Anyone who has had either a cat or a dog that they loved and cared for would know that their animals smile, and not because it was "learned".
I currently have a cat named Stormcloud who smiles on a regular basis, at least now he does. My roomie saw him one afternoon walking with his head and tail down, not even paying attention to the fact that it was raining and he was soaked. She bought some cat food and had me put it out for him to find. A day or two later, he was tame enough (and full enough), that he would let me pet him. I asked my roomie if we could keep him (he was nothing more than fur and bones) and she said yes.
For the next couple of days, he refused to leave the kitchen. Instead, he curled up on the rug next to his food dish and watched it to make sure it didn't disappear. During that time we also got a kitten to keep him company, and teach him how to have fun again.
Now? Both cats smile on a regular basis. Sometimes when they are wrestling around with each other (no hard biting and they keep their claws sheathed), and other times, when Stormcloud is sitting on my lap and getting petted, he smiles at me.
I've had dogs that did the same thing. If they were happy and content, you would see them smile.