system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do different animal species have similar anatomical structures?
gpt
Similar anatomical structures may be due to homology, which means the structure was inherited from a common ancestor that already had that structure.  Humans and sharks both have hearts and two eyes because their last common ancestor did.

Similar anatomical structures may also be due to  homoplasy, which means the structures independently evolved in each lineage.  Hippos and alligators both have eyes and nostrils on the top of their head, independent adaptations to living mostly submerged in water.