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Analogy: When is a thumb a thumb?

Through careful study, biologist can also identify analogies. For example, panda bears have a "thumb" on their hands. They use this "thumb" to hold onto bamboo as they eat. Is the panda's "thumb" homologous or analogous to the thumb on your own hand?

panda grasping bamboo

Studying the anatomy of panda hands and human hands shows that these "thumbs" must be analogous. When you look at the bones of each, you see that the thumbs are not very similar at all! The human thumb has joints and is made of many bones. The panda thumb is just one bone sticking out of the side of the hand.

diagrams of human and panda hands

Furthermore, the panda thumb is the sixth "finger" on its hand! If you watched the hand of a baby panda grow, you would see that the "thumb" develops from a wrist bone. The panda thumb and the human thumb don't grow from the same bones. This is more evidence that they are analogous structures. In fact, the panda thumb is homologous to a wrist bone in humans, and the human thumb is homologous to the first finger in pandas!

diagrams of human and panda hands, highlighting corresponding bones



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Panda photo courtesy of John White