A red panda (Ailurus fulgens, left, photographed at the Bronx Zoo) and a giant panda (Ailuropoda melanoleuca, right, photographed at the National Zoo).

As the paleontologist Stephen Jay Gould observed in one of his most famous essays, the thumbs of giant pandas (Ailuropoda melanoleuca) are nothing at all like the large digits on our own hands. Their 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.“

Surprisingly, however, these highly-modified wrist bones are not unique to the black-and-white bears. Red pandas