our biological past.
“I think it teaches us to use our computer programs in a different way. We’ve had the genome for years. What Bo discovered could have been found years ago,” he says. “So I think the scientific community will be inspired by this work.”