I was emailed by a reader of the site with a question about evolution. Specifically, she wanted to be able to answer challenges about how the eye could have possibly evolved. I think my email to her might also be useful to other readers, so I’ve adapted it a bit for the blog. I hope you find it helpful.
How can a thing as complex as the eye have evolved by successive steps? If you take away certain parts, then it doesn’t work…or does it?
This question is really part of a whole broader spectrum of questions about whether evolution can really explain complex things. This modern argument is basically a restatement of William Paley’s watchmaker argument, only now it goes under the name of irreducible complexity. This argument was popularized by Michael Behe and is widely used by Intelligent Design proponents. So we’re faced with the question of “How could the eye have come to form in its present highly complex state?”
First, I’d like to point out that this is an argument from ignorance, which is a logical fallacy. It’s basically claiming that, if we cannot explain the eye, then it must have been designed. This is clearly wrong and is no better than saying Zeus caused lightning in ancient Greece. This is all that is really needed to defeat the argument, although we should say more if we can. But I point this out because not everything is currently explained by evolutionary biology. This is natural and it doesn’t indicate that it is incorrect. Think of physics – we have relativity and we have quantum mechanics. Both are well accepted, but we’re still not exactly sure how the two fit together. The uncertainty in minor areas do not undermine the whole of observed facts.
So what can we say about the eye? I should let professional biologists speak to the technical details of the problem, so I’ll provide a few links toward the end, and I’ll just speak in fairly general terms. First, we should not assume that our eye is this perfect device. In fact, the octopus has a better eye than humans do. We also have some severe deficiencies in the range of the electromagnetic spectrum we can see, we have a significant portion of the population needing corrective lenses, we have blind spots, and on and on. We even have severe malfunctions like blindness and cataracts. So, it seems silly to think of how great the eye is – this is clearly a matter of perspective because it could be much better. But we still have a pretty good eye and it’s fairly well suited to our normal lives. We can then ask, “Are there precursors to our type of eye that are less complex and adapted to other scenarios?” The answer is yes.
As Richard Dawkins and others have often pointed out, eyes seem to evolve at the drop of a hat. Eyes have developed independently several times. There are eyes without lenses, eyes that use pinholes, eye cups, eye spots, and even bumps for where very simple eyes used to be (see the following for a few of several examples).
The fact of the matter is that these more primitive eyes from our perspective still served a purpose. Any light sensitive cells are better than none. A slight cup formation allows even better “vision.” An eye that doesn’t focus well is better than no eye at all. Hopefully you are starting to see how the argument doesn’t hold water. There are several advantages along the way to a camera lens. For these creatures that “lost” their eyes, they live in very dark places and eyes aren’t necessary for their habitat. You also have creatures like bats that use a type of sonar and see without actually seeing. That’s where evolution is key. It is the only theory that gives a plausible explanation for the sheer diversity of living things. To say that they were designed in their exact present form seems silly (Why not design all eyes on similar creatures the same way? Why make little modifications?). They were quite obviously adapted to living conditions. The ID proponent wants people to believe that there could not have been steps along the way that provided utility to get to our current eyes. We know of plenty of examples which show that to be false.
The evidence of eyes in our ancestors may not be complete – remember that eyes are soft tissue, so they do not fossilize. But again, to fall back on incompleteness is a dishonest and fallacy-driven approach. We have loads of good evidence about eye evolution. In fact, the people on the front lines, like Behe, know this and now turn to obscure things like the bacterial flagellum (which, by the way, biologist Ken Miller has also debunked).
Here are some links to sources citing professional papers and a few videos from biologists, so you know I’m not making up the varyingly complex eyes out there:
As long as there can be successive steps that lead to improvement in a given environment, then this poses no problem for evolution by natural selection. We can see quite clearly that there are benefits to having some form of eye and that there are many stages still around today to observe. People often just have a hard time imagining the probability to produce things like eyes, but you have to remember that our notion of probable is limited to what happens in a day, week, year, and lifetime. How often have you heard something about a once in a lifetime opportunity? Well, this is billions of years and many failed adaptations later, but a few successful ones are still around to leave clues.
- Doubting Science: The other tactic of creationists
- Mendeleev and the Fossil Record
- In Defense of Science