Homologous Structures

A human hand, a whale flipper, a bat wing, and a horse leg all do very different jobs, yet underneath they share the same set of bones in the same arrangement.^[1] A designer starting fresh wouldn't reuse one blueprint for grasping, swimming, and flying — but inheritance from a common ancestor, with each lineage tweaking the parts it already had, explains it exactly. The same logic covers vestigial structures — leftover parts like the human tailbone — which are puzzling as design but expected as leftovers from an ancestor that used them.^[2]