The antonyms for the word "more well developed" are less advanced, less evolved, less mature, less improved, less grown, less progressed, less refined, less sophisticated, and less complex. These words suggest a lack of progress or a reduced level of achievement in a particular context. For instance, if we refer to a less-evolved species, we mean it is less complex or less advanced than its counterpart. Similarly, less-refined or less-sophisticated clothes, manners or ideas suggest a lack of growth or development. Thus, antonyms of 'more well developed' reveal the limitations or drawbacks in a noun or idea.