Approximately 25 million years ago, an ancestor of both humans and apes genetically diverged from monkeys and lost its tail. No one had identified the genetic mutation responsible for this dramatic change in our physiology — until now.

In a new study published Wednesday (Feb. 28) in the journal Nature, researchers identified a unique DNA mutation that drove the loss of our ancestors’ tails. It’s located in the gene TBXT, which is known to be involved in tail length in tailed animals.

“Bo is really a genius because he looked at something that thousands of people, at least, must have looked at before — but he saw something different,” said Itai Yanai, scientific director of the Applied Bioinformatics Laboratories at NYU Langone Health and a senior author of the study.