The story of how apes (and humans) lost their tails!

How a ‘jumping gene’ caused apes (and humans) to quit monkeying around and ditch their tails.

The story of how apes (and humans) lost their tails!
Xia B et al. 2024. On the genetic basis of tail-loss evolution in humans and apes. Nature. DOI: 10.1038/s41586-024-07095-8
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You may have noticed that most animals have tails.

There are a few exceptions, like gorillas, chimps, orangutans, and humans!

This is kind of weird because other primates definitely have tails.

And you’re probably not alone if you’ve ever jealously watched a monkey swing from tree to tree with the assistance of their tail.

The fossil record tells us that we (apes) lost that 5th appendage approximately 25 million years ago.

And this likely coincided with when we switched from living in an arboreal habitat (trees) to one a little closer to terra firma (the ground).

Evolutionarily, this makes sense!

Why waste energy maintaining something you don’t use very often?

And that’s exactly how evolution by natural selection happens sometimes…if you don’t use it, you lose it!

But the physical disappearance of a tail also requires changes on the molecular level.

While previous work has implicated around 100 genes in tail development, we weren’t totally certain how tail-loss occurred in apes.

However, an accident reminded Boa Xia, first author on today’s paper, that humans still have a little nubbin’ where our tails used to be.

So, he set out to figure out why we (and other apes) only have a tail bone and not the full blown thing!

In the figure above you can see this displayed in A) which provides an overview of the presence or absence of tails in the primate family tree.

But Xia’s big discovery came when he looked at a gene, TBXT, in a genome browser (B) and he noticed that Apes have an extra sequence between exons 6 and 7 that monkeys don’t have!

This sequence was a primate specific Alu element.

Alu’s are little bits of DNA that were left behind by ancient retroviruses as they jumped in and out of our genomes.

We have about a million of these sprinkled throughout our genes and they are the most common ‘transposable element’ found in the primate genome.

They’re broadly implicated in promoting evolutionary change and the Alu element, AluY, in TBXT is no exception!

It appears that AluY collaborates with another primate Alu element, AluSx1, that’s conserved in all primates.

Panel C shows how this works:

AluY and AluSx1 bind to form a loop that excludes exon 6 during splicing and this creates a TBXT transcript that’s sometimes a bit shorter in Apes.

This exon skip liberated us from our tails!

Experiments in the paper show that removal of exon 6 in one copy of TBXT produces a range of tail lengths in mice, and with a little genetic tweaking of the other copy, the authors could produce mice that no longer had tails!

But complete removal of exon 6 in both copies of TBXT caused lethal spinal cord defects, underscoring the delicate nature of tail-loss evolution.

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