Optics of pinhole and lens eyes. Source here
In my opinion there are two other major difficulties with echolocation that favour vision. The first is the ability to locate objects: with eyes such as ours it is very easy to locate objects. Rays of light can be bent by lenses and can reflect from surfaces, but in between they follow nice straight lines. That is the reason why even a simple pinhole camera such as in the image above will produce a good image: any particular point on the retina can only be lit be rays coming from a direction specific to that point. Such a pinhole will not let much light in, and solving that by increasing the pupil will blur the image. If you put in a lens you can have a large pupil for lots of light with a sharp image. Problem solved. The point of all this is that seeing an object is almost the same as knowing where it is.
From: Animal Eyes (2nd Ed.), Oxford; copyright Land & Nilsson
Above you see an image of lineages of eye design, leading to pinhole eyes and eyes with lenses. Those who wish to read more about eye evolution should read the new edition of Land and Nilsson's 'Animal Eyes'. It also describes the very high number of eye designs (there are even eyes based on mirrors!). Another very nice book is Evolution's Witness, with hardly any physics but boasting numerous examples of wonderful eye designs.
From: Animal Eyes (2nd Ed.), Oxford; copyright Land & Nilsson
In 1994 Nilsson and Pelger calculated that a good camera eye with a retina and a lens could evolve from basic elements without any localizing ability in fewer than half a million generations. With one generation a year this amounts to a geological blink of an eye (sorry for that one), meaning just half a million years. Vision can apparently evolve so quickly and conveys such a large advantage that some say it explains the runaway evolution known as the Cambrian explosion. In a very short time things such as armour, speed and vision evolved. Giving animals an unobtrusive ability for precise long-range sensing may just have been the impetus to start this accelerated runaway evolution: claws, shells, teeth and brains co-evolved quickly. Perhaps vision was not the only factor jump-starting the process, but the idea is too powerful to ignore a role for vision altogether, I think. It is tempting to think that most planets with complex life would have their own 'Cambrian Explosion' in the early evolution of complex animals. Of course, the label 'Cambrian' would not apply on Furaha, Snaiad, Nereus, nor on real exoplanets. We need a more general name for the phenomenon; how about the 'Sight Spark'? (and if it sticks can I copyright it?).
Back to sound
Seeing how a pinhole eye is simple and works well, how about evolving a 'pinhole ear'? Suppose we place lots of microphones on the inside of a sphere and cut a hole in front of the sphere to let sound in. Wouldn't each microphone only pick up sound from the bit of the world it 'sees' through the opening? If so, we would have an ear with perfect localising ability. Alas, no. Sound does not travel in neat straight lines but travels around corners. You can hear people talking through an open door even when you cannot see them.
Sound 'bending' around a building. Source here.
How the size of an opening affects diffraction of sound. Source here.
When sound waves hit objects, those interfaces form new sound sources, a process called diffraction. In the misbegotten 'pinhole ear' idea, the 'pupil' would simply act as a new sound source, so all microphones on the 'retina' would receive sounds from all directions. As a location device this would be utterly useless.
The physical reason why sound bends around corners and light does is not that diffraction is limited to sound. But in fact diffraction affects light and hence vision too. The effects of diffraction depend on wave length, and the wave lengths of sound have a range of a few cm to 15 or more meters; those of light are measured in micrometers. The diffraction of light is seen at microscopic scales, but that of sound occurs at the scale we live in, that of doors and people.
So the physics of sound conspire against it providing an easy way to tell where a sound is coming from. Evolution solved that problem as it did others, but the solution requires combining the signals from two ears (I know that using two eyes improves distance detection, but you can do it with one eye, and locating the direction of an object needs just one eye). Tiny differences in arrival time of a sound at two ears allow a suitable brain to calculate the direction of the source of a sound in the plane of the ears, but not whether it is to the front or the back nor up or down. Finding out things like that call for ingenious trickery such as tilting heads or complexly shaped outer ears that subtly change the characteristics of a sound depending on where it is. Wikipedia has a nice article on the subject. Some animals (owls!) perfected the art of sound location, but theirs is a small niche compared to the ubiquity of good camera eyes.
Mind you, I have no idea why there are no animals with more than two ears. Having four, placed at the corners of a tetrahedron, would be nice. Then again, perhaps there are arthropods with more than two functional ears. I have never heard of any but have not looked either. Are there any?
An unfair advantage of sight over echolocation
I wrote above that I thought there were two more difficulties with echolocation. The second one is based on the fact that echolocating animals have to produce their own signal, limiting the range at which they can detect anything. How about vision? There was an omission in the discussion, and it is a glaring one: the sun! (sorry about that one too). Sunlight, free for all and there regardless of whether anything of anyone is using it, is what allows vision to work as a long range sense. Compared to echolocation this free gift to sight is not really fair.
Copyright Gert van Dijk
The images above were made for the previous post: the right one showed what echolocation might be like, with some energy coming from the 'camera', only illuminating objects close by. Compare that to the left image, a visual scene lit by the much more powerful sun.
But vision is not always available, and echolocation has a chance when there is no light. On rotating planets like ours, sunlight is only available for half the time, so the night would seem a good time for animals to start echolocating. But that is not the case; most animals prefer to more or less shut down at night. In previous discussions on when echolocation would be better than vision some dieas came up: caves, planets with permanent fog, planets without suns and seas underneath ice caps. One region seemed to have been forgotten though: the deep dark seas, where the sun does not reach. Shouldn't they be filled with echolocating animals, squeaking and pinging away? For Earth whales, the ocean floor may be too deep to reach, but fish were there a long time before the first whale ancestor took its first dive. Why are there no echolocating fish? I asked experts, but they did not know either. Fish have suitable ears and brains, and nothing seems to stand in the way of them evolving echolocation. But they have not. Or is echolocation simply too much like a burglar who enters a silent dark house and then starts shouting 'Hellooo!'? I do not know.
However, I do know of one final twist in the comparison of echolocation and vision; but I will keep that for the last post on this subject...