The origin of the human eye has long been a bone of contention between evolutionists and their detractors. How, ask the latter, could such a sophisticated organ have developed by chance? But evolution doesn't work by chance and probabilities; it operates under very narrow, circumscribed rules according to very specific environmental factors.Recently, evolutionary biologists have examined the relationship between key visual adaptations and specific survival pressures. For example, Nathaniel J. Dominy of the department of ecology and evolution at the University of Chicago is interested in the survival advantage bestowed by trichromatic vision. Based on inputs from three photopigments, trichromatic vision is found routinely in humans, and in apes and monkeys from Africa and Asia. All other mammals, including most monkeys in South America, cannot discriminate red and green because they possess only two photopigments.Evolutionary biologists investigating the spectra of foodstuffs suggest that trichromatic vision arose in the higher primates, such as this chimpanzee, because it helped the animals identify tender leaves with high nutritional content by their red color. Scientists had long believed that the ability to discriminate red and green was linked to the identification of ripe fruit against a background of mature foliage. Fruit, however, is conspicuous by its shape as well as its scent. Another potential food item might be young leaves, which tend to have a higher nutritional value and a redder hue than mature ones.Dominy spent a year in the jungles of Kibale National Park on the Congolese and Ugandan border in Africa, observing the eating habits of chimpanzees and three species of monkey. Using a spectrometer from Ocean Optics Inc. of Dunedin, Fla., he collected spectroradiometric and spectrophotometric measurements of the primates' foodstuffs. The Ocean Optics portable cuvette holder permitted colorimetric assays, such as the Coomassie Blue protein test, to be conducted on-site. A tungsten halogen lamp illuminated the food specimens at wavelengths of 350 to 700 nm, and the spectral response was conducted to the spectrometer with fiber optic cable. Reflectance spectra were modeled into the trichromatic perceptual space by calculating the quantal catch of each type of retinal cone. Dominy found that the primates were more likely to select the younger, redder leaves, which are both tender and nutritious.Although it also has been suggested that the pigment tuning in our visual system is optimized for life beneath a class G2 star like our sun, Dominy said that this does not account for nonprimate mammals' lack of trichromatic vision. Presumably, all species in this environment would benefit from such a visual system. Instead, he contends that trichromatic vision in these higher primates resulted from their unique foraging niche -- another example of evolution, rather than providence.