Your Inner Fish

In the 1980s, molecular biologists revolutionized the approach to anatomy and developmental biology, so much so that some molecular biologists suggested that their research would replace “dead end” disciplines like paleontology. Yet Shubin explains that the fossil record is still a valuable source of evidence, working with DNA records to fill in the gaps of information.

Shubin explains how to extract DNA from a plant, blending together the tissues, adding salt, dish soap, and meat tenderizer, then letting the mixture separate until a white goop forms on top. This white goop holds the DNA, which scientists then analyze and compare among many different species.

One of the most incredible features of DNA is that every cell in the body, whether muscle, bone, or organ, holds all the DNA information for every other cell in the body. For example, locked within each cell is the DNA humans use to detect odors, though those genes are only active in the nasal area. Smell is one of the most ancient abilities of the human body.

Humans can pick out 5,000 to 10,000 different odors, as the brain registers different molecules floating in the air. As we breathe, these odor molecules come into the nose and are trapped in a patch of tissue with millions of nerve cells. The nerve cells bind to the air molecules and send signals to the brain that are read as a specific smell. Each air molecule has a specific receptor that matches that molecule’s particular shape. A single smell might involve many different molecules and their respective receptors in the nasal cavity. Fish, reptiles, mammals, and birds all share the same general framework for a sense of smell. Fish, however, must smell molecules in water, not air.

Linda Buck and Richard Axel made a major breakthrough in the sense of smell in 1991 by identifying the genes involved in smell. They started from three assumptions: that human genes for smell would resemble the genes for smell in mice; that these genes would only be active in tissues involved with smell; and that there would be a large number of genes involved in smell (based on the idea that the sheer number of chemical smell receptors would require many genes to produce). Buck and Axel found genes for each of the receptors for odor molecules, representing a full three percent of the entire human genome.

The smell genes are actually an important record of major transitions in the history of life. These genes had to change significantly when animals stopped smelling molecules in water and started smelling molecules in air. In an interesting twist, the most primitive fish still alive on Earth actually have receptors that can handle both water and air molecules. Furthermore, these fish have a relatively small number of odor genes. It seems that as animals became more complex, the sense of smell became more refined.

The “extra” odor genes in mammals seem to be copies of the few odor genes in primitive fish. The large number of mammalian odor genes most likely came from thousands of generations of mutations and duplications in the fish’s odor genes. Yet, paradoxically, hundreds of odor genes in humans are useless due to mutations that render them ineffectual.

Dolphins and whales help explain why some of the human odor genes are useless. As mammals, dolphins and whales have the same huge number of odor genes as all mammals that are specialized for air molecules. Yet dolphins and whales use their nasal passages for their breathing blowhole, and none of their odor genes are functional. It seems that, because dolphins and whales do not use their sense of smell, random mutations in the odor genes built up in the population until all of the odor genes were useless.

Similarly, advanced primates (the evolutionary ancestors of humans) began to rely more on their sense of sight to find food and escape predators. Thus, the sense of smell was less important and mutations in the odor genes did not negatively affect individuals. These mutations were then passed down and built up in the population as a whole.

The sense of smell gives a good window to how closely related species are, because the copies of the olfactory genes seems to change each time they are duplicated. The more similar the odor genes are in two species, the more closely related those species are. Human odor genes are most similar to primates, then other mammals, then reptiles, followed by amphibians, and then finally fish.