The worm, C. elegans, has had its DNA totally mapped. Its development from a single egg to each functioning cell has also be documented. This article, To Build a Worm: Where Undirected Evolution Runs into Severe Difficulties, shows us one such problem that cannot be solved by pure, unguided chance.
Thus, if we want to explain the origin of the adult form of C. elegans, we need first to explain the origin of the developmental pathway that constructs the worm. And this is where undirected evolution runs into severe difficulties.
The origin of the earliest stages of the pathway (shown in Figure 1 in previous post), where the major cell lineages start (which will eventually divide and give rise to the various structures and tissues of the adult worm), cannot be explained by natural selection -- because there is no selective advantage to those stages existing until the adult worm has been constructed.
But those later stages, especially reproductive capability (a necessary condition of natural selection), cannot themselves exist…