Why don’t humans have tails? Scientists find answers in an unlikely place
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Humans have many wonderful qualities, but we lack something that’s a common feature among most animals with backbones: a tail. Exactly why that is has been something of a mystery.

Tails are useful for balance, propulsion, communication and defense against biting insects. However, humans and our closest primate relatives — the great apes — said farewell to tails about 25 million years ago, when the group split from Old World monkeys. The loss has long been associated with our transition to bipedalism, but little was known about the genetic factors that triggered primate taillessness.

Now, scientists have traced our tail loss to a short sequence of genetic code that is abundant in our genome but had been dismissed for decades as junk DNA, a sequence that seemingly serves no biological purpose. They identified the snippet, known as an Alu element, in the regulatory code of a gene associated with tail length called TBXT. Alu is also part of a class known as jumping genes, which are genetic sequences capable of switching their location in the genome and triggering or undoing mutations.