Just as a biplane and a jet share common features of wings, body, tires, engine, controls, etc.,... does not require that one must have evolved from the other naturally, without a maker. They argue it's more reasonable to conclude that similar design indicates a common, intelligent designer. An architect typically uses similar building materials for numerous buildings, and a car manufacturer commonly uses the same parts in various models. So if creation had a common designer, we could expect to find a similar "blueprint” used in many different creatures.