The Jumping Gene That Helped Snip Our Tails

For decades, scientists have wondered why humans don’t have tails.

A team of researchers from New York and Harvard universities traced the answer to a mutation in a key gene.

What researchers have learned

In short, they’ve decoded the genetic mechanism behind our taillessness.

Tails were a defining feature of most vertebrates for more than half a billion years. They began disappearing about 25 million years ago, possibly giving our ancestors an advantage as they moved from trees down to the ground. Tails helped with life in the branches, but on land that function became less important.

Overall, tail development across vertebrates involves more than 100 genes. The authors suggest humans lost tails because of changes to one or more of those genes in the DNA.

The team identified the TBXT gene as the key player. TBXT helps determine tail length in many animals. When a small piece of DNA called AluY inserted into TBXT, tails disappeared.

AluY fragments are known as “jumping genes” or mobile elements — bits of DNA that can copy themselves and insert into other places in the genome. They can also affect how nearby genes are regulated and expressed, the Daily Mail reported.

How the researchers proved it

The team found two AluY elements in the TBXT gene of primates, which led them to hypothesize that AluY inserted itself into the gene tens of millions of years ago.

To test that idea, researchers introduced the jumping gene into 63 mice. The offspring had shorter tails or no tails at all.

The AluY insertion stayed in the same spot in TBXT and produced two RNA variants of the gene. One of those variants appears to cause tail loss.

Researchers say gorillas, chimpanzees, and early humans lost their tails after splitting from Old World monkeys. After that evolutionary split, fewer tail vertebrae formed, eventually giving rise to the coccyx.

Charles Darwin noted anatomical changes in our ancestors in The Descent of Man (1871), but he was not convinced that the coccyx was a true leftover tail.

“Our research explains how evolution removed our tails. This question has intrigued me since my youth,” said lead author Bo Xia.

The results of the study were published in the journal Nature.