>>16700754Emotions allow for more general, nuanced responses to stimuli. The very smallest multicellular animals like this nemotode have dopamine and seratonin in their brains just like us.
The most basic emotions, beyond the simple pain/pleasure response, are excitement and depression. When a nematode finds food it's body releases dopamine which increases activity allowing it to exploit food. When it gets stuck in a hostile environment it becomes depressed and stops trying to flee.
Pain/pleasure responses aren't emotions at all. They are more like reflexes. Emotions affect a wide range of behaviors, they are basically "modes" of operation for animals. Without them we would stop reacting the instant the stimulus stopped.