A tip-off that evolution has been at work is the commonness of variations on a theme. Time and again we see a basic plan jury-rigged for different functions, rather than being designed from scratch, the way a competent engineer would do it. And sometimes we even see variations on the same theme reaching the same functional endpoint, but by obviously different routes.
Three of the four creatures that have managed to evolve flight have
done so by using forelimbs built on the same basic plan. The pterosaurs, extinct flying
reptiles, did it by stretching a membrane from an elongated fourth finger to ankle; the
bats by stretching membranes between fingers and to the ankle; the birds went a third
route by reducing the hand and depending on feathers for the flight surface. Yet, all
three wing types are easily recognizable as based on the same theme as your and my
arms. On the other hand, the insects, evolving independently from these three related
vertebrates, formed wings from the body wall—a theme belonging to an entirely different
tune.
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.