The New York Times, September 21, 2021
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 recognized this change in our ancient anatomy. But how and why it happened has remained a mystery.