Don't we have single celled organisms with more than one 'eye' now?
We've been able to sense light and shadow even before we became multicellular, didn't we? And this article seems to be implying rather otherwise.
We've been able to sense light and shadow even before we became multicellular, didn't we? And this article seems to be implying rather otherwise.
The vertebrates originally had at least 3 eyes, 2 lateral eyes and one in the middle of the head.
Most vertebrates of today have lost the median eye, though there are a few species, like tuatara, which still have it.
The research reported in the article has shown evidence that the retina of the lateral eyes of the vertebrates has evolved not from the original retina of those eyes, but from the retina of the middle eye, which had a different structure, i.e. different photoreceptors.
As another poster has said, most unicellular eukaryotes are either attracted or repelled by light and this is very simple to observe.
For some of them the mechanism of photoreception is partially understood, because they use photosensitive molecules related to those used in the eyes of animals.
Moreover, there are many bacteria that can sense light, so they are also attracted or repelled by light. Again, many of them use rhodopsins for sensing light, the same as our eyes.
Some light-sensitive molecules, like rhodopsins, are known to have existed in living beings for at least a few billion years, so the ability to sense light is that old.
Where the animals have innovated is in developing optical systems that can capture 2-dimensional images, instead of just sensing whether light is present or absent, like the other living beings can do.