The Evolution of Trichromatic Color Vision
published: Oct. 20, 2010, recorded: April 2009, views: 2867
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Sometime around 100 million years ago, when the continents of Africa and South America were still in touch, a female primate -- one of our ancestors -- was born with the capacity to see in vivid color. Jeremy Nathans describes the fortuitous genetic event that gave rise to this evolutionary leap, and links an ancient biological timeline to his very current research in human color vision.
Nathan’s talk, spanning eons and disciplines, starts with Isaac Newton’s astonishing 17th century experiments into the physics of colored light, and his prescient guess that the human brain could somehow translate colors the way it interpreted sound vibrations. The physiology behind vision didn’t coalesce until the 19th century, when a picture emerged of photoreceptor cells, with rods for night vision and cones for color. 20th century science finally cracked the photochemical mechanism behind light sensing.
In the 1980s, Nathans became interested in “making a dent in the area of identifying (genetic) sequences of the visual pigments.” He describes how he isolated the DNA behind the light sensors responsible for human color vision -- the short(S), medium (M) and long (L) wavelength receptors. He also discovered a diversity of genetic variations in normal, trichromatic vision. Indeed, he says the sequences lend themselves to all sorts of “mischief,” which can result in what’s commonly described as color blindness. When genes for the M or L pigments are not expressed, humans lose various degrees of color discrimination. When Nathans shows a picture of fruit from the perspectives of those with normal and abnormal color vision, it’s clear how “trichromats” enjoy an advantage in detecting ripe foods, or just enjoying scenery.
From his genetic research, Nathans became interested in how some mammals made the leap from dichromatic to trichromatic vision. Simple creatures such as honey bees and tropical fish are blessed with better color vision than humans, but among mammals, only a subset of primates have moved to trichromatic vision. Lower mammals lack one of the three dimensions for color vision. Nathans conjectured a “happy accident” on the X chromosome in primates likely resulted in the genes for the additional dimension. In a groundbreaking experiment to “recreate in a mouse the first step in the evolution of trichromatic color vision,” Nathans knocked into the mouse genome a human L pigment gene in place of its M pigment gene, resulting in an animal with the capacity for distinguishing colors a normal mouse could not. “This argues,” concludes Nathans, “that acquisition of a new dimension of color vision is not so difficult after all.”
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