It’s hard for me to form a direct question, so please bear with me as I poke all around my inquiry.
The way I’ve come to think about the electro-magnetic spectrum is that it is unbiased. Mankind has come along and decided to create meaningful divisions along the frequencies. So at one end you have your Radio waves, Micro waves, and Infrared. The other end you have your Gamma rays, X-rays, and Ultraviolet. And in this teeny tiny section somewhere between is the most important “visual spectrum”, AKA: light. Is there anything fundamentally intrinsic about this part of the spectrum, that us evolving humans chose to lock in on as we developed eyes? That is, was it a completely random evolutionary path, or is there something about those exact wavelengths, that make eyes possible that the other parts of the spectrum do not posses? Could we have just have easily detected infrared or microwaves as another “color”? Or even Ultraviolet or gamma rays? I know bees and other insects can detect ultraviolet light, so I don’t see why we couldn’t SEE radio.
Also a tangent for bonus points: Had humans not evolved eyes (say we can “see” like bats, through echolocation), what would we have made of our place in our universe? Not being able to directly detect the heavens, I think we would have been at a major disadvantage. Maybe this questions is best for IMHO.
One point, IIRC the size of the detection apparatus (eyeball or whatever) is going to be dependent on the wavelength being detected; visible light ranges from about 400 to 700 billionths of a meter, whereas radio waves range in wavelength anywhere from millimeters (thousandths of a meter) all the way up to tens of thousands of kilometers. So in order to be able to “see” in radio, you’d have to have inconveniently large eyeballs.
I don’t know much about evolution, but I do know about optics and detectors. Here are some thoughts:
[ul]
[li]The sun emits most of its energy in the “visible” wavelength band, so there’s plenty of light availble for illumination. There’s not much illumination in radio wavelengths, or X-rays and gamma rays.[/li][li]Infrared is difficult because any object near body temperature emits infrared. You can’t see anything if your eyeball is glowing.[/li][li]It’s difficult to make UV eyes because very few materials are transparent to UV. Water in particular is opaque to UV; it’s hard to imagine eyes that don’t contain any water.[/li][li]Soft X-rays are absorbed by air, so that’s out.[/li][li]Hard X-rays and gamma rays go right through the retina without interacting (i.e. without being detected). You’d need an inch-thick retina, or a retina that has a high concentration of heavy metals.[/li][li]The amount of detail (sharpness) you can see is limited by the wavelength of the wave; e.g. if you use radio waves with 1-inch wavelength, you can’t see detail much smaller than 1 inch.[/li][/ul]
And of course the eyeball size / resolution issue mentioned by MEBuckner.
fascinating. I don’t know why that restriction didn’t occur to me. So, if the rods and cones in our retinas were big enough, or small enough, we could detect those wavelengths. Man, I’d love to see what “color” the “light” would be when I throw my coffee in the Mic for a warm up.
Are there any other creatures that can see into the infrared at least? I want to say certain reptiles and snakes can do this, but isn’t that more of a heat detection thing, rather than actual sight, using their eyes?
It really sounds like there is a “goldie-locks” zone in the spectrum, then. Where everything just happens to fall in line and make biological vision possible. I truly feel enlightened, thanks MEBuckner and scr4!
p.s. This is what I’m thinking about while watching The Descent.
Of some relevance, the July 2006 issue of Scientific American has an articles titled “What Birds See”, analyzing the roles of cones (light-sensitive cells at the back of the eye). Birds have four, sensitive to various wavelengths. Mammals lost two of them early on when being nocturnal was more important (developing light-sensitive “rod” cells) but regained a third through mutation. The article doesn’t mention it, but I can imagine genetic engineering creating a human with four or even more types of cones, broadening the range of color vision upward into ultraviolet and downward into infrared. The latter is more difficult, since the wavelengths get longer the corresponding cells must be larger. It may not be possible to get useful infrared vision without a major redesign of the eye.
Fortunately, the genetically-engineered human will have more and better cones to take up the slack.
One key factor is that the atmosphere blocks out most wavelengths in sunlight. There are “windows” that allow through visible light, some infrared, and radio waves, but most everything else is blocked. This includes everything shorter than the near UV.
From here, , which also has a diagram of the radiation “windows” in Earth’s atmosphere:
Yep, the “pits” of pit vipers (e.g., rattlesnakes) are infrared sensing organs. Some other snakes can also sense infrared. I don’t know what sort of resolution they get, nor if they in some sense “see” an IR image or not, or if it would be somehow more analogous to one of our other senses like smelling or hearing prey.
So, there are good reasons why eyesight tends to be concentrated in what we humans think of as “the visible spectrum”; but living things have also evolved different sorts of ways of using the EM spectrum, including parts of it we humans don’t use too well with our Mark I eyeballs (of course, we can build tools to use just about any part of the EM spectrum we want).
Another point to consider is that current evolutionary theory is that eyesight has evolved independently around four or five times on Earth, and all of these groups see in roughly the same range of wavelengths (AFAIK). Some can see a little farther in one direction or the other, but generally, we all see around the same spectrum. This lends support to the idea that these wavelengths are either (A) easier to detect, presumably because they interact well with the stuff we’re made of, or (B) dense with information important to survival.
Some insects and birds can see ultraviolet; some birds have plumage colored UV, and some flowers also have UV coloring to attract bees. I can’t find a cite, but I read an article some years ago about how some types of owls track prey by a faint UV glow left by their feces/urine.
As for humans, not only do we lack UV receptive cone cells, but our eye’s lenses block UV from entering in the first place. Link
As for infrared eyes for mammals, I expect you’d have to add whole new structures. Perhaps something like a pair of corrugated ( for cooling ) horns sticking out from the head with the “infraeyes” embedded in them for protection, kept at lower than body temperature. Rather like testicles, but better armored.
You can see in the near infrared up to about 780nm by blocking out everything below about 700nm with filters. Your eyes are not very sensitive to this frequency, but will become accustomed to the low light once the filters are applied.
Bill Beaty’s site explains how to do this. Heed the safety warnings!!
I get the feeling that the info from the pit viper’s heat pits are somehow overlaid or crossbred in the brain with the snake’s regular vision. I’m sure someone’s done experiments where they temporarily blind the snake, then introduce the prey to see if it’s accuracy is at all reduced.
Now we know why aliens have eye-stalks! They can see in the infrared! A huge plus when you come from a distant frozen world. I wonder tho, do they have ‘shrinkage’ problems?
As an undergraduate, in Herpetology class we did an experiment with a Tree Boa, which has heat sensitive pits in its lower jaw, whose vision was blocked with tape. The boa still showed excellent ability to sense a hot water balloon that we used as a test object. The balloon, of course, had to be much closer than a visual object to be detected. The snake was also much more sensitive in the direction the pits were oriented in, that is forward and a little below the head. - exactly where they would be most useful for aiming a strike at a mammal or bird. But I would guess that vision gives much greater accuracy.
I doubt whether this detection system is hooked into the visual system perceptually. It is probably something perceived rather differently by the snake. I would guess it is more like our ability to sense the direction of a heat source with the skin, but far more sensitive.
For imaging eyes, I’m only aware of the compound eyes of arthropods and the lensed eyes of cephalopods and of vertebrates. What one or two am I missing? Are you refering to non-imaging “eyes” like the lightspots on some single-celled creatures or the tuatara’s “third eye”?
I can think of two examples that you are missing and one that you are neglecting through lumping.
The first one is the eye that evolved in the cnidarians and is present today in the cubozoans. These are true imaging eyes complete with lenses, retinas and occassionally irises.
Eyes with lenses capable of forming true images are also found in some annelids and the onycophorans and probably represent a single evolutionary innovation.
The one you are lumping is the compound eye of the uniramia cf. the biramia/chelicerates. The arthropods are polyphyletic, having evolved at least twice from annelid ancestors. So the imaging compound eyes of the arthropods evolved indendently at least twice from the non-imaging eyes of annelids. There are those who would argue that the chelicerates and crustaceans are also polyphyletic, in which case imaging eyes arose at least 3 times independently in the arthropod “group”.
So the imaging eye is presumed to have evolved independently in the cnidarians, molluscs, polychaetes/onychophorans, chordates and at least twice in the arthropods for a grand total of 6 different instances.
I haven’t read through this whole thread, so I’m sorry if this has been mentioned, but there’s a plot of incident solar radiation in Jackson’s book Classical Electrodynamics that shows a very clear “window” wghere visible light falls. There’s a plot in the RCA Electro-Optics Handbook (Fig. 6-1) showing the spectral radiance at sea level of sunlight that has passed through the atmosphere, and it likewise shows that the most intense portion on a semilog plot is the visible spectrum. It’s not surprising that our eyes are most sensitive there.
Infrared “vision”, as with pit vipers, relies on nearby heat sources, rather than the sun, and is a whole different thiong. By the way, I have seen pictures of blindfolded snakes – they definitely can sense and strike at hot targets using their “pit vision” alone.
Cnidarians are the jellyfish and their kin, right? I had no idea they had eyes at all, much less imaging ones. And I’ve no clue what the cubozoans are, unless they’re 10 feet by 10 feet by 10 feet and fill a dungeon corridor completely.
I didn’t know this, either… Can you give me some examples of some arthropods which fall into one or the other clade?