R
rossum
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From abiogenesis we have blind primitive cells. Then single cells with a light sensitive organelle. That carried over into metazoa, so we have light sensitivity in plants, though no brain and no nervous system. Jellyfish have light sensitivity and a nervous system but no brain. Amphioxus has a brain (just about) nerves and a few light sensitive cells on its surface. Add curvature on the surface – a cup eye – to allow detection of the direction light is coming from. Mostly close the cup and fill it with transparent gunk to protect the light sensitive cells. Harden the gunk nearest the outside to start making a lens. Add muscles to shrink or expand the opening as needed. Some of these changes happened in parallel and some species did not develop all of them. Nautilus do not have a lens for example, though squid and octopus do.But isolate just one species with a complex eye. Starting from abiogenesis; how did it go through a thousand plus incremental steps to get to this eye?
Other organisms – protostomes – generally started with a dome eye for detecting direction, not a cup eye. They followed a different path and ended up with compound eyes.