equipped not only with
a bone to give it strength but also with muscles to sustain its agility. As with
the radial sesamoid, these muscles did not arise de novo; like the parts
of Darwin's orchids, they are familiar bits of anatomy remodeled for a new function.
The abductor of the radial sesamoid (the muscle that pulls it away from the true
digits) bears the formidable name abductor pollicis longus ("the long
abductor of the thumb"--pollicis is the genitive of pollex,
Latin for "thumb"). Its name is a giveaway. In other carnivores,
this muscle attaches to the first digit, or true thumb. Does
the anatomy of other carnivores give us any clue to the origin of this odd
arrangement in pandas? Davis points out that ordinary bears and raccoons, the
closest relatives of giant pandas, far surpass all other carnivores in using their
forelegs for manipulating objects in feeding. Pardon the backward metaphor, but
pandas, thanks to their ancestry, began with a leg up for evolving greater dexterity
in feeding. Moreover, ordinary bears already have a slightly enlarged radial sesamoid.
In most carnivores, the same muscles that move the radial
sesamoid in pandas attach exclusively to the base of the pollex, or true thumb.
But in ordinary bears, the long abductor muscle ends in two tendons: one inserts
into the base of the thumb as in most carnivores, but the other attaches to the
radial sesamoid. Two shorter muscles also attach, in part, to the radial sesamoid
in bears. "Thus," Davis concludes, "the musculature for operating
this remarkable new mechanism--functionally a new digit--required no intrinsic
change from conditions already present in the panda's closest relatives, the bears.
Furthermore, it appears that the whole sequence of events in the musculature follows
automatically from simple hypertrophy of the sesamoid bone."
Theradial thumb of pandas is a complex structure formed by marked enlargement
of a bone and an extensive rearrangement of musculature. Yet Davis argues that
the entire apparatus arose as a mechanical response to growth of the radial sesamoid
itself. Muscles shifted because the enlarged bone blocked them short of their
original sites. Moreover, Davis postulates |
