Music stimulates emotion, too, but it typically takes an entire symphony or song to make anyone burst into tears. But fragrance, he points out, can do it to you with a single odorant, by the smell's associations, and the conscious mind need not be involved. Say the smell is purple petunias, which have a rich spiciness no other petunia has, and say your mother died when you were 3. You wouldn't need to identify the smell, or to have conscious memories of your mother or her garden, to feel sad when that sweet tang drifts up to the porch. Seemingly so, even if they lack a nose.
Moths have chemical receptors on their antennae, flies on their feet.
Even bacteria will cluster around one chemical and avoid others. Salmon and trout find their way back to their original spawning areas by smell. Smell in that broad sense is thought to be ancient, because its fundamental structures are similar in species all the way from fish to moths to primates. A rat has some 10 million, a rabbit 20 million, and a bloodhound 100 million. In a mouse, it's a little bigger.