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Octopuses, squid and other cephalopods have camera-like eyes with a lens and a retina, much like ours, yet the two designs evolved separately. The last common ancestor of vertebrates and cephalopods had, at most, simple light-sensitive spots.

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This is a classic example of convergent evolution, in which distantly related animals arrive independently at similar solutions to the same problem. The details give the separate origins away. In vertebrates the eye grows out of the developing brain, and the retina is wired back to front: light passes through nerve fibres and blood vessels before it reaches the light-sensitive cells, and where the nerves leave the eye there is a blind spot. In cephalopods the eye forms from an inward fold of the skin, the nerves run behind the retina, and there is no blind spot. In 1859 Charles Darwin admitted that the evolution of the eye by natural selection seemed “absurd in the highest possible degree”, then explained how it could happen in small steps. Simple light-sensing eyespots are thought to have evolved independently between 40 and 65 times.

Türkçe oku
Ahtapot, kalamar ve öteki kafadanbacaklıların gözü de bizimki gibi bir fotoğraf makinesine benzer: Önde bir mercek, arkada ışığı algılayan bir retina vardır. Ama bu iki göz birbirinden bağımsız evrildi; omurgalılarla kafadanbacaklıların son ortak atasında olsa olsa ışığa duyarlı basit lekeler vardı.

Bu, yakınsak evrimin, yani uzak akraba canlıların aynı soruna birbirinden bağımsız olarak benzer çözümler bulmasının en bilinen örneklerinden biridir. Ayrı kökenleri ayrıntılar ele verir. Omurgalılarda göz, gelişmekte olan beyinden dışarı doğru büyür ve retina ters kurulmuştur: Işık, ışığa duyarlı hücrelere ulaşmadan önce sinir liflerinden ve kan damarlarından geçer; sinirlerin gözden çıktığı yerde de bir kör nokta oluşur. Kafadanbacaklılarda ise göz, derinin içe doğru katlanmasıyla oluşur; sinirler retinanın arkasından geçer ve kör nokta yoktur. Charles Darwin 1859'da, gözün doğal seçilimle oluştuğunu varsaymanın “olabildiğince saçma” göründüğünü itiraf etmiş, ardından bunun küçük adımlarla nasıl gerçekleşebileceğini anlatmıştı. Işığı algılayan basit göz lekelerinin birbirinden bağımsız olarak 40 ila 65 kez ortaya çıktığı düşünülüyor.

Sources and further reading

Background reading: Evolution of the eye
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