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Asked by ruddockawh to David, Helen, Ian, rhysphillips, Sarah on 20 Jun 2011.
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David Corne answered on 17 Jun 2011:
The standard explanation is that it developed bit by bit over millions of years – although this might seem incredible, it is actually very plausible, but it’s difficult to explain in a short answer. Basically, it starts with light sensitive patches of skin, gradually evolving so that the light sensitive patches are in little craters/pits/depressions (like your bellybutton), gradually these tend to contain fluid, which naturally focuses the light, after a few million years they close up until there’s just a slit remaining for the light to get into, and meanwhile the light sensitive patches and the bits of brain that process them have got more sophisticated, and then, you have an eye. Right from the beginning, it was doing something that had survival value, and evolution kept on adding to that value with slight changes.
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Helen Fletcher answered on 19 Jun 2011:
Richard Dawkins has a very good explanation for the evolution of the human eye in one of his books “The Blind Watch Maker” (I think?). It’s along the lines of what David is saying but more detailed
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Ian van der Linde answered on 20 Jun 2011:
Science can explain that the eye could *feasibly* be created by showing how a series of small, incremental changes, all of which individually make the eye work just that little tiny bit better, could lead to the complex human eye that we have today. It is worth remembering that the eye isn’t perfect – it contains some “design faults” precisely because it improved very gradually over time. For example, the light entering the eye gets bounced off a sheet of cells before it hits the light sensitive part – if you were to redesign it from scratch, you’d flip this part around for a better quality picture. Plus look at how many people need glasses, even young children – if the eye was an engineered product like a camera, you’d take it back to the shop and get a refund on the warranty!
Comments
Helen commented on :
Sam who I work with wanted me to add:
How animals’ bodies grow is coded by the genes in their cells, rather like hypertext tells a computer what to make a website look like. If these genes work well then the animal survives longer and so has a longer time to have more offspring, passing on the ‘good’ genes to them. Every so often these genes get an error, or mutation, in them – sometimes this is a good mutation that gets passed on; sometimes it’s a bad error and the animal dies earlier, passing on the gene to few (or no) offspring. It’s likely that the first ‘eyes’ were just mutations that allowed that animal to sense the difference between light and dark – worms have cells that can do this. Being able to tell light from dark was an advantage to the animal that allowed it to survive longer and so pass the mutated gene on. Eventually over millions of years these light-differentiating cells became more and more specialised, eventually evolving into the eyes that mammals have today.