They have different coat colors, sure. But they all have the same baby heads. They’re round, and they don’t elongate as the animal matures, which is the standard developmental pattern for mammals. Dogs have short, round faces as puppies but long, snouty faces as adults. An adult cat looks pretty much like a baby cat but bigger. With dogs, breeders play off of that developmental variation to create breeds with different face shapes. But because cats don’t have that developmental variation, there isn’t much to play around with other than coat color.

This all goes back to the fact that cats are extremely specialized. Every member of the order of mammals known as the carnivorans, including cats and dogs, has an upper fourth premolar and a lower first molar that form what we call the slicing pair, which slices meat. A lot of carnivorans retain molars behind the slicing pair that can grind up stuff such as vegetation. But cats have lost pretty much everything behind their slicing teeth. They might have a little nub, a peg tooth, but it can’t process stuff. This is why foxes are perfectly happy going through garbage, whereas leopards will kill livestock instead.

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Cats have nailed this one thing so well that they all do it and just come up with slightly different sizes. That’s why they’re perfect, evolutionarily. They don’t need variation. They might get bigger or smaller, but they don’t change anything else because they’re doing it just right otherwise. They’re not jacks-of-all-trades; they’re masters of one.

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There are lots of things that have tried to be cats—other groups of mammals that have evolutionarily converged on cats. Marsupials have tried to be cats. An extinct group of carnivorans called creodonts have tried to be cats. Weasels have tried to be cats. There’s all kinds of stuff that has tried to be a bit catlike in different ways—mongooses, things like that. But they kind of dip in and dip out of being cats, and they can’t really outcompete cats in their space. They haven’t lasted. All of those things that have tried to be cats, they do other things, too, and those things are fine. But there aren’t a lot of things that are around today that do a very good job of being a cat.... You can’t just casually try to be a cat. You have to commit. Cats have committed to being cats. Everything else is just sort of dabbling, and it doesn’t work.