Your Inner Fish

Shubin describes the only time he has ever found a fossilized eye. In a small mineral shop in China, Shubin and his colleague Gao Keqin bought fossils of 160-million-year-old salamanders. Keqin spent considerable time negotiating in Chinese before Shubin was allowed to go into the back room and see a fossil of a larval salamander with its eye intact. Eyes are incredibly rare in the fossil record, as they are made entirely of soft tissue.

There are many different types of eyes still used by animals alive today. The eyes of invertebrates give an important look into the history of the parts that make up the complex human eye. Shubin compares the eye to a car, where the development of the car as a whole also incudes the development of pieces such as tires and the rubber that tires are made of.

Human eyes function like cameras. Light enters the eye and is focused on a screen (the retina) in the back of the eyeball after passing through the lens. Tiny muscles in the eye control the iris, a small opening that controls how much light is allowed to enter the eye, as well as the shape of the lens itself. The retina has two types of light receptors that send signals to the brain. More sensitive receptors see only black and white, while less sensitive receptors see color. All of these cells make up about 70% of the sensory cells in the body, showing how important vision is to humans.

Most animals with a skull have this camera type eye. Other animals have different eyes, from light-detecting patches, to compound eyes in insects, or simple versions of the camera eye. Shubin compares all these different kinds of eyes by studying the molecules that gather light, the tissues in the eye, and the genes that direct eye production.

Light-gathering Molecules. The molecule that collects light breaks into two parts when light is absorbed: Vitamin A and a protein called opsin that sends an impulse to the brain. Animals need three different opsins to see in color, and only one to see in black and white. Every animal with the ability to see light uses the same kind of opsin molecule to do so.

Opsins transmit messages by carrying a chemical across the membrane of a cell, then helping the chemical follow a specific twisting path through the cell to the nucleus. This same twisting path is seen in certain molecules in bacteria, tracing the history of vision all the way back to single-cell bacterium.

The development of rich color vision unique to primates (including humans) comes from a change in the gene that makes light receptor molecules. Primates have three of these genes, where other mammals have only two. It seems that primates copied one of these genes, just as mammals copied the odor genes and gained a better sense of smell. A mutation that increased color vision would have benefited primates who could better discriminate between different kinds of fruits and choose the most nutritious. Scientists estimate that color vision arose about 55 million years ago, at which time the fossil record also suggests that forest plants became more diverse.

Tissues. There are two main types of eye, the invertebrate eye and the vertebrate eye, each using a different method to increase the amount of light-gathering surface area in eye tissue. Invertebrate eyes have many folds in the tissue, while the vertebrate eye has bristles projecting from the surface of the eye. Scientists could not understand how to bridge the gap between the two types of eyes until 2001, when Detlev Arendt studied the eyes of a primitive worm called a polychaete.

Polychaetes are among the simplest living worms, but they have both a true eye and light sensing patches in their nervous system under their skin. Arendt studied these physical structures and the genes that created them, finding that the eye was a normal invertebrate eye but that the light-sensing patches had the opsins normally found in vertebrate eyes. These patches even had primitive versions of the little bristles of vertebrate eyes.

Genes. In order to understand how eyes that look different can be related, Shubin turns to the genes that create eyes. In the early 1900s Mildred Hoge studied flies with a mutation that gave them no eyes at all. A similar mutation in mice and humans creates individuals missing large chunks of eye tissues. In the 1990s, geneticists found that these mutated flies, mice, and humans had similar DNA sequences on a specific gene. Scientists then began to study this gene, then called “eyeless,” through fly populations, to pinpoint how this gene was responsible for forming eyes.

Walter Gehring isolated the eyeless gene and was able to insert the gene to form eyes all over flies’ bodies. Gerhing then used the mouse version of the eyeless gene and was able to insert that genetic code to make an eye on a fly’s body. DNA from a mouse was able to make the eye of a fly by acting as the “on” switch for a complex chain of gene activity in fly cells. This same gene, now called Pax 6, is responsible for the development of eyes in any animal that has eyes.