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.

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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?

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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 mapoflife.org

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.

A Dentist’s Guide to the Map of Life

By Eric Kilby from Somerville, MA, USA (Clouded Leopard Mouth Open) [CC BY-SA 2.0], via Wikimedia Commons
Clouded Leopard Mouth Open By Eric Kilby from Somerville, MA, USA [CC BY-SA 2.0], via Wikimedia Commons
Imagine two people from two different tribes, separated by space or time, coming up with the same idea or way of doing something. If it is a good invention it will surely spread within each of their families and communities. Is this a coincidence, or would it make more sense if Continue reading

Inevitable Humans?

© Ruth Bancewicz

Last weekend, the Faraday Institute ran a course on ‘Science, Religion and Atheism’. The aim was to examine questions of science and religion from both theistic and atheistic positions. The speakers were more than usually candid about their own views, which was both refreshing and a fascinating sociological exercise.  (See the Faraday website for recordings when available).

To continue last week’s theme of biology raising questions that in the past have been left to cosmologists, I will focus on Simon Conway Morris’s presentation at the course. I have already written on biochemical fine-tuning but the related theme of convergence, which is the focus of Conway Morris’s research, is also an important one.

Simon Conway Morris is one of those thinkers who prefer to limit themselves to the most risky fields of investigation. He is genuinely interested in upsetting the applecart of received dogma, and getting closer to the truth. As a PhD student, Conway Morris worked with Harry B. Whittington on the Burgess Shale in British Columbia, and was part of the huge revival of interest in the Cambrian Explosion.

At first the Burgess Shale fossils seemed to point to the randomness of the evolutionary process – a point that Stephen Jay Gould elaborated on in his book ‘Wonderful Life’. It has become something of a convention in the biological world to avoid using teleological language, and Gould famously suggested that if you ‘reran the tape of life’ you would get something completely different. That strikes at the heart of who we are, and the theological consequences are enormous. Is the existence of human beings a happy accident? How could people of faith even consider evolution if the implication is that intelligent beings such as ourselves are an accidental dot on the cosmic landscape, rather than lovingly created by God?

But as Conway Morris proceeded in his career in palaeobiology it seemed to him that evolution had repeatedly navigated towards similar solutions in different contexts. An example of this is the sabre-toothed cat. In North America this deadly cat was a placental mammal, and in South America it was marsupial – sabre-toothed cats evolved at least twice. Other examples include the ‘camera eye’ that has evolved many times independently, and the octopus’s tentacle that bends in a similar manner to a jointed arm.* The conclusion that can be drawn from the vast number of examples of convergent evolution is that there seem to be a limited number of ways of solving the same problem. One could describe these solutions as points towards which evolution navigates. So if you reran the tape of life, the emergence of intelligent beings like us might actually occur every time.

In his seminar Conway Morris took his thesis further, speaking about extraterrestrial intelligence, consciousness, and language. The fact that we can see beyond the limits of our inbuilt sensory systems, and have capabilities way beyond any other animal species has prompted many to ask questions about humanness that sound like religious questions.  I realise that this is a vague theological ending to a largely scientific piece, but Conway Morris is often tentative in his theological conclusions. Hopefully another book is on the way that will take this line of thinking further.

*You can see these and many other examples of evolutionary convergence at mapoflife.org.