Thank you. For those interested the paper this article is based on is:
https://www.cell.com/current-biology/fulltext/S0960-9822(25)...?
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.
Jellyfish (cnidarians) do phototaxis - https://journals.biologists.com/jeb/article/227/18/jeb247503...
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.
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.
my question has always been why (I think most vertebrates) stop at two? It seems that an extra eye here and there could be really helpful. Maybe it's because all verterbrates evolved from an ancestor that had two eyes, and once the template is in place, it was simply too deep a local maximum to evolve out of? Similar to the 5-digit hand design that all vertebrates share.
Well, from what I've read...
Spiders have 8 eyes. As with vertebrates, this number doesn't change, but there is variation in what it means.
A "normal" spider doesn't really use its eyes. It just has them.
Some spiders are different and rely on their vision. Those spiders have two primary eyes, which they rely on, and six secondary ones, which they don't.
Moving to insects, they often have compound eyes. Two compound eyes. A mantis has two primary compound eyes and three secondary non-compound eyes.
All this convergence suggests to me that even if you have the option to grow more eyes, the correct number is two.
There are also spider species with 6, 4, 2, and 0 eyes.
What benefit is an eye unless there is also the capability of processing and using the information? How would both evolve simultaneously?
A photosensitive patch of cells could be wired directly to motor cells/muscles on the opposite side, which would allow the organism to swim toward the light (maybe useful for feeding or migrating, etc.)
How would the photosensitivity and wiring to muscles come about at the same time?
As long as a mutation isn't strongly maladaptive, it can evolve prior to its being useful.
They didn't need to come about at the same time. Photosensitive proteins (opsins) and cellular motility both predate multicellular life entirely. Even single-celled euglena detect light and swim toward it with no nervous system at all. In early multicellular animals, cells were already chemically signaling their neighbors. A photosensitive cell releasing a signaling molecule near a contractile cell isn't a coordinated miracle. It is just two pre-existing cell types sitting next to each other in tissue, which is what bodies are. Natural selection then refines that crude coupling because even a tiny, noisy light response is better than none.
Each piece, light-sensitive proteins, cell-to-cell signaling, contractile cells, evolved independently and for other reasons long before being co-opted into anything resembling vision. The question "how could A and B arise simultaneously?" dissolves once neither A nor B was new.
Stated clearly (0) has recently started a fantastic series about evolution that aims to explain bacterial flagella. It starts from basic principles and aims to answer questions like yours in evolutionary biology.
A fairly simple chemical reaction could cause an organism to turn or move toward or away from light in the ocean, with various imaginable benefits.
And note that box jellyfish have 24 eyes, some of them highly complex, but no brain. You can look into their behavior to find out what they do with the information.
See also https://en.wikipedia.org/wiki/Parietal_eye, a remnant "third eye" on the top of the head in some species.
Speculation 1. The bicameral mind was created as a result.
Speculation 2. The earliest creatures with two eyes may have been conjoined twins -- which were more successful in life than their single-celled/bodied siblings.
That “cyclopean” eye is described as a patch of light-sensitive cells.
Darwin was not puzzled.
“To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree. When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science. Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case; and if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself originated; but I may remark that, as some of the lowest organisms, in which nerves cannot be detected, are capable of perceiving light, it does not seem impossible that certain sensitive elements in their sarcode should become aggregated and developed into nerves, endowed with this special sensibility.”
And in a letter to Asa Gray, professor of natural history at Harvard:
"The eye to this day gives me a cold shudder, but when I think of the fine known gradations, my reason tells me I ought to conquer the cold shudder.”
> "The eye to this day gives me a cold shudder, but when I think of the fine known gradations, my reason tells me I ought to conquer the cold shudder.”
I think Darwin might be, despite everything, underrated. I wish more people had this level of both intellectual and emotional strength and honesty.