Passage Summary: Some scientists thought land-walking limbs evolved after animals moved to land, but a newly discovered ancestor had those limbs while still living entirely underwater.

Reasoning: The discovery of Acanthostega shows that a creature with land-appropriate limbs was actually an aquatic animal that couldn't survive on land.

Analysis: We are looking for a statement that follows logically from the fact that an exclusively aquatic ancestor already possessed 'land-walking' features. This directly contradicts the assumption that these features evolved only after animals moved to land. Therefore, the most supported inference is that rotating limbs and digits likely evolved while these ancestors were still living in the water. It suggests that these traits weren't originally 'designed' for land movement, even if they later became useful for it.