QUOTE: “If your light-sensing organ is sensitive to some band of radiation that is, say, only half as intense as the visible band, then come dusk, you are pretty much dinner for the life forms that sense the intense visible band.”
I don’t quite understand this part. Is it somehow impossible for an eye to see a broad chunk of the spectrum and so it has to be adapted to the most needed part which happens to be around the yellow part of the spectrum?
Couldn’t an eye probably evolve to adjust itself to whatever is the peak frequency? Or have a few different eyelids that block out certain parts of the light spectrum? Or just have a pair of night eyes and day eyes?
Also, do any nocturnal animals see infrared instead of visible light?
The eyes of organisms on Earth have adapted to the intensity of light penetrating our atmosphere, which is determined by the concentration of various gases contained in the atmosphere. I guess what the quote means is that organisms that possessed light-sensing organs that detected light in ranges other than the most intense range on Earth would have a selective disadvantage compared to those that could see in the most intense range. “Come dusk” confused me at first – but I guess it means that, at a time when the intensity of incoming light is low, you’d be better off being able to see the most intense part of the spectrum (which would be still fairly detectable even at dusk) than in a less intense part of the spectrum (where radiation levels would be below your threshold at dusk).
I think another reason for our seeing what we call ‘visible light’ is chemical (this is my side of the argument :); the article is mostly physical). Electromagnetic radiation does different things to molecules depending on its wavelength (and hence energy). UV and visible light increases the energy of electrons in a molecule, causing a chemical change that I think a living system could easily detect, with the right proteins and pigments. IR radiation makes molecules vibrate (which is why it’s good for heat transfer). The time scale for this is quite short, but clearly it’s feasible for organisms to detect IR.
So why don’t we have IR vision? Maybe it’s just because we radiate such large amounts of our own heat that the background radiation would blind us. Also, the advantage gained by having both IR and visible-light vision isn’t all that great for the vast amount of resources that would be required (energy and ‘bandwidth’ in the brain, to start). Diurnal organisms wouldn’t have any use for IR, since visible light is plentiful and background radiation would be very high; nocturnal organisms can get away with some method of being able to see less intense visible light.
Our tragic inability to have X-ray or microwave vision results from their chemical infeasibility. X-rays, microwaves and radio waves just don’t produce the kind of changes in molecules that a biological system would be able to detect. So I imagine that most organisms in the universe with the ability to see would see in the visible or near to it (IR or UV).
The big thing here is resolution. Your eye works by having a whole lot of light sensitive gizmos at the back. As you can tell I’m not going to get too technical. These gizmos receive visible light and convert it into nerve impulses. The trouble is that these gizmos have a real size and take up real space. That is a problem because the more of them you have the higher the resolution you get. The eyes can’t possible see something smaller than the distance between any two gizmos. After that the object becomes microscopic. It’s invisible. As it is all the gizmos detect visible light. Imagine what would happen if the eye were adapted to see IR and UV as well as visible. Only every third gizmo would be able to see visible light. That would mean that the distance between visible light gizmos would be three times bigger, and objects would need to be three times larger just to enable us to see them.
You may not think that’s a great problem in exchange for being able to see IR, but it is. As objects get further away they become effectively smaller. That means they become microscopic sooner as well. Trying to see a grass seed or an animal footprint from 5 feet off the ground would be impossible because we wouldn’t have the resolving power. And of course that charging tiger would be just a blur until it was 50 feet away from you. All humans would behave as though they needed both reading and distance glasses, but no lenses could fix the problem. That penalty is too big for the minor benefits of being able to see IR and UV.
The other problem I the one that Roches mentioned. UV light and shorter wavelengths tend to shake organic molecules to pieces. Organisms have mostly evolved ways to keep them out to avoid being cooked. To be able to see with them you need to let them in, and that requires some re-engineering. Some insects and birds can see into the near UV, which is low enough in energy that it doesn’t cause massive problems.
IR light and longer wavelengths have their own problems because they won’t pass through water. Most of your body is water, including the stuff in your eye. You can’t see IR or radio waves without having some sort of antenna to capture the signal and pass it through the fluid of your eye and into the nerves. Kinda hard to evolve that system. Of course the fact that all ife evolved in the ocean makes it somewhat unlikely that any pigments able to capture IR even exists. The snakes ** Duck Duck Goose** mentioned use the standard heat receptors of the skin to detect prey, but these work by direct excitation rather than chemical alteration as is the case with the pigments in the eyes. That makes them much less sensitive, slower to react and harder to pack in to a small enough area to give decent resolution.
You could try overcoming these problems by having multiple sets of eyes but it won’t solve the fundamental problems associated with UV energy levels or IR water opacity. Of course eyes are very energy expensive organs, weight for weight more expensive than any other organ. You’d want to find a lot more food or avoid a lot more predators to make it worth the effort. Then you would need to wire the brain to decipher the signals from the different eyes. Evolutionarily these things aren’t likely to happen.
Actually, that’s not quite true. Because the eyes are constantly in motion, the eye-brain system is able to make out considerably more detail than the mere number of receptors would allow alone. Nevertheless, the basic principle still holds, that the finite receptor size places a limit on resolution; it’s just not a simple 1-to-1 relationship.
Another factor is that every wavelength of light is affected differently by lenses. If you have a high-quality camera lens (non-zoom), you’ll notice the alternate focus point for use with infra-red film. The wider the range of wavelengths handled, the worse the focus. Astronomers and photographers use complex systems of lenses made out of different sorts of glass to reduce the effect, but it’s asking for a lot to expect evolution to produce that.
I’m not sure about that. Differing distances make a lot bigger difference in focus than do differing wavelengths, but I can still make sense of a scene containing both near and distant objects. In actuality, I’m selectively focusing on near and distant objects, one at a time, but my brain does a pretty good job of merging the information so the scene looks seamless. Similarly, I imagine that an organism with both infrared and ultraviolet vision could selectively focus on one or the other, and merge the information in the brain.
Blake: Nice post. I didn’t think of the problems with transmission of IR radiation through water – reminds me of the times I’ve had residual water result in unreadable IR spectra. CO2 would have the same effect, so IR vision would have to compensate for absorbance by exhaled CO2, or your vision would blur every time you breathed out.
Do you know anything else about how snakes (or anything else) can see in the IR? Or how organisms with UV vision filter out the short wavelengths that would lead to decomposition of the receptors?
Nobody has mentioned that IR, while great AT NIGHT, is not really very good at all just around dawn/dusk, when the background IR level is roughly equal to the black-body IR spectrum given off by living organisms. Hence (perhaps) “come dusk…” in the original column (rather than “come nighttime”). Even the best military FLIR systems are worth f***-all just around sunrise/sunset. (Oh, and those that work during daytime almost always show IR targets as COOL [black] spots during daytime, WARM [white] spots at night, unless manually recersed by the operator - hence the cross-over points)
I’m not sure about the following, but it may also be well-nigh impossible to spot that Orange tiger behind that Green vegetation if you’re looking at both in IR (I think plants have some body heat, too :)). But I may be wrong about this part.
Picky picky. If I could be bothered I’d weasel out of it by saying that isn’t really seeing the object, it’s interpreting it’s existence.
Not much. AFAIK they just use the standard nerves that are designed to detect IR, the same ones that let you know not to stick your hand on a hot plate. These work by the simple method if heat causing them to fire simply by increasing membrane permeability. Pretty basic and very different from sight which utilises an actual chemical change brought about by light. As a result heat receptors take up much more room and react much more slowly. Tyring to ‘see’ using this system must be like using a webcam with something 4 x12 resolution and a delay of about ½ a second between what happens and what you see. Not a good system to avoid predators or trying to catch moving prey.
AFAIK they don’t, any more than we do. We just let the UV in and it passes more or less harmlessly into the ground material of the retina. The problem would come with trying to capture it with a pigment. The pigment itself would be shaken to bits. The thing is that early vision systems evolved in organisms without eyes as such. These organisms generally had pigments or proteins to block UV. Letting in UV to see by would involve letting UV into the cells generally, which isn’t a Good Thing.
If I could be bothered I’d weasel out of it by saying that isn’t really seeing the object, it’s interpreting it’s existence.