Inside story on the panda's thumb.

The panda's grasp is "one of the most extraordinary manipulation systems in mammalian evolution," according to Hideki Endo of the National Science Museum in Tokyo and his colleagues. They've examined the panda's opposable digit with magnetic resonance imaging and other advanced medical-imaging techniques.

Pandas handle their food with considerable dexterity, in part because one of their paw bones has evolved a padded spur that works like a thumb. Their five regular, clawed digits can pinch an object against the spur, as a person's four fingers press an object against the thumb. But while a human thumb is, in essence, a finger jutting out at an odd angle, evolution has jerry-rigged the panda's spur from bones unrelated to our thumbs. As Stephen Jay Gould described it in The Panda's Thumb (1992, W.W. Norton and Co.), the panda's so-called pseudo-thumb represents an outgrowth of the sesamoid, analogous to one of the bones below the fingers in our hands.

The pseudo-thumb can't move by itself, Endo's team learned more recently. Yet when the other bones in the hand curl into a grasping position, they reposition the pseudo-thumb. In its, new position it pokes out at just the right place and angle to provide a brace for a pincer grip.