Why did humans lose our tails? Blame a "jumping gene"

 

Most animals have tails, including almost all mammals. For some reason, we humans don’t. This difference has been the source of much speculation among scientists over the years, and many arguments have been made about why we don’t tails.

One line of reasoning goes like this: tails are very useful for animals that live in trees, but once our ancestors came down from the trees and started living on the open plain, they didn’t need those tails any more. But why lose them? Lots of animals don’t live in trees, and they still have tails.

Even among the primates, most species have tails, but chimpanzees, gorillas, orangutans, and bonobos–the great apes–don’t. In fact, one way to tell great apes apart from other apes is by the presence of a tail. We humans are simply great apes without so much hair. Or, as the English scientist Desmond Morris called us in his famous book, humans are “The Naked Ape.”

So why am I writing about this now? Well, in a newly published article in Nature, a group of scientists from NYU, led by Itai Yanai and Jef Boeke, seem to have figured out what made us lose our tails. It’s all due to a piece of DNA that copies itself and jumps around our genome.

It’s a bit geeky, but stay with me and I’ll try to explain.

It seems that sometime around the divergence of the great apes from other primates, about 15-20 million years ago, a “jumping gene” popped into a gene called TBXT in our ancestor. (The B in TBXT stands for brachyury, which means “short tail.”)

The jumping gene here is just a piece of DNA a few hundred letters long*, not really a gene all by itself. But once that jumping gene got into TBXT, it was in just the right position to make the cells in our ancestor produce a shorter version of TBXT. The shortened gene was missing one of its pieces, but it still worked – well, sort of. Our ancestors managed just fine, but they lost their tails.

(Aside: the piece that’s chopped out is called exon 6, for those who really want to know.)

Given that this happened over 15 million years ago, how did the scientists prove their hypothesis? Well, other mammals have the same gene, but they make a longer version. So the authors of the new paper created a version of the TBXT gene in mice that included the jumping gene–and, as predicted, some of the mice lost their tails entirely.

Admittedly, this doesn’t exactly prove that one jumping gene caused us to lose our tails. Without a time machine to take us back 15 million years (with a DNA sequencing machine in tow), we can’t truly prove what happened eons ago. But it’s a compelling story, because we know that our genomes, and those of other great apes, have this unique jumping gene that other mammals lack.

So now we know how we lost our tails. We still don’t know exactly why, though. Some scientists speculate that being tail-less might have helped us to walk upright, or that it might have been better to lose the tails once our ancestors stopped living in trees.

On the other hand, guinea pigs don’t have tails either, and they don’t walk on two legs. And koalas don’t have tails, even though they live in trees. Some of these questions may just have to remain a mystery.

*Technically, the jumping genes in this story are called Alu elements, and they occur all over our genome. Famed geneticist Haig Kazazian, a former Hopkins colleague who passed away just two years ago, explained in a 2004 paper that Alus are a form of “nonautonomous retrotransposon.”