Various ways to achieve the same end is common in evolution. Thus one group of animals does something one way and another group a different way. But these ways are not scattered haphazardly among groups; instead, members of any group sharing many basic characteristics tend to do things the same way.
Take ankles, for example. In vertebrate animals, the foot has to be
able to move upon the lower limb. In ourselves and in other mammals, the movable joint
itself is between the lower leg bone, the tibia, and the astragalus, the
"top" bone of a number of bones that form the ankle. All the ankle bones,
then, lie lower than the joint. In birds, though, the upper ankle bones are fused to
the tibia and the lower ankle bones are fused to bones of the foot. The movable joint
lies in between—within the ankle itself, between the upper and lower ankle bones. Thus
birds are different from mammals in this feature as with so many others—but similar to
another group with whom they share many features—the dinosaurs.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.