Cannabis has probably been smoked by humans for more than 50,000 years as charas, aka finger hash.
The discovery of charas
It is likely that hunter-gatherers of Middle-Paleolithic Eurasia ate hemp seeds, harvesting them from the abundant, wild cannabis growing in warm and arid climates of the region.
Hunter-gatherers have to be opportunistic, and hemp is a superfood dense in nutrients. These ancient people almost certainly took advantage of this nutritional payload growing around them.
Tearing through the cannabis flowers to get at the nutritious seeds, these ancient foragers no doubt found the sticky oils and pollen of the plant coating their fingers. Hey, do you think we can use this stuff for anything? was definitely asked. Again, hunter-gatherers have to be opportunistic.
We will never know what kind of experiments were performed with the the cannabis residue: look guys, I made a perfume with it... I formed it into a little ball... how does it taste mixed into hot water?... Again, who knows.
Eventually, somebody dropped it into the campfire...
In those days, people lived really close to their fires and inhaled a lot of smoke (we know because the lungs of preserved bodies from that time, like Ötzi the Ice man, are filled with carbon from campfire smoke).
It's pretty safe to suggest, then, that people breathed in the cannabis smoke in this situation. Thus, the world was introduced to smoking charas, or finger hash, which is still made in parts of the world today (see image of modern charas below).
After being alerted to the psychotropic effects of cannabis, it was only a matter of time before people started intentionally spreading the seeds -- and knowledge of the effects of cannabis -- across the ancient world.
This is why some ethnobotanical historians suggest cannabis was one of the first plants ever to be cultivated by humans.
All photos public domain unless indicated