In the past, the idea of optimization for some sophisticated purpose seemed to be the only conceivable explanation for the level of complexity that is seen in many biological systems. But with the discovery in this book that it takes only a simple program to produce behavior of great complexity, a quite different—and ultimately much more predictive—kind of explanation immediately becomes possible.

In the course of biological evolution random mutations will in effect cause a whole sequence of programs to be tried. And the point is that from what we have discovered in this book, we now know that it is almost inevitable that a fair fraction of these programs will yield complex behavior.

Some programs will presumably lead to organisms that are more successful than others, and natural selection will cause these programs eventually to dominate. But in most cases I strongly suspect that it is comparatively coarse features that tend to determine the success of an organism—not all the details of any complex behavior that may occur.

Thus in a very simple case it is easy to imagine for example that an organism might be more likely to go unnoticed by its predators, and thus survive and be more successful, if its skin was a mixture of brown and white, rather than, say, uniformly bright orange. But it could then be that most programs which yield any mixture of colors also happen to be such that they make the colors occur in a highly complex pattern.

And if this is so, then in the course of random mutation, the chances are that the first program encountered that is successful enough to survive will also, quite coincidentally, exhibit complex behavior.

On the basis of traditional biological thinking one would tend to assume that whatever complexity one saw must in the end be carefully crafted to satisfy some elaborate set of constraints. But what I believe instead is that the vast majority of the complexity we see in biological systems actually has its origin in the purely abstract fact that among randomly chosen programs many give rise to complex behavior.

In the past it tends to have been implicitly assumed that to get substantial complexity in a biological system must somehow be fundamentally very difficult. But from the discoveries in this book I have come to the conclusion that instead it is actually rather easy.

So how can one tell if this is really the case?