For some people, evolution is strangely hard to wrap their minds around. One key difficulty may be resolved by exaptation, well explained in an article which appeared in The Atlantic. Salient quotes:
The turtle’s shell, then, is a wonderful example of exaptation—the evolutionary process where a trait evolves for one function and is then co-opted to serve another. They began as digging platforms and then became suits of armor. Feathers are another example. They now help birds to fly, but they probably originated as ways of keeping warm or signaling to mates and rivals.
“A change in a structure of the body can only provide a selective advantage based on its current abilities, not potential future ones,” says Cebra-Thomas. “That’s very important, and not just for understanding the evolution of turtles.”
Read the whole article here.