As the paleontologist Stephen Jay Gould observed in one of his most famous essays, the so-called thumbs of giant pandas (Ailuropoda melanoleuca) are nothing at all like the large digits on our own hands. The bear’s accessory thumbs – visible on the surface as a differentiated part of the pad on the “palm” of the hand – are modified sesamoid bones derived from the wrist. They are jury-rigged bits of anatomy which cast nature as an “excellent tinkerer, not a divine artificer.”

These highly-modified wrist bones are not unique to the black-and-white bears. Red pandas (Ailurus fulgens), which are much more closely related to raccoons than bears, also have modified sesamoids which they use to manipulate bamboo. The same