More Vestigial Features

Make a fist and bend your wrist toward you. If a thin cord pops up in the middle of your inner wrist, that is your palmaris longus tendon. A meaningful share of people are simply born without it, on one or both arms, and notice no difference at all.

It once helped tree-climbing ancestors grip branches. Today it does so little that surgeons treat it as spare parts, harvesting it for tendon grafts elsewhere in the body without any loss of strength.