vl nly once in my entire career have I found the eye of a fossil creature. I wasn't in the field on an expedition, I was in the back room of a mineral shop in a small town in northeast China. My colleague Gao Keqin and I were studying the earliest known salamanders, beautiful fossils collected from Chinese rocks about 160 million years old. We had just finished a collecting trip to some sites Gao knew about. The locations were secret, because these salamander fossils have serious monetary value for the farmers who typically find them. What makes them special is that impressions of the soft tissue, such as gills, guts, and the notochord, are often preserved. Private collectors love them because fossils of this quality are exceedingly rare. By the time we ended up at the mineral shop, Gao and I had already collected a number of really beautiful ancient salamanders of our own from his sites.
This particular mineral dealer had gotten his hands on one of the best salamander fossils of all time. Gao wanted us to see it and spent the better part of a day trying to work the deal. The whole visit had a terrifically illicit feel. Gao spent several hours smoking cigarettes with the gentleman, speaking and gesturing in Chinese. Clearly there was some bartering going on, but not knowing Chinese I had no idea what offers were being put on the table. After lots of headshaking and ultimately a big handshake, I was permitted to go to the back room and look at a fossil on the dealer's desk. It was a stunning sight: the body of a larval salamander, no more than three inches long. In it, I could see impressions of the whole animal, all the way down to the little shells it ate as its last meal. And, for the first and only time in my career, I was staring at the eye of an ancient fossil animal.
Eyes rarely make it into the fossil record. As we've seen, the best candidates for preservation as fossils are the hard parts of the animals—bones, teeth, and scales. If we want to understand the history of eyes, then we can use an important fact to our advantage. There is a remarkable diversity of organs and tissues that animals use to capture light, from simple photoreceptor organs in invertebrate animals to the compound eyes of various insects and our own camera-type eye. How do we put this variation to use in understanding how our ability to see developed over time?
The history of our eyes is a lot like that of a car. Take a Chevy Corvette, for example. We can trace the history of the model as a whole—the Corvette—and the history of each of its parts. The 'Vette has a history, beginning with its origins in 1953 and continuing through the different model designs each year. The tires used on the 'Vette also have a history, as does the rubber used in making them This supplies a great analogy for bodies and organs. Our eyes have a history as organs, but so do eyes' constituent parts, the cells and tissues, and so do the genes that make those parts. Once we identify these multiple layers of history in our organs, we understand that we are simply a mosaic of bits and pieces found in virtually everything else on the planet.
Much of the processing of the images we see actually happens inside our brains: the role of the eye is to capture light in a way that it can be carried to the brain for processing as an image. Our eyes, like those of every creature with a skull and backbone, are like little cameras. After light from the outside enters the eye, it is focused on a screen at the back of the eyeball. Light travels through several layers as it traverses this path. First it passes through the cornea, a thin layer of clear tissue that covers the lens. The amount of light that enters the eye is controlled by a diaphragm, called the iris, which dilates and contracts by the action of involuntary muscles. The light then passes through the lens, which, as a camera does, focuses the image. Tiny muscles surround the lens; as these muscles contract, they change the lens's shape, thus focusing images from near and far. A healthy lens is clear and made up of special proteins that give it its distinctive shape and light-gathering properties. These proteins, known as lens crystallins, are exceptionally long-lived, allowing the lens to continue functioning as we age. The screen on which all of the light is projected, the retina, is loaded with blood vessels and light receptors. These light receptors send signals to our brain that we then interpret as images. The retina absorbs the light via sensitive light-gathering cells. There are two types of such cells: one is very sensitive to light, the other less so. The more sensitive cells record only in black and white; the less sensitive cells record in color. If we look around the animal world, we can assess whether animals are specialized for daylight or night by looking at the percentages of each type of light-sensing cell in their eyes. In humans these cells make up about 70 percent of all the sensory cells in our body. That is a clear statement about how important vision is to us.
Our camera-like eye is common to every creature with a skull, from fish to mammals. In other groups of animals we find different eyes, ranging from simple patches of cells specialized to detect light, to eyes with compound lenses such as those found in flies, to primordial versions of our own eye. The key to understanding the history of our eyes is to understand the relationship between the structures that make our camera-eye and those that make all the other kinds of eyes. To do this, we will study the molecules that gather light, the tissues we use to see, and the genes that make the whole thing.
Eyes come into focus: from primitive light-capturing devices in invertebrates to our camera-type eye with a lens. As eyes evolve, visual acuity increases.
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