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.
The progress of science cannot forget the past and look only to future projects, as Shubin has shown multiple times in the book so far. In Shubin’s eyes, the best way to further scientific knowledge is to build up from and collaborate with these “old” disciplines rather than just replace them.
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.
Shubin gives easy instructions that the average reader could use to separate DNA in a home kitchen, opening up the study of DNA – a seemingly prohibitively complicated line of research – to anyone with a blender.
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.
DNA is an essential similarity between all the various cells in an organism’s body, though the individual cells might look quite different. Shubin chooses to highlight that every cell in the human body holds the DNA for the sense of smell, but he just as easily could have made this argument with any other body system.
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.
Shubin gives a simple, succinct explanation for the mechanisms that allow humans to have a sense of smell. This basic template is shared by many creatures, another piece of evidence for Shubin’s argument that all animals have common traits most likely inherited from a common ancestor. Yet these shared characteristics must always respond to an organism’s environment, such as a fish’s smell being tuned to water instead of 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.
Three percent may sound small, but it is actually a statistically large portion of the human genome. Though the sense of smell as a whole is complicated due to the sheer number of odors a human can identify, Buck and Axel’s assumptions were able to cut through the noise and find incredible results. Shubin does not explain how the genes for smell in mice were already known, but it is worth noting that the entire mouse genome has been sequenced, so isolating the genes for smell would be as simple as comparing a “normal” mouse genome to the genome of a mouse who had a dysfunctional sense of smell.
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.
Shubin does not explain why primitive fish would have had genes capable of identifying odors in air when they lived their entire lives in water. These explanations may come as there is more study on the individual genes of living fish. Starting from this basic template, it seems that animals in water and then on land adapted the same odor genes to their specific environment.
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.
The more a gene is copied, the more likely it is that some mutation will take place. The mutation will then be passed down to the next generation if it is beneficial to the animal and helps the animal have more children. Yet mutations can also be passed down simply if they are not harmful to the animal and don’t prevent it from having children.
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.
Dolphins and whales are different from humans in huge ways, but there is a fundamental similarity in our shared classification as mammals. In the developmental path of life, dolphin and whales’ mammal characteristics suggest that they are descended from land animals that returned to living solely in water, and then had no more use for a land animal’s air-specialized sense of smell.
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.
Shubin’s argument depends on the idea that humans are descended from advanced primates and therefore inherited their sense of smell while continuing a lifestyle that de-emphasized smell for survival. Shubin doesn’t say whether the sense of smell is as specialized as possible in animals, or whether it is still possible that smell-dependent animals could acquire new odor genes through the same copying mechanism that refined the mammalian sense of smell in the first place.
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.
The odor genes support the path of descent that Shubin originally traced through limbs and other body systems in the first few chapters. The more different sources of evidence that support this lineage, the more likely it is that this proposed developmental path is correct.