For some background on vision, particularly color vision, read my previous post.
Millions of years ago, before the dinosaurs roamed, the remote ancestors of mammals probably had magnificent color vision. When the dinosaurs came to ecological power, these ancestors (mammal-like reptiles), were banished from the daylight hours and became creatures of the night. During this period, their visual systems evolved be maximally sensitive to light, allowing their proficiency at color discrimination to deteriorate in exchange. Although many mammals have returned to reclaim their diurnal dominion, their color vision still pales (get it?) in comparison to that of other vertebrates, such as reptiles, fish, and birds, whose ancestors did not suffer this period of exile.
Most mammals (some primates, including humans, are exceptions) rely on a dichromatic visual system, with cones that respond optimally to either short (S) or medium (M) wavelengths. (For more on this, see the primer) Our remote ancestors, along with the aforementioned other vertebrates, had three, four, or possibly more types of cones. A broader range of cones enables an exponentially enhanced ability to distinguish colors, but was unnecessary during our ancestors' tenure as nocturnal animals and was subsequently lost.
Our more proximal ancestors, apes and Old World (African) monkeys, regained a third cone, the long (L) wavelength-responsive cone, an adaptation that was instrumental to our evolutionary success. This cone allowed primates to distinguish red from green, which would have been advantageous for purposes of foraging. There are two major questions that arise from this trichromatic revival: How did it happen? and How did our higher visual systems adapt to interpret/perceive this new dimension of sensory experience?
The first is a question of genetics, which I'll answer briefly before delving into the neuroscience. Outside of the lab, new genes aren't added to the genome de novo; they arise from duplications of extant genes. Over time (on an evolutionary timescale), mutations change one or both copies of the gene such that they end up coding for different proteins. The genes that code for the M and L cones are very similar, and are on the same chromosome; it is thus likely that when Old World monkeys regained trichromacy, the gene for the M cone duplicated and mutated to form a gene that coded for the L cone (the divergence of color sensitivities would have been favored by natural selection). It is further believed that the M cone originated from a duplication/mutation/diversion combo from the S cone (which is on a different chromosome).
So that's how it happened on a genetic/peripheral level, but what about the mind? What good is it to be able to have this increased diversity in the retina if the brain is only capable of processing a dichromatic visual world? How much time had to pass before we could not only see this new dimension of light, but could interpret it as well?
A very exciting paper came out recently in Science, in which a group led by Gerald Jacobs at UCSB (the king of color vision, I'm told) used genetically engineered mice to model this crucial adaptation to trichromacy. Mice are, like most mammals and our ancestors, dichromatic, with only S and M cones. The group furnished the mice with the gene for the human L cone, resulting in mice with all the retinal components of trichromatic vision. They then sought to determine whether the mouse nervous system would be able to capitalize on this new information, thus endowing the mouse with an enhanced sensory experience. Alternatively, the visual system may be genetically wired such that it could only handle inputs from the two existing classes of cones, implying that the homologous primate adaptation would have required generations for adaptive rewiring.
To address this question, the group wanted to see if the "trichromatic" mice were able to distinguish between two colors which their dichromatic siblings (and ancestors) would have considered the same. To do this, they put them in front of three colored panels, on which two different wavelengths or intensities were displayed (they should be able to distinguish, for example, a dim green light from a bright red light...for background see primer). The mice were trained to identify which of three panels was illuminated with a different color than the other two; a correct choice was rewarded with drops of soymilk. After about 17,000 tests (literally), most of the mice with the S, M, and human L cone were able to learn the task, and chose correctly 80% of the time! (Mice with only the S and M cones still performed no better than chance after a similar number of trials.)
This demonstrates, quite wonderfully, that "the mammalian brain is sufficiently plastic that it can extract and compare a new dimension of sensory input." It follows, they speculate, that the primate that first inherited an L cone would have immediately reaped the benefits, and would likely have enjoyed a selective advantage.
I think this is quite a marvelous find, and the paper made me happy and excited, but I am also not entirely surprised by the result. It is merely a reminder of the essence of evolution, in that it tends to be guided by everyday biological mechanisms; things that seem special or mysterious in our natural history and our living world are made possible by these same mechanisms. I've previously discussed this idea that the parsimony of evolution would allow novel adaptations in the periphery to exploit previously evolved central circuitry. For this study, I think it's helpful to supplement this idea with a brief discussion of the striking plasticity of the developing nervous system.
When an animal is developing, the pathways of the visual system are highly sensitive to certain visual stimuli, and grow and refine under the guidance of the animal's experience. Some of you may have heard of experiments in which kittens were raised in boxes in which their only visual stimuli were black and white vertical stripes. Their visual pathways developed such that neurons in the visual processing areas responded primarily to vertical lines, and failed to respond to horizontal lines when presented later in life. In a sense, their brains were not able to see horizontal planes.
It seems to follow that if an animal is endowed with a "new" element of visual stimulation, rather than deprived, its developing visual pathways would accommodate this visual world by wiring up accordingly. As the brain of a dichromat was already capable of comparing information from two different classes of cones to compute a color representation in the brain, it may not have been too demanding a request to factor a third into the equation. Importantly, this adaptation would take place during the development of the first animal that possessed the new cone, as opposed to an evolutionary timescale.
This brings up an intriguing thought: what would happen if a mutation arose that endowed humans with a fourth cone? Our resulting percept of the world is a bit difficult to fathom...would we be able to distinguish light mixtures that normal people cannot? What would these colors look like? What if this new cone was able to detect light outside our visible spectrum--ultraviolet light, like bees and some birds, or infrared light, like rattlesnakes?