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What benefit is an eye unless there is also the capability of processing and using the information? How would both evolve simultaneously?
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.
Earlier quoted context omitted.
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?
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…
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.
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.
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.
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.
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.
Earlier quoted context omitted.
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?
This communication ability has evolved before the multicellular animals, in the colonies of unicellular ancestors of animals (e.g. choanoflagellates).
The intercellular communication is a prerequisite for the development of multicellularity, like a common language is a prerequisite for a group of humans to be able to work as a team.
In an unicellular organism, a part of the cell senses light and another part, like flagella or contractile filaments reacts, moving the cell. In a multicellular organism, a division of labor appears, the cells from the dorsal side of the animal sense first light and other stimuli from the environment, so some of them specialize as sensory cells. Originally, the cells from the ventral side were more effective for locomotion, by using either cilia or propulsive contraction waves, so some of them specialized for locomotion, becoming motor cells, either muscles or ciliary bands (which in many simple animals are more important than muscles).
With this division of labor, the older intercellular communication methods have been improved, resulting in synapses between the sensory cells and the motor cells, which ensure that a chemical message that is sent reaches only the intended recipient, instead of being broadcast into the neighborhood.
For better reactions to external stimuli, the behavior of the sensory cells had to be coordinated, e.g. even when light is sensed only on one end of the animal, for the entire animal to move an appropriate command must be sent to all motor cells, not only to some of them, which has lead to synapses between the sensory cells themselves, not only between sensory cells and motor cells.
Eventually, there was a further division of labor, a part of the sensory cells has specialized to be middlemen, i.e. to relay the sensory information between the cells that have actually received it and the motor cells. These third kind of cells have become neurons. Initially the neurons were in the skin, together with the sensory cells from which they had derived, but later they migrated inside the body, where eventually they formed ganglia instead of a diffuse net, because this minimizes the reaction times, by shortening the connections between neurons, leading to a centralized nervous system.
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.
Most vertebrates, with the exception of a few species, like tuatara, have lost the middle eye.
The subject of the parent article is that it was expected that if the third eye was lost, the retinas of the 2 lateral eyes that have been preserved are derived from the retinas of the 2 ancient lateral eyes, but despite this expectation, the retinas of the modern lateral eyes of the vertebrates are derived from the retina of the ancient middle eye.
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.
See other comments.
In most cases the evolution of eyes in animals has been from more eyes to fewer (but more complex) eyes, and not the opposite.
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…