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Eyes are so complex that some say natural selection couldn’t possibly be how they formed. So, how did eyes evolve?
"The eye to this day gives me a cold shudder," Charles Darwin once wrote to a friend. If his theory of evolution was everything he thought it was, a complex organ such as the human eye could not lie beyond its reach. And no one appreciated the beautiful construction of the eye more than Darwin—from the way the lens was perfectly positioned to focus light onto the retina to the way the iris adjusted the amount of light that could enter the eye. In The Origin of Species, Darwin wrote that the idea of natural selection producing the eye "seems, I freely confess, absurd in the highest possible degree."
For Darwin, the key word in that sentence was seems. If you look at the different sort of eyes out in the natural world and consider the ways in which they could have evolved, Darwin realized, the absurdity disappears. The objection that the human eye couldn't possibly have evolved, he wrote, "can hardly be considered real."
Today evolutionary biologists are deciphering the origins of not just our own eyes but the dozens of different kinds of eyes that animals use. Fly eyes are built out of columns. Scallops have a delicate chain of eyes peeking out from their shells. Flatworms have simple light-sensitive spots. Octopuses and squids have camera eyes like we do, but with some major differences. The photoreceptors of octopuses and squids point out from the retina, towards the pupil. Our own eyes have the reverse arrangement. Our photoreceptors are pointed back at the wall of the retina, away from the pupil.
For decades, most scientists argued that these different eyes evolved independently. The earliest animals that lived over 600 million years ago were thought to be eyeless creatures. As their descendants branched out into different lineages, some of them evolved their own kinds of eyes. It now turns out, however, that this is not really true. All eyes, in all their wonderful variety, share an underlying unity in the genes used to build them. By tracing the history of these shared genes, scientists uncovering the steps by which complex eyes have evolved through a series of intermediate steps.
Source How the Eye Evolved