Chapter 8: Making Scents

This was by far my favorite chapter title because it combines making sense of how smell works and the word 'scents.' I found it extremely clever and appealing to my curiosity. He began this chapter by teaching us how to extract DNA in our kitchens, which was particularly interesting although I have already extracted DNA before reading this book. Shubin then went on to state that by extracting DNA from any tissue, you can decipher the history of any part of our body...including our sense of smell! We can do this because the apparatus used to detect odors in the environment are locked inside our DNA. He defined smell as the brain's response to molecules floating in the air (with the molecules binding to our nerve cells sending signals to the brain, which we interpret as smell). He compared the molecular part of smelling to a lock and key mechanism, only working once the molecules (key) connect to the nerve cells (lock). And if this comparison did not help readers understand completely, Shubin went on to compare smell to music, stating it was like a chord. By tracing how organisms smell, we can trace our history as fish, amphibians, and mammals. Through experimentation scientists discovered odor receptors have a characterized structure with a number of molecular loops that help convey information across the cell. These genes (in receptors) have a very specific activity (and has to be lots of these genes since many kinds of chemicals can stimulate different smells). Buck and Axel discovered that three percent of the entire genome is devoted to genes for detecting different odors (each makes a receptor for molecule), and these olfactory receptor genes are found in major transitions in the history of life. For example, research shows that smelling genes eventually split into two types: one picks up chemical scents in water, while the other one picks up these scents in the air; while the number of odor genes present increases over time (from relatively few to enormous numbers). As the organism has more genes, they have a more acute ability to discern different kinds of smells, with large number odor genes arising from many duplications of small number genes, present in more primitive species. However, many of humans' odor genes are rendered completely functionless by mutations that altered their structure beyone repair (these mutations crop up in genomes from generation to generation as time goes on). This sparked my curiosity because I did not understand why we have so many odor genes, but most were functionless. Some scientists, such as Gilad, were just as curious as I was when this was discovered, and through experimentation found that primates that develop color vision have large numbers of knock-out smell genes, trading use of smell for sight, or, in other words, relying more on sight than smell. All in all, this chapter taught us that our DNA carries baggage in our nose that are silent witnesses to our past (basically contains a veritable tree of life!).