3 ms·

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?

• emmelaich 8mo ago
  I was looking for that too. I'm sure I've read that single cell animals were sensitive to light (and/or heat). I guess it's a speculation though, because we'd have no physical evidence.
  • hinkley 8mo ago
    We know that modern flagellates can steer to or away from light. When they started doing that is, as you say, pretty difficult to establish since they haven't left archaeological evidence. Unlike shellfish.
  • adrian_b 8mo ago
    There is plenty of evidence. 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.
• ckemere 8mo ago
  Jellyfish (cnidarians) do phototaxis - https://journals.biologists.com/jeb/article/227/18/jeb247503/362131 https://journals.biologists.com/jeb/article/227/18/jeb247503...
• adrian_b 8mo ago
  The article is about vertebrates, not about animals in general. 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.