Here's why:
* Humans are mammals, which are vertebrates within Chordata.
* Snakes are reptiles, also vertebrates within Chordata.
* Sharks are cartilaginous fish, making them vertebrates within Chordata.
All members of Chordata share these key characteristics at some point in their development:
* Notochord: A flexible rod that provides support.
* Dorsal hollow nerve cord: A tube of nerve tissue along the back.
* Pharyngeal slits: Openings in the throat area, used for filter feeding in some species.
* Post-anal tail: A tail extending beyond the anus.