The Light of Our Sun

Human eyes, and all vertebrate eyes, are actually protrusions of the brain. Michael Sobel explains, “Embryological studies show that the eyes begin to develop very early as two small buds on the neural tube that eventually becomes the brain.”35 We often think of ourselves as the pinnacle of evolutionary adaptation, but the human eye is adapted to detect a very narrow slice of the overall electromagnetic spectrum. This is not at all accidental. Had our Sun been a brown dwarf, emitting most of its light as infrared, our eyes would likely be adapted to infrared wavelengths. Some animals and insects, in fact, can see in wavelengths altogether in­visible to us. Pit vipers have a sensory organ located near their eyes that allows them to detect infrared or thermal images of prey. But­terflies are thought to have a wide range of vision and can identify potential mating partners by ultraviolet markings on their wings. Bees, many types of fish, and some birds also see in ultraviolet light. Scientists have recently discovered that flowers and plants, when viewed in ultraviolet light, often display patterns invisible to us and color quite distinct from the color we see. By imaging flow­ers in ultraviolet light, scientists realized that the patterns revealed in ultraviolet apparently serve as “landing strips” or markers use­ful in guiding insects to pollinating portions of the flower.36

At least forty to sixty different types of eyes, with up to ten dis­tinct means of forming images, have independently evolved, from simple constructions like a patch of skin acting as a photorecep­tor, to the compound eyes of flies and spiders, to the sophisticated eyes of the hawk or squid. “The result is an enormous range of eye types using pin-holes, lenses, mirrors, and scanning devices in various combinations to acquire information about the surround­ing world,” write Land and Nilsson. “Not all eyes are paired and placed on the head: there are chitons with eyes spread over their dorsal shell, tube worms with eyes on their feeding tentacles and clams with eyes on the mantle edge.”37 Various cave-dwelling ani­mals, because of a lack of light, have evolved without eyes. Some lizards, frogs, and fish, though they have complex eyes, also sport at the top or back of the head a third eye, or parietal eye, that at the least can indicate the presence or absence of light caused by the passing of a predator’s shadow. Even such a rudimentary eye offers valuable survival information.

At the other end of the spectrum of eye development are box jellyfish that have twenty – four eyes of four different types, two of which are extremely similar to human eyes. Existing for 600­700 million years, jellyfish have survived five mass extinctions but they’re often incorrectly characterized as not having a brain. Re­searchers recently found that neurons in jellies occur in neuronal centers distributed throughout their bodies and that they purpose­fully navigate within their environment. Box jellyfish use the acute vision of their pseudo-human eyes to see under and above water in navigating to and from underwater mangroves in order to feed, as a story in the New York Times reports: “Not only are the eyes equipped with a cornea, lens and retina, as human eyes are, but they are also suspended on stalks with heavy crystals on one end, a gyroscopelike arrangement that ensures the eyes are focused un­erringly skyward. . . . Every morning they must return to the roots or risk starvation. They rise toward the surface and their upturned eyes scan the sky, until at last they spy the mangrove canopy.”38 Beyond even these complex eyes are those of the cephalopods, like the octopus or squid, believed to be the most sophisticated eyes on the planet. Cephalopods focus their eyes with fine movements of the lens, unlike vertebrates, and they can automatically keep their pupils horizontal and even sense the polarization of light.