Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body

This chapter describes the evolutionary history of our sense of smell. Noses typically are not preserved in fossils, as they consist of “soft tissues” with few bones (186). An understanding of the development of humans’ sense of smell must come from studying human DNA and comparing it to the DNA of other animals. Shubin explains that smelling works via a “lock-and-key mechanism” in which molecules floating in the air lock into unique sense receptors located on nerve cells within the nose (188). The nerve cells then send a signal to the brain, which interprets it as a distinct smell. Experiments conducted by biologists Linda Buck and Richard Axel demonstrated that three percent of the human genome consists of olfactory genes. Buck and Axel further showed that each unique odor receptor is controlled by a single, distinct olfactory gene.

Following Buck and Axel’s discoveries, scientists identified two general subsets of olfactory genes in other organisms, split between genes controlling water-based scents and those controlling air-based scents. Mammals contain far more olfactory genes than fish, which means animals developed a heightened sense of smell after leaving the water to live on land. In spite of humans’ large number of olfactory genes, roughly 30 percent of the genes are no longer functioning. These genes are evolutionary vestiges—made useless by genetic mutations but still carried in the DNA structure. Shubin hypothesizes that the large amount of useless smell genes in humans may be due to the development of eyesight: many of the olfactory genes could have become “functionless” because humans rely more on sight than smell in daily life (196).  

“Vision” opens with Shubin remembering a particularly rare fossil he encountered at a mineral shop in China: a “larval salamander” with a fossilized eye (198). Shubin then describes the general structure of human eyes, exploring their similarities and differences with those of other creatures. Eyes must be understood both as whole organs and as consisting of “constituent parts” (199), each of which has its own evolutionary history.

All animals with “a skull and backbone” (199), including humans, have a similar eye structure. Sight occurs when light travels through the eye’s lens, projecting an image onto light-sensitive cells in the back of the eye (or retina). These cells transmit information to the brain. Within these light-sensitive cells are molecules called opsins, which change their structure when exposed to light. Opsins exist within all forms of eyes and consist of a “twisted” structure that bears similarities to ancient bacteria (203). While most animals’ eyes contain two types of opsins, humans and some monkeys have three opsins, which allow them to see more colors. A heightened sense of vision may have developed in response to the evolution of colorful plants, as monkeys would have needed to “discriminate better among many kinds of fruits and leaves” in order to survive (204).

An experiment by the biologist Mildred Hoge found a gene called Pax 6 in fruit flies. Mutations of the Pax 6 gene could lead to eye deformities or a lack of eyes. Biologists later discovered that Pax 6 exists in all animals, including humans and mice. Biologist Walter Gehring experimented with activating the Pax 6 gene on various places of flies’ bodies, leading to the growth of an eye wherever the gene was activated. The experiment still works when a mouse Pax 6 gene is injected into a fly, causing the fly to grow an extra eye. The experiment shows that the same gene controls the growth of all animal eyes.