This chapter explains why the molecules that plants make are able to work in the human body. It is not by chance that these plant molecules fit into the slots in our brains, it is because we share a common ancestor and origins with plants, as well as insects: we are all eukaryotes. The genes we share define the core mechanisms for cell membrane signaling, including the gating mechanisms of neurotransmitters.
Plants are autotrophs meaning the can create new molecules from materials in their environment, and for a few hundred millions years, plants had no competitors. During this time they experienced a number of "DNA duplication events," where their DNA created nearly full DNA copy within themselves. This created a lot of "working but dormant" DNA which was then able to mutate over time and eventually create what we today call "secondary metabolites." Secondary metabolites are all the molecules that plants make that are not required from their normal growth, and the ones we are interested are the ones that alter our biology, such as caffeine.
Caffeine, for example, works similarly in bees to how it works in humans. It gives the bees a "buzz" and boosts their memory so they can remember where they got that enjoyable feeling so they will return to pollinate the plant again. At the same time, caffeine is used in other areas of the plant in higher doses to acts as a pesticide against herbivore predator insects. The reason caffeine works as a stimulant in the brain has to do with its shape.
This chapter helps lay the foundation for why plant molecules work in the human brain so that we can understand the way other molecules might work in the brain, and maybe understand how we can use molecules to make ourselves better.