shkrobius (shkrobius) wrote,

Why are we afraid of the dark?

Many of us are afraid of the dark. All children are afraid of the dark. Remember how scary it was? Why?

Human eye, adapted to darkness for 30 min, can reliably detect 5-9 photons arriving in 100 ms; there are occasional reports of single photon detection. The rod cells in our eyes were shown to respond to single photons. The adaptation to darkness increases sensitivity of rod cells by a factor of 10,000. In fact, we have one of the most sensitive eyes in the entire animal kingdom. The only problem with our night vision is that it is scotopic: we cannot tell colors apart. Our eyes are as good as cat's (the typical nocturnal predator) in terms of contrast sensitivity but beat cat's eyes at greater spatial frequency. Nocturnal mammals typically have tapetum lucidum, a reflective membrane beneath the retina that collects and re-emits light back to the retina a second time, giving the rods a second chance to absorb the light. Night predators have large eyes with large pupils packed with rod cells, sacrificing color vision during the day for greater sensitivity during the night. And still they do not appear to beat us at night vision. That's because we also have more rods than cones; not that long ago, the primates were nocturnal, too. Cats and dogs are not afraid of the dark. So why are we afraid of the dark, if our eyes, under low light conditions, are as good or even better than those of the mammalian predators?

Why do we have good vision in the first place? is believed that several key traits that defined primates evolved together: large brains, front facing eyes with great color vision, and dexterous hands. There are two schools of thought on why this should be so, the “visual predation” and the “leaping” hypothesis. Advocates of visual predation believe that early primates feeding on insects were forced to develop great vision to spot well camouflaged food, plus hands that could grasp to catch it. The leaping hypothesis is similar in that it relies on the same combination of vision and dexterity, except the main driving force behind the changes was the need to navigate (i.e. leap about) the primordial forest canopy away from predators that lurked on the ground.

Alas, not all predators lurk on the ground, and leaping is not of much help during the night when most predators hunt. The primates, our ancestors, lived in the trees where they were out of reach to most predators except for constricting and venomous snakes. The recent thinking is that our uncommonly advanced, for mammals, visual system has evolved in response to this ever present danger:

...The initial change in primate eyes occurred when they had to deal with constricting snakes, about 90 Mya. That ended up with primates that have forward-facing eyes, whereas other mammals tend to have eyes on the sides of their heads. Forward-facing eyes allow better depth perception. When poisonous snakes evolved about 60 Mya, primates further specialized. That resulted in the anthropoid primates that had better vision all around. Some mammals evolved a physiological resistance to venom, but it was only the primates that went the visual route, strengthening the visual system to detect snakes. Neurological studies suggest that the visual system is actually connected to the "fear module," a group of brain structures in mammals involved in vigilance, fear, learning, and other behaviors. If you look at the brains, you find cells that respond to things that might represent snakes, like diamond-shaped patterns. (L Isbell, UC Davis)

Here is the problem in a nutshell: our night vision is no worse or better than that of predatory mammals but it is much worse than that of the snakes. Snake eyes are not only as sensitive in the dark as ours, they actually retain some color vision in the dark and have in addition ultraviolet (UV) sensitive cones and infrared (IR) pit organ sensors. We are afraid of the dark because the snakes and the dinosaurs are still lurking in the night. How did it happen that our vision is so much worse than that of the "primitive" reptiles? And it is not only the reptiles. Birds, frogs, fish, and even some insects have more advanced visual reception systems than us. In fact, mammals have one of the worst visual systems around. You can look at the diagram of opsins among different animal species here It is a sad story.

...the human eye is of an uncommon type, only shared by Old World monkeys and apes; it is trichromatic, as our color vision involves three distinct classes of cones (424, 530, and 560 nm). Retinas with four classes of cones involved in color perception (tetrachromatic vision) have been reported in birds, fish, and reptiles. Due to an additional class of cones, tetrachromats have the ability to see twice the number of colors compared with trichromats. Humans may hence be blind to many critical aspects of animal coloration and perception. We may not only perceive slightly different hues compared with other animals but also are possibly missing major components of animal coloration. Compared with humans, birds have an additional color channel located in the ultraviolet (UV) to near ultraviolet range (they have 370, 445, 508, 565 nm cones).

...the color vision of most birds and many kinds of lizards, turtles and fish is superior to that of mammals. In the 70's it was discovered that birds and other vertebrates were capable of seeing in near UV. The vertebrate line developed a vision system based on four color sensing pigments. The last common ancestor of birds and humans had tetrachromatic vision, with an extra cone pigment in the near ultraviolet. Unfortunately for us, mammals went through a stage where we were small nocturnal animals trying to avoid being eaten by the reptiles. Color vision has no survival value in the dark, so in a classic case of "use it or lose it" mammals lost two of the cone pigments. Once nature had dropped a rock on the dinos and primates were evolving, a third cone pigment was "reclaimed" through genetic duplication and mutation, leaving us with our current trichromatic vision. There's a gender aspect to this as well. The genes for our long wavelength cones lie on the X chromosome. Women have two X chromosomes and men have only one, so men are more vulnerable to mutations which leave them less capable of distinguishing reds from greens (color blindness.)

Our trichromatic vision is the result of a rare opsin mutation on the X chromosome that occurred ca. 40 Mya. Because the spectral difference in the absorption of light by the cones is very small (530 vs. 560 nm) we need large brain to process the slight image difference. Even now 8-12% of European males and 1% of females are red-green color blind. New World monkeys that developed trichromatic vision independently of the apes, have all of their males and some of their females dichromatic, whereas other females are trichromatic. This polymorphism persists supposedly because dichromatic vision helps to spot some types of camouflage better than trichromatic vision. Together, the polymorphic group can respond to a greater range of dangers and forage more fruit. In 'traditional" human societies color blindness rate is < 1% (except for some islands). Mark Ridley suggested that high rate of color blindness in Europeans is the onset of another period of regressive evolution in the absence of selective pressure (in "Mendel's Devil"). We are going backwards again.

For now, we still have color vision. The fortuitous opsin mutation backed up by our large brain is still no match to what non-mammals have in their store. We are better off than most other mammals (e.g., rodents) that have only two types of cones (dichromic eyes), but that's little consolation. Other vertebrates have three critical adaptations increasing color perception and sensitivity: color filters that narrow the photoresponse of the cone cells(greatly improving color perception), retractable rods, and double cones:

...The mammals diverged from reptiles at the beginning of the Mesozoic era--the same time during which other reptiles were transitioning into dinosaurs. Mesozoic mammals were small rodent-like creatures that were most probably nocturnal. The color vision of primates is not homologous to the color vision of fish, birds, turtles, etc. Much of the machinery used for primate color vision arose independently long after similar systems developed (without being lost) in other vertebrate lineages. Many features of ancestral retinal anatomy are lost in mammals. The phylogenies of the opsin molecules suggest that mammals have always retained two cone pigments, but any mammals that, like us, have more than two pigments (re)gained the third relatively recently.

...Many vertebrates have oil droplets at the bases of the light sensitive parts of their photoreceptors. These oil droplets often have pigments in them that absorb (i.e. filter out) some of the light that would otherwise stimulate the cell. What this does is to modify the spectral sensitivity of the photoreceptor bearing that droplet. This feature is not found in mammals. Many vertebrates have double cones--two cones that are joined along their long axes by tight junctions, gap junctions or both. Nearly all classes of vertebrates have some variety of this form of receptor in their retinas. This feature is not found in mammals. The photoreceptors of many vertebrates perform a sort of circadian dance. During the day, the rods are extended on long stalks so that their sensitive parts are buried in a layer of pigmented epithelium. This epithelium shields the rods so that very little light reaches them from the sides, and the cones basically shield them from axially propagating light. At night the cones are extended out into the pigmented epithelium, and the rods are contracted back to where the cones were during the day. This feature is not found in mammals.

We are afraid of the dark because Nature endowed us with an eye that is inferior to that of the predatory reptiles (snakes and dinosaurs?)and birds (that feasted upon mammals in the Eocene), our (primate) sworn enemies. That is who we are afraid of: they'd spot us first. The problem was not that the early placental mammals were nocturnal (nocturnal birds and reptiles have perfect color vision), but that they underwent rapid regressive evolution. Perhaps they were burrowing animals, like moles, that had almost no use for their eyes. The oddest part of the story is that our mammalian brothers, the marsupials (often denigrated as "primitive" mammals) have trichromic eyes, see, which is ancient and reptile-like. They can see in the UV, while the placental mammals cannot. That's how bad things are. We really got the bum deal.

Why is it us and not them?
Why are we afraid of the dark?

Suggested reading:
TH Goldsmith, What Birds See SciAm 295(2006)69
Goldsmith, T.H. (1990). "Optimization, Constraint, and History in the Evolution of Eyes", Quarterly Review of Biology, 65(3):281-322.
Lythgoe, J.N., and Partridge, J.C. (1989). "Visual Pigments and the Acquisition of Visual Information", Journal of Experimental Biology, 146:1-20.
R Dawkins, "The Ancestor Tale" pp 145-155
Tags: whys
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