Cologne, Aftershave, Eau de Toilette, Eau de Parfum, etc.

[Bugbear]

The olfactory sense was actually the first sense developed in the biological world, except back then it was a simple primitive reflex of retraction or flight from certain noxious and potentially harmful chemicals or an attraction to other chemicals which might signal either food or mating. Virtually all multicelled species that precede humans in evolution have a far greater developed sense of smell than we possess. Olfactory sense is very primitive which preceded the closely related sense of taste, then tactile, auditory, visual senses which developed as the evolutionary advancement occurred into higher organisms. However, those olfactory senses still retain their primitive connections and relations to our more base and primitive neurological responses, and as well to stronger memories. This is why certain smells strike us with such visceral strength, and why we often have very strong memories stimulated by certain smells, in comparison to our other senses. Our senses developed evolutionarily first to assess our immediate nearby environment (initially chemical---smell then taste), followed by physical stimulus (tactile sense) with further sense development allowing higher organisms to evaluate their environment at ever greater distances from the primary organism (hearing and sight/light response). At the same time those later developed senses became more prominent and necessary in species survival, required more complicated neurological apparatus, and thus took over ever more and more of our brain capacity, at the detriment of the more primitive sense of smell, which becomes progressively less sensitive and less utilized in higher organisms (who has the better smeller, a dog or a human?). But those base primitive neurological connections are still there, which is why smells can evoke some pretty profound responses in most people (think how people react to manure, vomit, rotting or burnt flesh and you will get the idea).

Mr Science signing off.

Ya, what he said. Except, I would add kin selection, and perhaps the most powerful of all, sexual selection (Darwinian terms not mine) to this, which is probably a bit of what we are talking about in this thread, as being a driving factor in the olfactory evolution of the more complex organisms, as well as, some of the posts in the thread.

* Edit: looking again, ForresterModern wrote "food or mating," so he brought it up first.