How are sharks equally related to bony fish and us?

This is one of my favorite evolutionary riddles.

The Chondrichthyes, encompassing sharks, skates, and rays, look like fish. They have gills and swim in the ocean like fish. We call them fish. So they must be fish, right?

Well, It depends on how you define fish. If you assign fish-dom based on things they all share, i.e. floppy slimy creatures who live underwater, breathe through gills, and swim with pairs of fins instead of limbs, then sure, sharks are fish and humans aren’t. However, if you define fish like we do in evolutionary biology, humans, dogs, tortoises, parrots, and sharks are friends, not food right alongside Dory and Nemo.

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