There is considerable evidence based on archeology that ancient peoples were intentionally fermenting grains and water in clay jugs as early as 10,000BC. No kidding. There's actually a wonderful documentary titled "How Beer Saved the World" that demonstrates that the quest for alcohol quite possibly inspired the neolithic revolution (based on archeological evidence of what exactly the first purposely planted crops were). The documentary then slippery-slopes that initial interest into the invention of the plough, domesticating animals, village like settlements, medicine being discovered as a result of fermentation, etc etc etc.

The first recorded use of cannabis for its pain relieving, intoxicating benefits is 2737 BC by Emperor Shen Neng in China.

To answer your question, though, alcohol was certainly discovered as an intoxicant first. Rain collected in jugs of grain gathered for food. Ancient people thought "hey we should drink this shit". They drank it, they liked it, they planted those grains as much as they possibly could.