Complex (biological) organisms have complex internal structures; all sorts of cells, proteins, amino-acids, neurons, a gazillion things working together. Simpler ones have simpler bodies and other internal mechanisms. We also have single-celled life forms.
In the same way, I feel, the more complex a life, as in full of complications and problems and troubles, the more wholesome it is, for lack of a better word that is evading me right now; or more “higher”. It’s a “higher” form of life, for it’s giving us (soul) beings more opportunities to experience (different things) and learn and grow and evolve; which is why we have come here in the first place.
That’s one way to look at it.