WASHINGTON (AP) — Our very ancient animal ancestors had tails. Why don’t we?

Somewhere around 20 million or 25 million years ago, when apes diverged from monkeys, our branch of the tree of life shed its tail. From Darwin’s time, scientists have wondered why — and how — this happened.

Now, researchers have identified at least one of the key genetic tweaks that led to this change.

“We found a single mutation in a very important gene,” said Bo Xia, a geneticist at the Broad Institute and co-author of a study published Wednesday in the journal Nature.

The researchers compared the genomes of six species of apes, including humans, and 15 species of monkeys with tails to pinpoint key differences between the groups. Once they identified a significant mutation, they tested their theory by using the gene-editing tool CRISPR to tweak the same spot in mouse embryos. Those mice were born without tails.