Final answer:
Scientists likely used fossils of ancient penguins' wings and feathers to conclude that penguins evolved adaptations for powerful swimming early in their evolutionary history.
Step-by-step explanation:
When examining the evolutionary history of penguins as powerful swimmers, scientists have likely used fossils showing the shape of ancient penguins' wings and feathers. These fossils provide evidence of physical adaptations over time, reflecting changes in the penguins' environment and behavior. The wing and feather structures would be indicative of changes from flying to swimming, as seen with the transition to flippers that enable penguins to navigate aquatic environments efficiently. Furthermore, because fossils directly demonstrate past anatomical structures, they provide the most concrete evidence for understanding the historical adaptations of penguins for a life spent mostly in water.