As author Richard Firn reminds us in the first chapter of Nature's Chemicals: The Natural Products That Shaped Our World, “all organisms are made up of chemicals.” This is the starting point of a fascinating book about natural products (referred to as NPs) and how they have evolved and shaped the trajectory of civilization. Hundreds of thousands of individual chemical compounds are produced by plants and microbes; their variety within a single species accounts for the difference in taste between a pear and an apple, for example. The majority of NPs have their origin in three categories of chemicals: isoprenoids or terpenoids; polyphenols, phenylpropanoids, or polyketides; and alkaloids. These groups form the base of the metabolic pathways that produce the immense chemical diversity found today in living things.
Firn engages the reader by posing questions that we have all wondered about and then addresses them using examples that touch our daily lives. For example, he reflects on the eighteenth-century idea that naturally occurring chemicals are different from those made synthetically, and therefore, the former are to be favored: Mother Nature is, after all, the chief chemist. Individual molecules—synthetic or derived from natural sources—can have the same structure and properties, but when constructed into chemicals, they may not. Vanillin can be synthesized, but natural vanilla extract differs chemically (and monetarily) from synthetic vanilla flavoring. Natural vanilla extract contains compounds other than vanillin, and many consumers prefer the flavor and texture produced by the pod of the vanilla orchid. Some of these consumers have a desire to support developing-country farmers—even though the price of synthetic vanilla extract, made from an industrial fermentation process that converts ferulic acid into vanillin, might be much lower.
Controlling the supply of certain NPs—those that are in limited supply or restricted geographically in their production—can provide money and thus power to those who manage the resource. Human obsession for plants that produce coffee, tea, spices, morphine, cocaine, and quinine has influenced world history, and when these NPs are controlled by a monopoly, the geopolitical landscape of the world is altered. One example is cocoa, which contains theobromine—a stimulant similar to caffeine—as its key NP. With its early origins in Central America, the bean reached Europe in the late 1500s. Bolstered by its reputation as an aphrodisiac, chocolate became popular and spread throughout other parts of the world, where it was appreciated as a healthful drink. Chocolate merchants became wealthy as a result of their trade in this bean, which could only be grown in the tropics. The story of the coca bush— the source of cocaine—is much darker. Traditionally used by indigenous populations, the plant is now the raw material for the multibilliondollar cocaine trade. Coca is also one of the important ingredients in some soft drinks, considered by Firn to be “a very profitable way of selling water by adding a few cheap NPs.”
Nature's Chemicals summarizes the history of scientific thought addressing another question: Why are NPs produced by plants in such a variety of structures? An early belief was that plants and their compounds were created with the purpose of benefiting humans. Another belief was modeled on observations of animal physiology combined with the notion that plants must also eliminate waste. Without livers or kidneys, plants were thought to excrete their waste as “weird chemicals.” Another theory suggested that NPs were a sort of test chemical needed to generate variation in species, maximizing the number of differences in the population of a plant so that natural selection could identify the fittest individuals. Other theories are discussed, including the widely held chemical convolution model, whereby plants use NPs as a way of fending off predators, which at the same time are developing defense mechanisms to overcome these chemical protections. Firn points out weaknesses in this model and discusses a newer model to explain the extraordinary diversity of NPs.
Dubbed the “screening hypothesis” and proposed by Clive G. Jones and Firn in 1991, the new model seeks to explain NP diversity through the evolution of specific metabolic traits in plants: “There could be ways that organisms could maximize the production of chemical diversity and increase the chances of retaining even some “redundant” NPs. The retention of some “redundant” chemical diversity is necessary in order to seed the generation of new chemical diversity.” A section of a chapter in Nature's Chemicals is devoted to a complex and compelling explanation of this theory. As a student of ethnomedicine, I found this book to be fascinating and rich in information helpful for understanding the diversity of plant chemistry, its historic and contemporary importance to human society, and how and why plant organisms will continue to evolve novel compounds in a process that is both dynamic and ongoing.