There are two things that human beings, since the time of homo erectus, have always had close at hand. One is fire, which we need to keep warm and to cook our food.

The other is plants, which humans have always needed for our sustenance. People in all cultures have manipulated plants to release their alimentary and medicinal properties: chewing them, drying them, rubbing them, boiling them, combining them.

Given the availability of fire, it was inevitable that humans would discover that burning can sometimes release fragrant and therapeutic chemical agents locked up in a plant - i.e., there are secondary metabolites that can be used as flavourings, medicines, or ceremonial drugs.

According to Pennacchio, Jefferson, and Havens, Uses and Abuses of Plant-Derived Smoke: Its Ethnobotany as Hallucinogen, Perfume, Incense, and Medicine, plant-derived smoke may have been used as far back as 1.6 million years ago, and human beings have made use of the smoke of some 1,460 plants, in virtually every part of the world.

Many of these uses are external, but given the effectiveness of inhalation in allowing chemicals to enter the blood stream, it makes sense to smoke them directly.