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About Those Thumbs: Pandas

"The Other Panda's Thumb" and "The Panda's Peculiar Thumb" Were essays by Stephen Jay Gould published in Natural History in 1978. He was an amazing writer. Stephen Jay Gould died of cancer. He was a Harvard professor, the recipient of over a hundred honorary degrees, and a member of the National Academy of Sciences. In 1981 he was a witness for the ACLU in a successful fight in Arkansas to push back a Creationist law. A law that insisted creationism be taught and taken literally alongside evolution.

Gould said that Darwin himself spent a lot of effort uncovering the strange contortions by which organs change shape and took on new functions. And to Gould, the thumb of the giant panda was the epitome of a funny solution. Before a giant panda eats a piece of bamboo it grabs the shoot between its flexible thumb and finger and strips off the leaves. But this useful thumb is not really a thumb at all, at least in our sense of the word. Pandas descend from carnivorous mammal ancestors, as reflected in the many traits they share with bears, dogs, and other relatives. For one thing, its true thumb is lined up with its other fingers. What looks like its "thumb" is actually a wrist bone (the sesamoid) that evolved until it was so big that it stuck out to one side of its forepaw. The muscles that control the bone have become rearranged so that now it can move much like our own opposable thumb. Gould pointed out that the corresponding bone in the panda ankle is somewhat oversized, which he suggested was the result of genes that controlled the growth of the sesamoid in all its limbs. The large size of its anklebones serves no function. Instead they're merely the by-products of natural selection acting on other parts of the panda body. Scientists who study the evolution of optimal design don't deny that evolution may be optimizing things with weird origins and which may have once had other uses. Of course, it's largely thanks to Gould that the views on adaptation are widening.

"The Panda's Peculiar Thumb": the red panda was not included in the essay. This bushy-tailed creature is the size of a small dog and climbs in trees. Found in East Asia, it eats bamboo, along with lichen, acorns, and even bird eggs. It earned its name from the similarities it bore to giant pandas. Those similarities include a false thumb that is actually an exaggerated sesamoid bone in the wrist.

More detailed studies raised doubts about whether the two pandas were closely related. So, for a long time scientists had a difficult time determining their kinship. The picture has cleared up dramatically in the past few years thanks to large-scale studies of mammal DNA. These studies indicate that giant pandas and red pandas are only distantly related. Their common ancestor lived 40 million years ago. One lineage gave rise to bears, including giant pandas. Another lineage gave rise to red pandas as well as skunks, raccoons, and weasels.

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