For the first few billion years the earth was just a barren rock.
Then microbes appeared, they broke down the rocks and made soil which led to plants flourishing.
Next came animals that ate the plants which were full of microbes so some microbes ended up in the guts of these animals
These microbes digested the animal’s food, manufacturing the spectrum of complex chemicals the animals needed and regulating appetite, sending out specific hormones to make the animals hungry f when they were low in a particular food and yet other hormones so they would stop eating when full.