Crystal Clear: The evolution of the eye, part 2

“I am inclined to believe that in nearly the same way as two men have sometimes independently hit on the very same invention, so natural selection, working for the good of each being and taking advantage of analogous variations, has sometimes modified in very nearly the same manner two parts in two organic beings, which owe but little of their structure in common to inheritance from the same ancestor.”

Charles Darwin, The Origin of Species

To put Darwin’s statement in other words, it looks very much as if the processes of evolution have hit upon the same solution multiple times. We’ve had posts on ‘convergent evolution’ on Science and Belief in the past, so I will leave you to read A Dentist’s Guide to the Map of Life, or Inevitable Humans? if you want a little more explanation of this phenomenon. In this article I want to concentrate on the makeup of eye.

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Italy-1453-Murano Glass by Dennis Jarivs. Flickr. (CC BY-SA 2.0)

The furnaces for making glass are heated up to a roasting 1000oC or more. So how is it that the optical machinery of the eye can be made at 37oC? A living lens is made when cells fill up with a material that makes it transparent, squeezing out everything else – including the DNA. But what is it that makes the lens so clear, strong, and long lasting?

Crystallins are small proteins that come together to form not crystals (despite the name) but a smooth and tightly packed random arrangement that is both flexible and can transmit light. Several different types of crystallins are fitted closely together like a beautifully even dry stone wall – except this one is perfectly clear. In the same way that gems in a bowl of water can look more glassy, the crystallin proteins take on a transparent appearance in the liquid contents of the cell.

The name crystallin is ironic, because if the proteins in the lens clumped together to form crystals, it would lose its transparency. One of the types of crystallins even acts as a ‘chaperone’, holding on to proteins that have become damaged and are therefore more likely to bunch together. Later in life, the broken proteins can become numerous enough to overwhelm their chaperones and aggregate together, forming cataracts.

Different forms of crystallin proteins. David S. Goodsell and the RCSB PDB- Molecule of the Month. (CC-BY-4.0 license)

Although the different types of crystallin proteins bear the same name, they have very different structures. These structures look very similar to other proteins elsewhere in the body, suggesting that they have been ‘recruited’ from other roles to become part of the lens.  So while crystallins are grouped under the same title because of their function as tough transparent proteins, in fact they appear to have very different origins. This is a great example of convergent evolution: different proteins following a variety of evolutionary paths to end up with the same function.

Often the crystallins look like the enzymes that help organisms handle different stresses (such as overheating) inside their bodies. In the process of being recruited as crystallins, they have changed and lost their original function. These changes have to be subtle, so they don’t destroy the structural qualities that make them so useful for forming a stable arrangement – one that lasts even when environmental conditions (sunlight, for example), are harsh.

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Developing eye in a chick embryo. Inner red section will become the lens. by Kate Whitley. Wellcome Images. (CC BY-NC-ND 4.0)

With their separate origins and same destination, the crystallins are a great example of the phenomenon that Charles Darwin spotted more than 150 years ago. The Cambridge palaeobiologist Simon Conway Morris has been collecting stories of convergence for a number of years now, and believes they are a sign that evolution is more predictable than some people think. He also thinks that convergence is compatible with the existence of a creator. In other words, if there is a God who wanted to create, surely that creation could have happened through a long slow process? And if that long slow process contained a little predictability, hitting the same solutions to biological problems again and again, perhaps that is compatible with there being some sort of plan for the whole show?


Part 1: Imagination and Incredulity: The evolution of the eye

Charles Darwin, The Origin of Species

Crystallins, on the Protein Database education portal

Crystallins, on

Simon Conway Morris, The Runes of Evolution, and Life’s Solution

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© Faraday Institute

Ruth Bancewicz is a Senior Research Associate at The Faraday Institute for Science and Religion, where she works on the positive interaction between science and faith. After studying Genetics at Aberdeen University, she completed a PhD at Edinburgh University. She spent two years as a part-time postdoctoral researcher at the Wellcome Trust Centre for Cell Biology at Edinburgh University, while also working as the Development Officer for Christians in Science. Ruth arrived at The Faraday Institute in 2006, and is currently a trustee of Christians in Science.

Imagination and Incredulity: The evolution of the eye


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

Charles Darwin, The Origin of Species

The ‘yet’, in Darwin’s introduction to ‘Organs of Extreme perfection and complication’ is very telling. The ideas that follow this oft-quoted sentence remind me a little of Paul’s letter to the Ephesians Continue reading

Earthworms and Orchids: Why the founders of modern science cultivated virtue

© Alfred Borchard,
© Alfred Borchard,

I met a man at a conference this year who said he has spent his whole life studying. I have no idea how he funds his insatiable appetite for new knowledge, but it seems he has spent his days going from one topic to the other, modelling himself as a renaissance man. He told me stories of people in 1970’s Germany who spent ten to fifteen years on a single undergraduate degree, often taking just one class at a time. For him, learning was of such value that it was worth approach it steadily and patiently, as a means in itself. I find this attitude a bit extreme, but it’s an interesting way of looking at life!

I recognised this perspective when I heard Richard Bellon, Assistant Professor with a joint appointment in the Department of History and the Lyman Briggs College at Michigan State University, speak recently on values in the scientific community. Bellon has been studying Victorian scientists, or – as he says on his website – ‘obsessing about men with muttonchops who obsessed over the sex lives of plants’. Continue reading

For the love of wisdom of natural things

Photo by John Bryant
Galapagos tortoise, © John Bryant

One of the people who set Charles Darwin along the road to evolutionary theory was not a scientist, but the Governor of the Galapagos Islands, Nicholas Lawson. When Darwin and the Beagle crew landed on Charles Island, Lawson invited him to dinner. As they talked, Lawson mentioned that the giant tortoises for which the Galapagos chain was named varied noticeably between islands. In fact, said Lawson, if any tortoise was brought to him, he could identify which island it came from.

It turns out that the tortoise-naming party trick was not exclusively Lawson’s. Whether he was just repeating what the locals said, or had actually studied the tortoises personally, the fact remains that the person who set Darwin on the course of studying variation among species on the Galapagos islands was not a scientist.

John Bryant, the author of last week’s guest post, told this story during his lecture at this year’s Faraday Summer course, and I enjoyed it because Continue reading