© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

13.5.1  Visible Vision

The "visible" range of light is actually slightly broader than the visual spectrum for human eyes. Our sight is normally limited to 4000-7000 Angstrom. Bees, however, are fully sensitive to ultraviolet radiation down to about 2500 Angstrom but they cannot see red. All animals have somewhat different reactions to visible stimuli, as shown in Table 13.2 below.

Many have advanced the interesting notion that ETs evolving under the light of other suns will necessarily have eyesight which is most sensitive to the frequency of peak output of their home star. Hotter stars, under this scheme, would spawn creatures with bright-light accommodation and heightened sensitivity to blue light. Cooler stars would give rise to organisms more attuned to the red. (See especially Anderson,⁶³ Clarke,⁸¹ Clement,²⁹² Macvey,⁶¹ and Shklovskii and Sagan.²⁰)

There are strong reasons to doubt the above hypothesis. Consider Table 13.3 below, which shows the visible radiation from various stars incident on planets located near the center of the Earth-normal habitable zone. Note that among all stellar classes of xenobiological importance the shift in the visible power spectrum is not dramatic. While some small variation exists in the relative intensities of blue, green and red from star to star, the differences are decidedly underwhelming -- hardly sufficient to represent a decisive evolutionary selective factor.

Furthermore, the peak sensitivity of terrestrial lifeforms is highly variable, ranging from 3600 Angstrom for bees (the equivalent emission peak of an A7 sun) up to 6500 Angstrom for seagulls (the equivalent emission peak of a K3 star).

And yet all of these creatures evolved under the same sun. It seems clear that eye sensitivity probably relates more closely to other environmental factors than the stellar class of the home sun. For example, it is often suggested that the wavelengths of light to which humans are most sensitive are virtually identical to the color of sunlight filtering down through a dense forest canopy of green vegetation and foliage, thus reflecting our arboreal origins. Aliens will answer in similar fashion to their own peculiar evolutionary heritages.

If sight is so important to living beings, then what kinds of photoreceptors might we expect to find on other worlds? Will ETs have eyes like ours, and if so, how many?

One of the most striking examples of parallel or "convergent" evolution in the terrestrial animal kingdom is the incredible similarity between the eyes of creatures with vastly different evolutionary histories. Animals in many separate phyla -- Chordata (amphibians, fishes, reptiles and birds), Mollusca (octopus, squid), Annelida (the alciopid worms Torrea and Vanadis²⁴⁸²), Coelenterata (the cubomedusan jellyfish²⁸¹⁶), and even Protista (the elaborate lensed eyes of dinoflagellates²⁸⁵⁶) -- have independently evolved photoreceptors surprisingly similar in structure to the mammalian eye. There are some discrepancies; for instance, the photoreceptor cells in molluscs point towards the light, the opposite of vertebrates.¹⁶⁹⁷ Nevertheless, close comparison reveals that the adjustable lens, retina, pigments, focusing muscles, iris diaphragm, transparent cornea, and eyelids all are immediately recognizable. The ubiquity of the eyeball is perhaps the clearest indication that it’s simply the best design for the job. Similar evolutionary forces and physical laws should lead intelligent ETs down much the same path.

Of course, the "camera lens" eye used by mammals isn’t the only imaging system lifeforms can use (though it’s probably the best for larger organisms). The next most common -- indeed more common if you just count species -- is the compound eye of insects and crustaceans.²⁵³⁵ Each organ looks like a small multifaceted jewel, but is actually a bundle of optical tubes. Light is directed down each of these tubes onto a large matrix of individual photosensitive spots on the retina. The image appears as a composite mosaic of tiny light-bits. Dragonfly eyes, to take an example, have more than 28,000 facets each.⁷⁹ Motion may be discerned as far away as 12 meters.

Unfortunately, the compound eye has only very poor resolving power. (See Table 13.4.) It has been pointed out that an insect, poring over this page of print, would be quite unable to make out the individual letters.⁸¹ This is why larger creatures who need large amounts of accurate, well-resolved visual data will probably find the compound eye an unattractive alternative on any planet. Yet for smaller organisms it is ideal. Part of the reason for this is the laws of physical optics. If a flea had a spherical lens eye like that of humans, the pupil would be so minute that diffraction effects would make clear imaging impossible.⁹⁵⁸ Once again we see that the worlds of size are truly worlds apart.

Other image-forming techniques are of limited importance on Earth, but this is no guarantee that the emphasis on other planets will be the same. For instance, alien species may utilize the pinhole camera concept. Such a system uses a open optical pit without lenses that is exceptionally useful in water. The beauty of the pinhole eye lies in its simplicity, and it has been adopted by at least one group of animals on Earth: the chambered nautilus.⁵⁸⁴

Another curious mechanism is the scanning eye of the snail. The image formed by a simple crystalline lens is scanned by moving a single retinal nerve sensor over the field of vision. The entire picture is slowly reconstructed from a series of these scans. Scanning eyes aren’t particularly useful for spotting rapid movements, but may be of value in highly viscous environments.

The principle of the optical reflector telescope has never been directly employed to form images by terrestrial lifeforms. But it is clearly possible to do so. And while pinhole and compound eyes cannot gather light but merely serve to redirect it, both lenses and reflectors can.⁴³⁹

Some animals have developed biological reflectors for other purposes. The luminous squid has retractable reflectors which, when coupled with its bioluminescence, produce a kind of searchlight. This elaborate apparatus, constructed of lenses, concave mirrors, diaphragms and shutters, emits a beam of illumination with which it sweeps the vicinity in search of prey.¹⁰⁰⁰

The common house cat also makes use of curved reflectors. When light enters the feline eyeball not all of it is absorbed by the visual pigment in the retina. In most mammals this light is simply wasted. But the cat has evolved a mirrorlike coating on the inward-facing front side of its eye, called the tapetum. This specialized device reflects unused photons back into the retina for another try at absorption. Efficiency is increased and sensitivity heightened, although the image is blurred very slightly.

Another specialized adaptation is the split-pupil eyeball. This allows an organism swimming along the surface of a liquid body to enjoy bifocal vision.⁶¹ The Four-eyed Fish, one of the largest tropical tooth carps (Anableps), actually has this remarkable feature. It is an elongate fish with eyes projecting upwards, each member of the pair divided into an upper part adapted for vision in air and a lower part adapted for vision in water. Any creature so equipped -- alien or Earthly -- can keep tabs on events above and below at the same time!

Another kind of visual perception is the sensation of polarized light. Many animals on Earth have this sense -- insects, crustaceans, birds -- although most mammals do not.*

Radiation coming from the sun is unpolarized. But when these rays are intercepted by a planetary atmosphere the plane of vibration is altered depending on the position of the star in the sky during the day. Honeybees, to take one example of many, monitor these shifting patterns of light and use this information to map out the shortest direct air route home. Sky polarization thus serves as a highly accurate navigational aid for these creatures.²⁵³⁰

Since sunlight reflected off a lake or ocean also is polarized, sentient avians of other worlds who must seek their prey by diving through a shiny water surface may be equipped with internal Polaroid filters. (Herons, kingfishers and other terrestrial birds have these.⁹⁶¹) Species inhabiting a "glassy" desert planet (with large expanses of volcanically or electrically fused surface sand) might also find this sense quite useful as sunlight bouncing off glass is polarized. And in constantly swirling irregular media, the detection of polarized light could help to maintain a proper orientation of "up" and "down."

We can imagine still more exotic schemes. Nature may have use for an eye with the ability to integrate light from a scene over a long period of time, much like photographic film. Such a system might be useful on a torpid, dimly-lit world (such as Venus) or in an environment populated by creatures with very slow response times. Since human rod cells are able to soak up dim light like a sponge for seconds at a time,⁸² there is no reason why aliens could not go us an order of magnitude or so better. This same ability could be used to design "wraparound eyes" for panoramic viewing.

What about eyes on stalks? Despite the attractiveness of this idea among devotees of the outré, both omnidirectional viewing (as found in some insects and terrestrial fishes) and independently swiveling eyes (like the chameleon) are generally seen by most xenobiologists as rather unlikely adaptations for thinking animals. It is said that eyes on mobile stalks are a feature more of animals likely to be preyed upon, and that "no superior intelligence could evolve in circumstances in which it lived in constant fear of being struck down and eaten."⁵⁰

Bonnie Dalzell has produced three additional arguments against eye stalks for intelligent ETs that are very persuasive:

1. Eye stalks require an hydraulic support mechanism, which is very inefficient except in small animals.

2. Eye stalks are too dangerous. Eyes are often the most vital sense, yet a predator could just clip them off with the stroke of a claw or pincer. And such organs are more prone to normal injury -- in an accident, stalks could be bumped, slammed, or squashed rather easily.

3. Stalks, in fact all independently-targetable eyes, present severe problems for the brain in making the correct parallactic computations necessary for binocular vision.⁴⁰⁰ (However, the chameleon seems to triangulate on his food quite easily.⁴⁷⁴)

How many eyes are best? It’s clear that nature usually chooses the cheapest way to do a given job. Certainly for senses that are not highly directional, it would seem that a single receptor organ should suffice. This is perhaps why most larger organisms have but one organ of smell and one of taste.

On the other hand, senses that are highly directional can make good use of the benefits of stereo. The ability to accurately triangulate a source depends on the simultaneous operation of at least two physically separated receptors. One organ of sight or sound gives 2-D resolution; a second organ, by making use of binocular or binaural sensing, gives three-dimensional coverage.

But there appears to be little to gain from using more than two sensors.

A single pair is sufficient to ensure spatial resolution with depth perception -- a quantum leap over single organs but not much poorer than three or more. Notes one xenologist: "Three eyes represent not nearly the same improvement over two that two represent over one."²⁰

Since the Principle of Economy tells us that nature invariably chooses the cheaper of several ways to do the same thing, this may partly explain why we have exactly two eyes and two ears. The advantages of having more than this number are slight or nonexistent, a conclusion bolstered by the apparent convergent evolution of stereoscopic vision among mammals, birds, and other animal groups on this planet.²⁵²⁰ Still, there is plenty of biological precedent for alternatives.

Cyclopean organisms are common in the microscopic world, but these are mere eyespots, useless for imaging or any discrimination between all but fuzzy patches of light. Scorpions and water fleas have a pair of compound eyes set so close together that they appear almost as one -- but these are not truly monocular.

It is possible that monocular vision might suffice even for large alien creatures. For example, a single eyeball that vibrated slowly from side to side could provide limited depth perception. Nearer objects would seem to move faster across the field of view than more distant ones, giving clues to their relative positions in space. (The human eyeball constantly quivers to prevent "visual accommodation," but the effect is too small to permit stereoscopy.)**

Very few animals have more than two imaging eyes, because third eyes don’t tell the being anything it didn’t already know. Reptiles (and man) have an ancient third eye called the pineal gland. In today’s reptiles it is only a day/night sensor and has no imaging capability whatsoever,²⁵²¹ and in humans it is fully degenerate. But such may not always have been the case. The skull of Cynognathus, an extinct Triassic Period theriodont reptile, definitely shows three eyes. This animal was mammal-like, possibly warm-blooded and probably hairy.⁶⁰⁰

Nereis, the common sandworm or clamworm, has four eyes. The Horseshoe Crab (Xiphosura) is also tetraocular, though two of the four are degenerate. Most insects are pentaocular, with three small eyespots called ocelli located on the upper part of the head between the two compound eyes. Once again, how ever, there are only two image-forming eyes.⁹⁶⁵

In a few cases, several batteries of eye-pairs are used during the successive phases of the hunt for food. Dr. Norman J. Berrill describes the dinnertime antics of the spider, which has four pairs of eyes:

The rear pair serve to watch behind for either food or danger. The other three pairs work together but in succession. If something comes within the range of vision of one of the outermost pair, the head turns until the object is brought into the field of the two pairs of eyes in the middle, and the spider then advances. When the object is brought into focus of the forward pair, the spider jumps to attack. The whole business is much like a self-operated mechanism with seeing instruments, and the eight eyes together do not compare with the camera or the compound eye of a bird or a bee.⁸⁹

The ultimate limit is probably reached by the scallop, whose literally hundreds of tiny, beautifully constructed "eyes" are spread around the circumference of its mantle like running lights on an ocean liner. These sense organs are very limited in function, however, as they cannot image. They serve only to initiate an automatic escape reaction at the approach of a hungry starfish (the scallop’s natural predator) heading in from any direction.

Specific environments can be imagined in which more or less than a single pair of eyes might be selectively advantageous. But the cost in added neurological equipment will usually be prohibitive. From the many examples above we know that more or less than two eyes have seldom proven more adaptive than exactly two.

Large sentient aliens, if they see by visible light, will most likely be binocular.

* Humans can see polarized light, but just barely. In natural skylight "Haidinger’s Brushes" appear as a small yellow and blue Maltese cross in the center of the visual field, the blue segment oriented parallel to the plane of polarization. The phenomenon is so close to the borderline of perception, so faint and unreliable, that it cannot serve a navigation function for man.²⁵³¹

** It should be pointed out that the possession of two eyes does not guarantee stereoscopic vision. The rabbit and the woodcock, for instance, have eyes located on opposite sides of the head -- which provides almost no binocular field.⁸²

Last updated on 6 December 2008