A study published online Sunday by Nature says it's a bit of both.
The stemcells in birds that normally produce the first digit die off during early stages of embryonic development, it found, while cells programmed to manufacture the index unit give rise instead to a thumb-like appendage.
Member No. 2, in other words, has undergone a shift in digital identity.
All four-legged animals with backbones -- vertebrates -- share an ancient template of five digits per limb. But that has not kept evolution from generating a numerically diverse menagerie for grasping, clawing and walking.
Human and primate hands and feet normally have five fingers or toes each; birds have three in their wings and two, three or four digits on their feet; two-toed sloths speak for themselves. Snakes shed their limbs entirely, while Pandas have five clawed fingers and a sixth bigtoe-like appendage, the better to grasp bamboo stalks while dining.
In general, it is easier to lose a trait through evolution than