Explained: How human beings lost their tails

 Charles Darwin first recognized this change in our ancient anatomy. But how and why it happened has remained a mystery

For half a billion years or so, our ancestors sprouted tails. Like fish, they used their tails to swim through the Cam­brian seas. Much later, when they evolved into primates, their tails helped them stay balanced as they raced from branch to branch through Eocene jungles. But then, roughly 25 million years ago, the tails disappeared.

Charles Darwin first recognized this change in our ancient anatomy. But how and why it happened has remained a mystery.

Now a team of scientists in New York says they have pinpointed the genetic mutation that may have erased our tails. When the scientists made this genetic tweak in mice, the animals didn’t grow tails, according to a new study that was posted online last week.

This dramatic anatomical change had a profound imp­act on our evolution. Our an­cestors’ tail muscles evolved into a hammock-like mesh across the pelvis. When the ancestors of humans stood up and walked on two legs a few million years ago, that muscu­lar hammock was ready to support the weight of upright organs.

Although it’s impossible to definitively prove that this mutation lopped off our anc­es­tors’ tails, “it’s as close to a smoking gun as one could hope for,” said Cedric Fesc­h­otte, a geneticist at Cornell not involved in the study.