I mean it's really silly to imply that creatures loose their legs because they are no longer needed. Well, we all know enough biophysics to recognize the abject silliness of that scenario. And yet I've had this exact discussion with educated, enlightened people when it comes to evolution. I just can't understand why it's such a mystery to so many. Thanks for the soap box, no charge for the lecture.
Evolution is a science.
In fact, Biology cannot presently be truly studied without incorporating evolution in almost every aspect of the endeavor. I'm sorry Mr. Carroll, but you've stepped on a pet peeve, here. Evolving an individual forward to its 'endpoint' is as silly as looking at a typewriter and a stack of paper and deducing the Foundation Trilogy. What happens over time is absolutely crucial. Technically, our evolution could take us right back to a creature indistinguishable from Australopithecus.