Tony Sidaway discusses a unifying property of theistic evolutionists: the desire or need for there to be some kind of universal plan for their existence. It’s not an attitude I understand very well; I don’t think it makes life better to believe that there is some ineffable teleological intent behind the events in your life, and no one ever bothers to explain why it would be preferable to be a pawn to a cosmic puppetmaster. Their reasoning also tends to be incredibly bad, as can be seen in the article by Mark Vernon that inspired Tony’s musings.

The work of Conway Morris, and now many others, is showing that evolution keeps coming up with the same solutions to natural problems. One of the better-known examples is that sabre-toothed cats. They evolved on at least three different occasions along independent Darwinian paths. And yet they look almost exactly the same. Dozens of examples of convergence have now been documented across a wide variety of biological phenomena, from animal and plant physiology to molecular biology.

Convergence raises the possibility of directionality in evolution. This is anathema to the old school. Strictly speaking, even to talk of adaptations being advantageous is to risk a false sense of teleology. The sense of “advantage” only comes because we have hindsight. As Stephen Jay Gould put it: according to this interpretation of evolution, if you re-ran the “tape of life”, life would look very different.

Convergence challenges this, because in a way, evolution has already re-run the tape of life several times, and it looks strikingly similar.

The argument from convergence is wrong and makes no sense, yet somehow it appeals to smart people like Simon Conway Morris and Ken Miller, who have both made it themes in their books. Convergence occurs, of course, but “dozens of examples” is not very impressive and does not imply that this is a dominant mode of evolution. The examples also exhibit the constraints of contingency; yes, several mammals have evolved saber teeth, which seem to be tools for a particular kind of predation that involves deep tearing to induce bleeding in prey. If we get away from mammals, though, it doesn’t appear very often, if at all. Raptors, for instance, probably used an overdeveloped claw in the same way. Convergence is often a consequence of limitations in anatomy and physiology that make a narrower range of solutions to common problems available.

Another good example is the eye. Eyes have independently evolved multiple times, and we do see examples of convergence — molluscs and vertebrates have simple camera eyes that are not related by ancestry. It’s not because of some master plan, however, but because using a lens to focus light on a sheet of photoreceptive cells is a simple, easily evolved strategy for putting an image on a neuronal array. This is a case where physics itself imposes some limitations on how a receptor organ can function. At the same time, though, life explores a wider set of solutions than we can imagine. Mollusc and vertebrate eyes differ in all the details of their development and anatomy, and obviously enough, other organisms, such as arthropods, have put together radically different solutions with compound eyes. Did a god have a plan that involved eyes forming as orbs with single lenses? Why? And does that make dragonflies satanic, for defying the plan?

Vernon is also completely wrong. The tape of life has not been replayed, except in a small scale and with historical limitations. You could argue, I suppose, that the Permian and Cretaceous extinctions represented a catastrophic rewinding of life’s tape for large terrestrial animals, but do note that each produced different solutions. Dinosaurs became ascendant (in a megafauna sense) after the Permian, but very different vertebrates took over after the Cretaceous.

It’s all very peculiar. This particular breed of teleologist seizes upon small functional similarities in organisms, tooth size or body shape or color pattern, and declares that because two species independently generate similar solutions to common problems, it must be because there is a guiding force producing these solutions. They want the guiding force to be a deity, but unfortunately, Darwin long ago identified the force as short-term local adaptation to environmental forces, nothing more, no grand planner, no deep purpose, and these instances of convergence provide no evidence otherwise.

There must be some psychological need in the teleologists that I lack. I don’t feel any a priori requirement that complexity and adaptation and similar solutions must be driven by any kind of master blueprint, and I find any kind of deterministic explanation for earth’s history to be personally horrifying (not that that is an obstacle to such explanations being true, but it does confuse me that some people think such an answer to be desirable).

We are each our own individual engines of purpose, operating in a hostile universe where randomness can shape our fates. There is no grand scheme behind our existence, other than the same function that all our ancestors had: to order our local environment to allow each to survive and to make the world a little better for our progeny. And that’s enough — that’s all that is needed to make a rich, diverse, living planet, and it’s all I need to live a satisfying life.