I'm assuming it started small and simple (perhaps just intentionally dropping seeds in a location and hoping for the best) and then problem-solving for higher yields is what led to the great amount of work in the end. It's like how making a weapon can range from simply picking up a stick to refining and material science to mass production methods (to outfit an entire army), all the way up to splitting the atom. There's a wide gulf between wanting to have some extra food growing in a convenient location and wanting to feed an entire village throughout the entire year solely on cultivated food.