I can just see how it all happened. The cave man was sitting in front of his kill. That great big, hairy beast. He had discovered fire so he had his dinner well done (because there were no pesky cooks to tell you medium rare was the only true way to eat red meat.) There was nothing left but the pelt and the bones.
And he plucked some of the long hair from the beast, looked over and saw two leg bones (OK, they could have been arm bones) said to himself, "Hmmm, I wonder." (Whatever the cave man equivalent of that is.) And before you know it, he was the fashion plate of the cave condo community.
I'm telling ya; that's how it happened.