Just wondering …

A thought occurred to me recently regarding evolution. Before posting anything about it, though, I wanted to make sure my understanding of evolution was sound. I'm still not certain of that, but at least my understanding turned out to coincide with what Wikipedia has to say on the matter: "Evolution by means of natural selection is the process by which traits that enhance survival and reproduction become more common in successive generations of a population. … The central concept of natural selection is the evolutionary fitness of an organism. Fitness is measured by an organism's ability to survive and reproduce, which determines the size of its genetic contribution to the next generation."

Fitness and survival constitute the alternating current of genetic transmission. This, however, makes me wonder on what grounds life progressed beyond unicellular plants and animals. Those are still with us, and have been here from the start. They are obviously fit, and just as obviously have solved the survival problem. Indeed, given that they reproduce by division into two, they could be said to have solved the problem of immortality as well. 

Fitness and survival do not seem to necessitate further development. Is there a biochemical principle of complexification? It would seem the theory of evolution — at least if understood in a completely materialistic manner — seems woefully incomplete without providing a raison d'être for complex life forms. Wikipedia says that "evolution has no long-term goal and does not necessarily produce greater complexity. Although complex species have evolved, they occur as a side effect of the overall number of organisms increasing …." I don't get this. I don't see how an increase in the number of simple life forms makes the development of complex life forms even likely, let alone necessary. There ought to be a better explanation than that.