Are human eyes also efficiently coded? They don't seem to be. The sky and sea make up much of our natural scenes, yet only 6% of our cone cells detect blue, and they are mostly located around the edge of our retina. Of the remaining cones, the ratio of red to green cones varies wildly between individuals.
To find out why this is, Tkačik, along with neurobiologist Vijay Balasubramanian of Penn and colleagues, created a database of more than 5000 high-resolution photographs taken at various locations in Botswana, a place near where humans likely evolved and other primates still live. The same scenes were shot at different times of day, with different exposure lengths, apertures, and distances from the camera. Using an algorithm they developed from previous studies of how human cones detect light, the researchers calculated how many photons of different wavelengths the camera had captured and what cone arrangement would pick up the largest number of them.
The actual pattern of cones in the human retina matches the algorithm's predictions, the researchers reveal in a paper uploaded to the arXiv database this month and another published in PLoS Computational Biology. Red and green cones would pick up more photons from the images than could blue cones. That explains why the eye makes so few blue cones and places them around the periphery of the retina rather than at the center, where light focuses, Balasubramanian says. Red and green cones, however, pick up about the same amount of information, so there's no evolutionary benefit in keeping their ratio tightly regulated.
Now it seems to me that some caveats are in order. The images captured here are from contemporary Botswana. While we're a young species in geological terms, there still could be some significant changes from the African landscape that our protohuman ancestors saw.
Still, the idea that Africa shaped the evolution of our eyes is a valid hypothesis, and interesting. It's also a good reminder that our vision is selective.