Different materials produce unique qualities in a fire. It's likely that early man noticed these traits and began producing fires for the sake of smell or effect or look. I know that in the Urals there's a tradition of smoking hemp which has been placed into a communal fire in the center. In Northern Canada, descendants of the red clay peoples will throw sacred herbs on a fire in a sweat lodge and "smoke" that way. This is likely a close approximation of how the idea of smoking began, but of course it's almost completely divorced from how we smoke today. So, to answer your question, "by accident" is the likeliest answer.

I'm on mobile atm but I'll give some sources for you to check out when I get home. The