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It’s not something that crosses our minds every day; but yes, the primate ancestors we were derived from, according to the theory of evolution, had—and still continue to have—tails.
So that should mean we would’ve had tails at one point, right?
Even Darwin called it. Once, he wrote, “I believe the Os coccyx gives attachment to certain muscles, but I cannot doubt that it is a rudimentary tail.” Imagine that: a human with a tail. Multiple, even.
But clearly, we don’t have them now. And the apes that appeared in the fossil record, which date back 20 million years, didn’t either. The true, concrete reason for this evolutionary change has never been recognized.
Until now, at least. A team of scientists based in New York believes they have narrowed it down to a specific genetic mutation, one that caused mice to stop growing tails when they were investigated in the study published last week.
“This question—where’s my tail?—has been in my head since I was a kid,” Bo Xia, a graduate student in stem cell biology at the NYU Grossman School of Medicine and an author of the paper, tells The New York Times.
Still determined to find an answer, he set out on the grand quest to find out. To start, he looked at how other animals’ tails were formed.
In the early stages of an embryo’s development, a set of master genes directs parts of the spine to begin growing the identifiable protrusions that come in the form of a neck, for example.
At the other end of the spine, a tail bud is developed. This contains a chain of vertebrae, muscles, and nerves. Further growth ensures the tail sprouts. So, it should mean that not having this bud would hinder the growth of a tail.
An alteration in the master genes must’ve changed this, Xia thought, and he studied the DNA of six species of apes without tails to nine species of monkeys with tails. His findings revealed a mutation shared by humans and tail-less apes in the form of a gene called TBXT.
To test this, Xia and his colleagues genetically engineered mice with the TBXT mutation found in humans. This comes in the form of 300 genetic letters in the middle of the gene.
When the mice embryos developed, many of them didn’t develop a tail. Others only managed to grow a short one. It seems to confirm Xia’s theory almost perfectly.
What must’ve happened, the findings suggest, is that the mutation must’ve accidentally occurred to some ape who grew a coccyx (tailbone) rather than a tail.
But what fascinated the team was the fact that not only did a seemingly disadvantageous mutation allow the ape to survive, but it also thrived to the point where it was able to pass it on to future generations. In turn, the offspring were able to thrive so well without a tail that it became the norm for apes; and consequently, humans.
It surprised scientists so much because, as it turns out, a tail isn’t “just” a tail. It’s a pretty great built-in tool, in fact. Apart from helping to balance weight when the primates began to walk upright instead of on all fours, it also let monkeys hang from trees.
Apes were, and are, larger than monkeys. It should’ve been more crucial for them to have a tail because their size made them more likely to fall. And those falls, in turn, were more likely to be fatal or injure them greatly. A tail for balance—isn’t that supposed to be what they needed more than their relatives?
One question appears solved, and that’s the question of how and why us humans don’t have a tail. That can be attributed to the TBXT gene.
But the other question is why our ape ancestors lost them when they would’ve been much better off having one. Unfortunately, Xia and his team’s findings don’t provide any hints.
And for us, we would’ve maybe had the opportunity to still hang from trees after a long day at work.