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.
Shubin says he “found” a fossilized eye, but this discovery depended more on negotiation and people-skills on the part of Keqin than any fossil finding expertise of Shubin’s. Shubin suggests that scientific discovery takes many pathways and always includes some element of luck, especially in circumstances as rare as finding a fossilized eye.
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.
The eye as an entire organ has a developmental path through many different species, but the parts of the eye can be traced even further back to invertebrates. Shubin’s car analogy helps clarify how the many simple parts can build on each other, leading to the development of a complex piece of large machinery. Detailing the entire history of a car is difficult, but following the history of one tire is much easier and provides a lot of information about the car as a whole.
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.
Shubin gives a simple run-through of the function of the human eye, glossing over many of the trickier aspects of sight to give a basic understanding of the entire mechanism. Vision is by far the most important sense to the average human, and the human eye is one of the most fine-tuned sight organs on Earth.
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.
Though the human eye is incredibly complex, Shubin draws the similarities that human eyes have to other animal eyes through their component parts of. Calling back to the car analogy from earlier in the chapter, Shubin is comparing cars to motorcycles and bicycles by focusing only on tires.
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.
In another example of similarity in animals despite perceived difference, there is no grand new mechanism for seeing in color, just more of the same opsin proteins that see in black and white, and that are slightly tweaked for color vision.
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 path of opsins through cells calls back to Chapter 7 and the mechanisms that cells use to communicate with each other. Opsins just have a more specialized version of this same basic practice. This feature is shared with even the most simple life on Earth that has no real sense of sight.
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.
Again, the mechanism of copying genes with mutation creates a new ability out of old, shared parts. Mutations that change the way an animal’s sense works are only passed down to the next generation if they are beneficial to the animal. Shubin explains why better color vision would have been helpful to advanced primates, leaving the door open to whether humans could continue to improve this sense if there were an environmental reason to do so.
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.
Shubin constantly seeks to bring back together groups of species that biologists have deemed separate based on physical traits or outward appearance. Shubin does not explain why one method of gathering more light molecules might be better than another, but the vertebrate eye has more room to improve vision by increasing the number of bristles, whereas invertebrate eyes can only handle a specific number of folds.
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.
Here, the polychaete worm acts as the “bridge” for vision the same way that Tiktaalik is the bridge for limb formation. Polychaetes bring together vertebrates and invertebrates by combining features of both distinct groups. Significantly, Arendt had to look at both physical structures and genes to see these similarities that were not fully apparent on the surface of the worm.
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.
As when studying mutations in hands or body plans helped to isolate genes for limb development or Hox genes for body plans in chapters 3 and 6, geneticists can use the same approach to isolate the genes for vision. Crucially, animals as different as flies, mice, and humans seem to have the same gene for eye development, though the eyes of these animals look quite different.
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.
The similarity between the Pax 6 eye gene in many different species is a huge boon to scientific research, as it is much easier to accept experiments on flies that cause mutations in eyes than it is to accept experiments on animals such as mice. Gerhing’s study of Pax 6 shows one way that the fundamental similarity between all animals (even flies and humans) can benefit human health care by teaching us how our same genes work.