Does evolution of a single body part suggest that there is a single best design? Why or Why not?

Answer 1

No, since a given body part is adapted for different uses and will have radically different forms in different species. Bat wings, horse forelegs, whale flippers, cat paws, and human hands all began from the same point (and relatively recently; they're all mammals) but are adapted for flight, running, swimming, hunting, and gathering.