There was an entirely new system discovered in the human body thanks to marijuana.
Its scientific name is the endocannabinoid system and it is found in the human brain. It just so conveniently happens that the chemicals found in weed is able to exclusively target the system. In fact, it was down to our understanding of the chemicals found in weed that scientists were able to discover the system back in the mid 1990s. The system plays a part in learning, appetite, pain and a whole lot of other things, too.