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Our textbooks
like to illustrate evolution with examples of optimal design�Xnearly
perfect mimicry of a dead leaf by a butterfly or of a poisonous
species by a [tasty] relative. But ideal design is a lousy
argument for evolution.... Odd arrangements and funny solutions
are the proof of evolution�Xpaths that a sensible God would
never tread but that a natural process, constrained by history,
follows perforce.... Which brings me to the giant panda and
its "thumb."
Giant
pandas are peculiar bears, members of the order Carnivora.
Conventional bears are the most omnivorous representatives
of their order, but pandas subsist... almost entirely on bamboo.
They live in dense forests of bamboo at high elevations in
the mountains of western China. There they sit, largely unthreatened
by predators, munching bamboo ten to twelve hours each day.
As a
childhood fan of Andy Panda, and former owner of a stuffed
[panda] toy won by some fluke when all the milk bottles actually
tumbled at the county fair,
I was delighted when the first fruits of our thaw with China
went beyond ping pong to the shipment of two pandas to the
Washington Zoo.
I went
and watched in appropriate awe. They yawned, stretched, and
ambled a bit, but they spent nearly all their time feeding
on their beloved bamboo. They sat upright and manipulated
the stalks with their forepaws, shedding the leaves and consuming
only the shoots.
I was
amazed by their dexterity and wondered how the [heir] of a
[lineage] adapted for running could use its hands so adroitly.
They held the stalks of bamboo in their paws and stripped
off the leaves by passing the stalks between an apparently
flexible thumb and the remaining fingers. This puzzled me.
I had learned that a dexterous, opposable thumb stood among
the hallmarks of human success. We had maintained, even exaggerated,
this important flexibility of our primate forebears, while
most mammals had sacrificed it in specializing their digits.
Carnivores run, stab, and scratch. My cat may manipulate me
psychologically, but he'll never type or play the piano.
So I counted
the panda's other digits and received an even greater surprise:
there were five, not four. Was the ''thumb'' a separately evolved
sixth finger?... D. Dwight Davis,...of... Chicago's Field Museum
of Natural History...had the answer....
The panda's
"thumb" is not, anatomically, a finger at all. It
is constructed from a bone called the radial sesamoid, normally
a small component of the wrist. In pandas, the radial sesamoid
is greatly enlarged and elongated until it almost equals [the
length of the first bones of the true fingers. It] underlies
a pad on the panda's forepaw; the five digits form the framework
of another pad.... A shallow furrow separates the two pads
and serves as a channelway for bamboo stalks.
The panda's
thumb comes equipped not only with a bone to give it strength
but also with muscles to sustain its agility.�� [This adaptation
did not arise by magic, but from parts of the organism already
in place. The panda��s opposable sixth digit arose from a wrist
bone already enlarged and supported by musculature found in
all bears and raccoons, the panda��s closest relatives.]
The panda's
true thumb is committed to another role, too specialized for
a different function to become an opposable, manipulating
digit. So the panda must use parts
on hand and settle for an enlarged wrist bone and a somewhat
clumsy, but quite workable, solution.��
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