"Vestigial structures are entirely consistent with intelligent design, suggesting structures that were initially designed but then lost their function through accident or disuse. Nevertheless, vestigial structures also provide evidence for a limited form of evolution. From both a design-theoretic and an evolutionary perspective, a vestigial structure is one that started out functional but then lost its function. Yet, in the case of evolution, vestigiality explains only the loss of function and not its origination. Vestigiality at best documents a degenerative form of evolution in which preexisting functional structures change and lose their function."
Chapter 5, The Design of Life
So much for the mysterious vestigial structures...
