How human beings lost their tails

For half a billion years or so, our ancestors sprouted tails. As 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 recognised this change in our ancient anatomy. But how and why it happened has remained a mystery.

Now a team of scientists in New York say 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.