Millions of Years Ago, Our Ancestors Had Tails — But We Can Thank a Gene Mutation for Why We Don't


Daisy Hernandez
Mon, October 18, 2021

Photo credit: Daniel Day - Getty Images

A new study seems to have found the reason why humans don’t have tails even though our ancestors did.

In their study, a group of researchers found that a mutation of the TBXT gene caused rat embryos to develop stunted tails or no tails at all.

The researchers hypothesize that 20 million years ago, a random human ancestor was struck by the TBXT gene mutation and passed the tailless trait to its offspring for several generations. Eventually, humans evolved with this mutation which is why we don't have tails.

Millions of years ago, our ancestors had tails. So, why don’t we?

The short answer, of course, is we lost the ability and need to grow tails thanks to evolution. The longer, more accurate explanation is one that scientists have been working to figure out and now we might finally have an answer: genetic mutation.

In a new study, New York-based researchers theorize that the mutation was mediated by the addition of a short segment of DNA—known as an Alu element—and is the reason why humans and apes do not have tails but monkeys do. In fact, the question of why humans lack tails has plagued Bo Xia, an NYU Grossman School of Medicine stem cell biology graduate student, since he was a child, he told the New York Times.

In an effort to find some answers, Xia studied embryo development with a particular focus on which genes were activated and which were turned off at different points during the growth within the womb. He also analyzed tail development in other animals and compared the DNA of tailless apes to monkeys with tails.

Because scientists have previously discovered over 30 genes responsible for tail development in various animals—including Manx cats who famously have no tail or a small nub for a tail caused by a genetic mutation—Xia theorized that the same had happened to our ancestors, and eventually, us.

Photo credit: Angie Selman / EyeEm - Getty Images

After comparing the apes and monkeys, Xia made an exciting discovery: a mutation in the TBXT gene was evident in humans and apes but not seen in monkeys. To test the theory that this gene was the genetic mechanism responsible for tail loss in humans, Xia and his team genetically modified mice embryos to see what would happen.

They found that the addition of the TBXT gene resulted in some mice having no tails while others developed short, stubby tails.

This led Xia and colleagues to hypothesize that 20 million years ago, a random gene mutation in an ape caused it to either develop a shortened nub of a tail or no tail at all. That ape then passed the trait down to its offspring and the mutation continued to rapidly make its way down several genealogies until it reached us.

Still, the question remains as to why the loss of a tail was beneficial to our ancestors. Did tails get in the way more than they helped? Plus, there was the increased risk of developing neural tube defects (NTD) which could adversely affect the brain and spine. Developing NTDs is still a concern during pregnancy which is why the CDC recommends “all women of reproductive age ... get 400 micrograms (mcg) of folic acid every day” in an effort to prevent the defects.

We may not yet know what the advantage was to losing our tails, but further research may one day be able to pinpoint the answer.