Eyes evolved before brains, how did that work

I’m on my phone so I can’t link, but on Wikipedia the page ‘timeline of human evolution’ mentions that the first eyes occurred 580 mya but the first brains were 550mya. So how would an eye work without a brain? Was it just a photoreceptor cell or can something more complex work without a brain.


Can’t give an actual answer, but I seem to recall that this point was well explained in Richard Dawkins’ ‘The God Delusion’ and ‘The Selfish Gene’.

(note to self: must improve long-term memory)

The brain is basically just an outgrowth of the nervous system (as I understand it). All the nervous system does is transport signals from one place to another. With a simple nervous system, the pathways are basically hardcoded. “If movement spotted on left side, turn left.” The creature has no will and there’s no logic behind what it does in response to a signal. It’s a biological clockwork machine. If the machine works, then the creature’s mechanism continues on in the evolutionary cycle. If the machine doesn’t, then the creature dies, and other mechanisms are passed on.

The brain adds the ability to try random responses and over time, develop a “best” strategy of response. Now instead of testing strategies via hard evolution, each creature “evolves” a strategy during its own life. It’s able to respond to changes in the environment faster, though presumably the time to process signals and react to them is longer.

Plants can be positively or negatively phototaxic without a CNS/brain.

Look up the box jellyfish for a living example of true eyes (retina and all) without any “brain” in the nerve net.

It’s worth noting that eyes in most creatures (but not jellyfish and I suppose not scallops) are a manifestation of cephalization. Even the eyespots of flatworms, which are at the “head end” was taught to me as representing rudimentary cephalization, although they do not have brains.

Further, there is a lot of processing which goes on within vertebrate eyes, even before signals are transmitted to the brain proper. Vertebrate eyes are actually outgrowths of the central nervous sytem.

I don’t know about arthropod or cephalopod eyes.

Nitpick: retina is CNS, the rest of the eye not really.

Cephalopods don’t have inverted retina (theirs isn’t ass-backwards like ours), and isn’t considered CNS.

Arthropods often have simple eyes or compound eyes. Note that compound eyes means the animal sees a blurry image. It does not mean that they see an identical, clear image in each ommatidium, cartoon style. That sort of evolved characteristic would be stupid and pointless.

Is the distinction whether the tissue is an outgrowth of the brain or not?

Many people are inclined to think of the eye as being like a video camera, with its function being to send a picture (or, at the least, a stream of information) back to the brain to be analyzed, and ‘experienced’. It is not really like that. Eyes (and brains, come to that) are really there to modulate and control behavior. As such, in simple cases, an eye can operate without need for a brain, connecting more-or-less directly to muscles or other actuators.

A very simple eye can consist of nothing but a single light sensitive cell, coupled with a pigmented cell next to it, that shades it from one side. The Platynereis larva, a plankton animal, has eyes like this. Its main behavioral need is to swim toward sunlight, and its eye enables it to do this (and very likely does little or nothing else for it) because when it spins its body as it swims (as it does) the stimulation of the eye spot changes in an appropriate temporal pattern only when it swims in the right direction. (It does have a, very small, brain, but I do not think it plays any very significant role in this phototactic process. Certainly it is not doing “information processing” in any meaningful sense, on the signal from the eye. Link.

Many invertebrate eyes have this sort of very specific and limited function. Wolf spiders have eyes that are specifically adapted and specialized for recognizing other spiders’ legs. The retina has an elongated and bent or curved shape that matches the shape of a spider leg and its joint. If this retina gets stimulated all over, which can only happen when it is confronted with something shaped much like a spider leg, this tells the animal that the little blobby thing it sees in in front of it (with a different set of eyes - spiders have several sets of eyes, all for different special purposes) is a potential mate rather than potential food. (I do not think it can tell which sex it is, but hey, spiders are very open about stuff like that.) It may be that the best way to think about vertebrate eyes, including ours, is as organs that consolidate a large number of specialized visual functions like this into one organ that can make efficient use of the same receptor cells for various purposes [PDF], rather like the way that an electricians multimeter can be used in three different ways, to measure voltage, current, or resistance, but all making use of the same galvanometer coil is as its primary “sensor”. From this perspective, the brain’s main role in vision is not so much to analyze or process the incoming signal from the eyes, but, like the electrician deploying his multimeter in different ways as needed, to deploy the eye in such a way as to test for the information we currently need for the ongoing control of our behavior.

Well, that depends on what you mean by “see” and “image”. Vision happens in the brain. It’s quite possible that the signals from each receptor are combined and interpreted by the brain into a single “image”. That is what vertebrates do, after all.

Just because the physics of the light collection in a compound eye does not form an image in the same way a camera does, does not mean the “image” in the brain of the invertebrate is not acute.

There is no image in the brain, blurry or otherwise, certainly not one that we, or any other type of animal, see. (Yes, I do know about retinotopically mapped cortex. That is not an image and we do not see the state of our visual cortex.) The notions that seeing takes place “in the brain” and that we see “images” are both sloppy fallacies long overdue for retirement. Even if there were images in the brain, there is no-one in there to see them. We do not see inner images, we see the world around us, and we do not see in the brain, we see with the brain (as well as with the eyes of course).

See my post above yours for some explanation of how seeing actually does work, with some citations (of which I could provide many, many more).

This is why a started my previous post with “that depends on what you mean by ‘see’ and ‘image’” and selectively used quotes around “image”. I was not talking about the physiology of sight, which you cover quite well. I’m talking about perception.

Despite there being no image in the brain, there is an “image”. That is, a perception of a field of color and intensity. What each receptor in a vertebrate or invertebrate eye senses is a separate issue from the acuity of the mental “image” that the brain perceives.

I think you are trying to read into things too much. At the receptor level, the information is not terribly useful, as we really need contrasts between multiple receptors to form any meaningful percept. And yes, the more complex stuff occurs in the cortex, but there are still many layers before that.

I was saying: flies don’t see the Hollywood/cartoon version. The ability to see form isn’t nearly as good as ours, but they make up for it in other ways like rapid detection of moving objects.

To make this whole thing a lot simpler, all the cells of the eye do is send a signal when triggered by the appropriate wavelength/quantity of light. In the simplest terms, then, you have a binary on/off switch.

The switch can be connected to anything. Connect that binary switch to a muscle group and now you can close your shell at night and open it up in the morning (or vice versa). No brain required to make that happen; you just need the muscle to relax when it gets signal 0 and to clench when it gets signal 1.