Eye evolution made easy

20 July 2012 by Pete Etchells, posted in Uncategorized

The National Trust hit the news recently over the controversial inclusion of creationist theory in a new exhibit looking at the history of the Giant’s Causeway. The exhibit claimed that:

“Like many natural phenomena around the world, the Giant’s Causeway has raised questions and prompted debate about how it was formed.
This debate has ebbed and flowed since the discovery of the Causeway to science and, historically, the Causeway became part of a global debate about how the earth’s rocks were formed.
This debate continues today for some people, who have an understanding of the formation of the earth which is different from that of current mainstream science.
Young Earth Creationists believe that the earth was created some 6000 years ago. This is based on a specific interpretation of the Bible and in particular the account of creation in the book of Genesis.
Some people around the world, and specifically here in Northern Ireland, share this perspective.
Young Earth Creationists continue to debate questions about the age of the earth. As we have seen from the past, and understand today, perhaps the Giant’s Causeway will continue to prompt awe and wonder, and arouse debate and challenging questions for as long as visitors come to see it.”

The good news is that the National Trust have listened to criticisms about the exhibit, and are currently reviewing it (with the hopeful conclusion that it will be removed). However, the deeper problem here, as Dr Adam Rutherford points out, is that it has allowed creationist lobby groups such as the Caleb Foundation a means through which they can appear to have a legitimate view about how the world was created. This wedge strategy is more political than religious in design; it aims, via getting a foot in the door, to slowly distort scientific facts, appeal to sentiments about freedom of speech, and peddle pseudoscience as legitimate evidence against evolution.

One of the classic ways that creationists attempt to distort science is to use the example of the eye. In fact, they even quote Darwin on the matter:

“…to suppose that the eye … could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree.”

- On the Origin of Species, 1859

If Darwin himself was stumped by the eye, it sort of undermines the whole concept of evolution, doesn’t it?

Nope. In a classic case of cherry-picking, the above quote wasn’t Darwin’s last word on the matter. If you read a bit further:

“…if numerous gradations from a perfect and complex eye to one very imperfect and simple, each grade being useful to its possessor, can be shown to exist … then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.”

So, how do we get from ‘being blind’, to eyes as we know them today? Well, in order to answer this, we need to think a little bit about what the basic point of vision is: to provide the necessary information about our surroundings, in order for us to behave more appropriately. Certainly, if I’m something that moves, having visual information at my disposal would definitely help me to navigate around my environment properly, and avoid things that might injure or kill me.

How do I get that information? From light.

We know that pretty much every living thing is sensitive to light – for instance, plants have a light-sensitive hormone called auxin, which promotes cell growth and tends to collect on the darker side of a plant’s stem. Because these cells grow faster than the ones on the side that is exposed to light, then the plant ends up bending towards the light (a process called phototropism).

So to begin with, we just need some simple, light-sensitive cells:

Step 1: Get some photoreceptors
Photoreceptors are specialised types of neurons that are capable of transforming light into an electrical signal. This signal can then be sent somewhere to stimulate a biological process. If you put a group of these together, you can sense light coming from a particular direction:

eye1.jpg

This is handy, because it now means that you can move either towards or away from a light source, and detect in very basic terms, whether something gets in the way of your light source. You won’t be able to see what it is, but it still gives you some rudimentary knowledge of what’s going around you.

Step 2: Directional sensitivity

The next step might be to figure out which direction the light is coming from. In order to get this, all you need to do is arrange the photoreceptors in a pit, like so:

eye2.jpg

In step 1, our basic eye would detect light no matter where it was coming from in the 180 degree arc in front of it. By adding a little pit, we can now limit this field to an area that is (vaguely) more head-on. As this pit deepens, the direction in which light can be detected from becomes more and more specific, until eventually, a chamber is formed.


Step 3: Make a pinhole camera

When this chamber is formed, it has an added benefit, over and above the fine directional sensitivity – it turns the eye into a pinhole camera.

eye3.jpg

Pinhole cameras project light that is coming in from the scene in front of it to form an inverted and reversed image on the back wall. In the case of the eye, this means that very basic shape-sensing can take place. So not only can you see whether something is blocking your light source, you can roughly see what shape it is, and in fact, you can still see this type of eye today, in the Nautilus:
nautilus.png

Step 4: Close the chamber

The chamber that forms part of the pinhole camera is filled with water at the moment, but as the pinhole gets smaller, it becomes harder and harder to maintain a good flow of water through it. Ideally, we want to close off this chamber with a transparent window, in order to prevent parasitic infections or contamination with debris that either damages the photoreceptors, or blocks off incoming light.

eye4.jpg

This has a further benefit of allowing a specialised fluid to develop in the eye, instead of just having water in there. This specialist ‘humour’ can serve useful purposes like blocking out ultraviolet radiation, or providing a higher refractive index, or allowing the organism to see out of water.

Step 5: Add a lens to improve it all

We know that lenses have evolved independently in a number of separate species. One way in which it might have happened in the case of our lineage is that the transparent window covering the eye chamber split into two layers, with a layer of liquid in between to act as a filtering system to keep the layers clean. Eventually, this secondary layer forms a lens, giving the ability to focus on near and far objects, and provide a wider viewing angle as well as higher image resolution. On top of this, a cornea eventually develops in order to passively increase the eye’s refractive power (meaning that the lens has to do less work).

eye5.jpg

And as complicated as it all sounds, it really doesn’t take that long to develop. Nilsson and Pelger showed in a 1994 paper that it would take about 364,000 generations to get from a basic eyepatch to a modern-looking eye. In the grand scheme of things, that’s no time at all.
nilsson.png

So there you go, next time a creationist bangs on about irreducible complexity, you can tell them that’s how you evolve an eye in 5 easy steps.


2 Responses to “Eye evolution made easy”

  1. Mike Haubrich Reply | Permalink

    And there is great utility in "Half an Eye," That's the creationist "quandary for the evolutionist" that always makes me laugh when a creationist thinks he has me stumped.

  2. rl Reply | Permalink

    Would be a slightly more useful resource if you gave examples of each type, otherwise, well done and thanks!

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