I understand that vision has evolved numerous times in different animals. Does it start when a single-celled blob engulfs some photosensitive chemicals and then the blob can move towards the sunlight to get more food? I seem to recall reading this obviously simplified explanation.
But this blob isn’t “seeing” or “aware” of light, so how does the presence of the chemical direct the blob towards the light? What is the photosensitive chemical doing to benefit the blob before its genetic material starts creating the pathways and proteins and stuff to make good use of it?
I’m in no way doubting evolution, and the rest of the development of the eye makes sense to me, I’m just trying to get a better understanding of how it started. There are a lot of websites that look scientific and informative but turn out to be creationist. I’d appreciate recommendations for websites or books that can explain this topic on a layperson level. Thanks!
A science teacher in middle school suggested that the initial mutation might have been a small patch of sensitive flesh- extra nerves or whatever. Over time the species became capable of detecting sunlight from above verses nothing from below. Eventually it’s capable of detecting movement because of creatures intersecting it and the sunlight, etc. Sounds plausible to me.
I don’t see how a blob engulfing material would then begin reproducing that material in its offspring. If I eat a ham sandwich my children won’t start growing ham sandwiches. But I’m not an expert so I might have it wrong.
Well, if you’re interested in a evolutionarily-talk, biochemically worded, “just-so story” (credit goes to Blake: for that last one but I really like it), check out the wikipedia article on crystalins.
Basically, water soluble clear proteins with one purpose – to be clear and function the the lens of animals with eyes. Except, they have, or may have evolved from enzymes, particularly those that prevent damage. So one current model is vision arose from preventing light damage to tissues. That seems to be borne out by some of the references on the wikipedia page.
(I am even less of an expert.)
Maybe if you are swimming in the ham sandwich sea then you and all your offspring will be ingesting ham sandwiches and one of them will develop a mutation to process the ham?
I think vision may have originated once, or a very few times, and that seeing animals generally descend from a few original ancestors.
Brine shrimp are an example of an animal that is on the edge of having vision. They can sense light and sense the direction it comes from, and tend to move towards it. This accomplishes some kind of navigation that is useful to them. But I don’t think they can form images to speak of. Not sure whether their ability to sense light originated with a different ancestor than did our own, but I bet they didn’t evolve it recently if so.
Pit vipers have primitive eyes for seeing in the thermal infrared, in addition to their regular eyes that see visible light. By “primitive eyes” I may be using poetic license, but they have pits, little dimples that are lined with heat-sensitive cells. Because these are in a dimple, and wired separately on different segments of the dimple, they are a bit direction sensitive. We can sense where a warm fire is by the sensation of heat on our skin. These pits are a more advanced form of this sense. I think these evolved completely independently of the regular eyes, and pretty recently.
Not an expert on any of these things - if anybody can correct or elaborate, please do!
If by “vision”, you mean “imaging”, it’s evolved at least four times: The compound eyes of arthropods, and the lensed eyes of vertibrates, cephalopods, and cubazoans (though I’m baffled as to what the latter could actually use them for). If you just mean the ability to distinguish between ambient light and lack of light, though, I’m not sure where that first evolved, or how many times.
I am going to recommend the book In the Blink of an Eye by Andrew Parker which discusses the evolution of vision and its impact on life. It’s not a proven theory, but it does go over a lot of the current evidence and discusses possible ways for eyes to evolve.
On the other hand, if a living “blob” surrounds, or accepts the entry of, other living organisms, with their own DNA, there can be a genetic exchange which results in the formerly separate smaller organisms actually becoming standard and replicable parts–organelles–of the host organism. Endosymbiotic theory argues that chloroplasts in plants, and mitochondria in all eukaryotes, originated as entirely independent beings.
To put it another way: plants didn’t invent photosynthesis, they just let green bacteria move in.
I’m not aware of any similar theory applying to any of the evolutionary tracks for eyes, though.
No. In fact it’s become clear that vision, in a general sense, is so useful that we might say evolution is “eager” to find ways to make eyes (I hope we all understand how that’s a figurative statement). There are several entirely different kinds of eyes (maybe nine–and here I mean eyes operating on separate principles, not just variations of design on the corneal eyes that humans share with many animals), all of which seem to be have arisen more than once in different evolutionary lines. Collectively, life on Earth has apparently invented vision of some kind on more than forty separate occasions! I believe Dawkins discusses this in Climbing Mount Improbable.
I haven’t read it, but I’m told that right now the definitive book on the extant biology of eyes and their various evolutionary origins is Animal Eyes.
This phenomenon is called convergent evolution:
“One of the most famous examples of convergent evolution is the camera eye of cephalopods (e.g., squid), vertebrates (e.g., mammals) and cnidaria (e.g., box jellies).[5] Their last common ancestor had at most a very simple photoreceptive spot, but a range of processes led to the progressive refinement of this structure to the advanced camera eye - with one subtle difference: The cephalopod eye is “wired” in the opposite direction, with blood and nerve vessels entering from the back of the retina, rather than the front as in vertebrates.[1] The similarity of the structures in other respects, despite the complex nature of the organ, illustrates how there are some biological challenges (vision) that have an optimal solution.”
I read somewhere that it is estimated that eyes evolved independently between 40 and 60 times. I don’t where that comes from. But I do know that the cephalopod (octopus and squid) eye differs from the mammalian eye in that the nerve in the cephalopod exits from the rear of the eye and thus they have no blind spot as we do.
It is even simpler than that. We animals are all evolved from plants. Or more correctly, we are all evolved from single celled photosynthetic organisms. So the photosensitive chemicals didn’t need to be engulfed. We have retained these photosensitive chemicals from our plant ancestors. The clearest evidence of this is that rhodopsin, one of the primary photosensitive chemicals in the mammalian eye, is just a few amino acids different from the rhodopsin used in the photosynthetic pathway of certain bacteria.
Us plants have come a long way, and we’ve taken out photosynthetic pigments along for the ride.
To avoid the usual SDMB nitpicking, the following explanation is highly simplified but remains accurate.
Basically, photosynthetic bacteria are little chemical batteries. When light strikes photosensitive chemicals are embedded in the cell wall, those chemicals start to pump material with the correct electrical charge from outside the cell to the inside of the cell. That causes a build up of electrical potential between the inside of the cell and the outside, exactly like the charge potential across two electrical cells. When the electrical charge inside the cell gets high enough, the bacteria discharges the electrical energy, and uses the energy released to produce sugar.
The whole thing manages to be amazingly complex and astoundingly elegant at the same time. But the main thing to note is that the chemicals evolved specifically to cause a charge to build up inside the cell.
Next you need to understand how single cells manage to manouevre in the environments. If you place an obstruction in front of an amoeba, it doesn’t keep banging its head into the wall. The damn thing goes around the obstruction, just like a multi-celled organism would.
So how does a single cell even know the obstruction is there? Once again, the answer lies in maintaining an electrical difference between the inside and the outside of the cell.
You can imagine a cell membrane as being like a sheet of latex. When the latex is relaxed the rubber acts as an insulator, and the inside and the outside of the cell can maintain different electrical potentials. When the latex gets stretched, and tiny holes become stretched and get really big. Water and salt run in and out through the hole and the latex stops being an insulator.
This is almost exactly what happens with cell membranes. When they get stretched, they leak and the cell becomes electrically neutral with the surrounding water. So when an amoeba bangs into a wall, the membrane gets stretched by the collision and starts leaking. Amoeba have evolved to move away from those areas of the cell membrane that have low charges, and move towards those with high charges. That ability to orient towards areas of the cell membrane with the highest electrical charge is the basis for all protozoan orientation. Its evolutionary origin is is fairly basic, simply deriving from movement away from collisions, or environments, that cause the cell wall to leak.
So we have two unrelated processes, but both rely on maintaining a charge difference across the cell membrane. From their it should be fairly simple to see how photosensitive chemicals allow a single cell to move towards (or away from) light. When light strikes the chemicals in the cell membrane, the electrical charge in that area starts to build up. Because the organisms has already evolved to move towards those directions with the highest electrical charge, their presence will cause the organisms to move towards light.
In fact experiments have been done where photosensitive chemical systems have been artificially embedded into single celled organisms that normally lack them, and even there the organism starts to move towards light. It’s not as robust as phototaxis in organisms that have evolved with those chemicals, but the fact that it works at all is proof positive that the underlying basis is very simple.
So you don’t need any pathways or proteins to make us of it. If you have the photosensitive chemicals that allow you to generate electrical energy from sunlight, you already have all the pathways and proteins to make use of it. Of course it can be perfected over time, but the basic system works fine with just the ability to generate that membrane potential
I suspect that what confused you most is that you didn’t realise that “animals” are all descendants of “plants”, and not the other way around. Once you realise that we have all evolved form a photosynthetic ancestor, the origin of sight is fairly straightforward, since plants obviously need to possess chemicals that respond to light.
Some of the single celled plants lost their photosynthetic systems over time, but they retained the photosensitive chemicals because they allowed them to orient themselves towards the light. That is the basis of all vision, and it has never been lost from out plant ancestors.
Of course respiration is also an atavistic hangup form our plant ancestors. It is just photosynthesis running backwards. If we hadn’t evolved from plants we probably wouldn’t be able to do that neat trick with oxygen. But that’s a story for another day.
Vision evolved multiple times from multiple ancestors that could not see. Precisely how many times we don’t know, probably 6 or more times.
Brine shrimp evolved from ancestors with highly developed compound eyes and visual abilities as good as any living crustacean. They have since lost most of their visual ability.
As examples of organisms “on the edge of having vision” they are about as useful as moles, cave fish or Mister Magoo.
While the pits are an interesting analogy of one of the likely stage in the evolution of the eye, they don’t tell us anything at all about how vision originated, since vision relies on wildly different mechanisms, being based on chemical detection of wavelengths that can’t cause changes in reaction times.
On what do you support this theory we evolved from plants? Because what I’ve been hearing/reading for a couple decades is that the last common ancestor of plants and animals occurred prior to photosynthesis evolving.
But they might not need to - they can just keep on eating ham sandwiches. The vision-allowing substance would be a vitamin, something the blobs can’t make themselves but need to find and eat.
Read it again.
This time see if you can make it past the second sentence.
I mean, I know it’s a bit long winded. But come on.
Well, Blake, how about *you *read past the first sentence?
In the third and second sentences respectively of the two posts, you stated “…we are all evolved from single celled photosynthetic organisms” and Broomstick said “…what I’ve been hearing/reading for a couple decades is that the last common ancestor of plants and animals occurred prior to photosynthesis.”
I fail to see how that’s not a cogent and relevant response to your post. If my reading comprehension has somehow failed, please enlighten me.
Yes. It is chlorophyll that is the distinguishing characteristic of plants, not rhodopsin. As you point out, rhodopsin occurs in some bacteria, but bacteria are not plants. So I don’t see how you go from rhodopsin occurs in bacteria, animals and plants to we’re descended from plants. Rhodopsin is something shared across very broad divisions of life and is not a distinguishing feature of the domains of life, or superkingdoms or whatever terminology you’re favoring these days.
The fact that we are discussing prokaryotes neatly sidesteps the entire issue.
And if he is claiming what he has *literally *posted… well that’s just silly. He would have to be saying:
-
That eukaryotism evolved independently multiple times, with one group of bacteria giving rise to the plants, and a second line independently giving rise to animals at a later date. Despite this paraphyeltic evolution they share perfect similarity in cell membrane and organelle structures that are found in no prokaryote lineages. This would be the most astounding example of convergent evolution ever known. In fact it would pretty much prove that Darwinian evolution can not account for 90% of biological diversity.
-
That cellular organelles such as mitochondria were independently incorporated from exactly the same prokaryotes by the two separate lineages in independent events. This is not totally impossible, but it does raise numerous problems.
-
That both eukaryote lineages evolved independently, yet remained hidden for over a billion years, since the oldest known eukaryote is about 1.5 billion years, and phs goes back at *least *3 billion .
-
That, despite existing as obligate anaerobes for over a billion years, aerobic respiration and oxygen protection systems evolved in both the lineages independently, using exactly the same enzyme systems. Obviously it could not have been the result of a common ancestral pathway, even for mitochondria, if the common ancestor existed before phs, and hence free oxygen.
-
That despite having no common ancestor for at least 3 billion years, plants and animals share 50% of their *nuclear *genes. Either the genomes evolved to be 50% identical through convergent evolution, or else the two lineages managed to retain 50% similarity for 3 billion years. I’m not sure which explanation is sillier. The first is merely ludicrously improbable. The second requires that we believe that the two lineages preserved genes for things like lipid bi-layer proteins from a prokaryotic ancestor that didn’t *have *any lipid bilayer proteins. It also requires that, while we have diverged 20% from other mammals in just a few million years from a common tetrapod ancestor, we haven’t diverged 50% from plants in 3 billion years from a prokaryote ancestor. That is actually impossible, so I guess that makes the second option the most silly.
So apparently plants and animals convergently evolved genomes that are 50% similar, at a probability that is slightly less than the odss of a monkey banging out a script for ‘Hamlet’ in one afternoon.
I assume that this is not what Broomstick is saying, and that he meant that the last common ancestor evolved prior to the evolution of photosynthetic eukaryotes, which is probably true.
Did you actually read the post?
Did you read the bit where I said “We animals are all evolved from plants. Or more correctly, we are all evolved from single celled photosynthetic organisms.”
Did you read the bit where I put plants inside quotation marks?
Do you realise that “Plant” is not a scientific term? Do you realise that most people consider kelp or cyanbacteria, for example, to be plants? Do you realise that when I say “We evolved from plants. Or more correctly, we evolved from single celled photosynthetic organisms”, not a single evolutionary biologist in the world would gainsay me as you appear to be doing?
And…???
What is your point? That rhodopsin didn’t evolve as a photosynthetic pigment in prokaryotes? That animal rhodopsin isn’t almost identical to rhodopsin in sulfur bacteria, and isn’t universally agreed by evolutionary biologists to be homologous? That rhodopsin isn’t a key component in animal visual systems as well as in the bacterial photosynthetic systems in which it first evolved?
In short, what precisely is your point here Broomstick?
And can you clarify what you meant by that nonsense about the prokaryote lineages diverging 3.5 billion years ago and provide a reference to these source who have been writing that for decades?
Because at this point you are making no sense at all and promulgating ignorance.
For those of who need things spelled out all slow-like:
So the common ancestor of modern plants and animals did not have photosynthetic capabilities. Since you wrote this:
can I assume that this common ancestor evolved from an organism that was photosynthetic, but which lost this ability?