I think I might be able to elaborate a little bit on this. Sharks are a category of fish that are very interesting, biologically and systematically, because they give us some insight about how vertebrates evolved. At some point, the Chordata group joined the party raging party of early animal evolution and really kind of ruled the pool because they had a stabilizing proto-spine in their backs that gave them, as one might imagine, some serious survival advantages. Now, here's a great thing for phylogeny: How these groups looked can often be reconstructed easily, because they usually split into two or more sub-groups and very oftenly one of them doesn't really change for millions of years. And the subgroups of Chordata are the Acrania and the Vertebrata/Craniota. Now, you may know the second group, because those are the vertebrates, the animals that everyone knows amd cares about. As the name says, they have a spine and a cranium, a skull. Now, you can probably imagine why their sister group is called Acrania, because they don't have a cranium. And those didn't really change all that much, which is why we know pretty well about how the ancestors of vertebrates probably looked.

The same goes for the sub-groups of Vertebrata/Craniota. These guys split into Gnathostomata (gnathos: jaw, stoma: mouth; jaw-mouths, animals with a jaw) and Agnatha ('No-jaw-animals'). And again, we are the descendants of Gnathostomata, while the Agnatha pretty much stayed the same for millions of years. This is why we look at Agnatha to see how early vertebrates looked. Now, the Gnathostomata split into sub-groups again, the Chondrichtyes (chondros: cartilage, ichtyes: fish; cartilage-fish) and the Osteognathostomata (Osteos: bone; 'bone-jaw-mouth-animals'). Again, we are part of the latter, while the first group can be used to look at our ascendants of half a billion years ago. And again, that group didn't change all too much.

Now, after that wall of text, the interesting and relevant part is: Chondrichtyes are sharks. And they are, as you see, one of the oldest groups od vertebrates there are. They don't even have real bones (compared to their sister group, the osteo-blablabla), but only cartilage. This is interesting because it gives us an idea of not only how early vertebrates looked, but also of how bones evolved and the early stages of skeletal evolution.

We are looking and the history of how we evolved.

So, if you are interested in early vertebrate evolution, look at an Acrania, then at an Agnatha and then at a Chondrichtyes, that will give you an idea of how the early vertebrate evolution happened.