Interspecies Connectivity and Adaptation Theme Analysis

Survival of the Sickest focuses on the interconnectedness of all life on Earth. Moalem examines not only how humans pass down genes from one generation to the next, but also the constant and parallel evolution of other types of beings—plants, animals, bacteria, viruses—with which we share the planet. Even though many humans might believe that we live and evolve relatively independently of other organisms, in reality we frequently cause adaptations in other beings and vice versa. Moalem thus argues that no species exists in a vacuum: our evolution can often change the organisms around us by making some of their traits more advantageous, and this in turn can spur evolutionary changes in us. As such, all species are locked in a constant feedback loop of adaptation.

Moalem describes the broader connection between evolution in the animal kingdom and evolution in the plant kingdom, and how each group has adapted to find a symbiotic relationship. Plants that produce edible fruit evolved that way for their own benefit: animals want to pick the fruit and eat it, and then those animals deposit the seeds within the fruit somewhere else. This allows plants to spread out and grow in other places. Evolution favored the plants that developed this fruit, and in turn, animals that recognized the fruit as a food source were more likely to survive. This kind of exchange introduced mutually-beneficial adaptations between the plants and animals. As more animals developed habits to eat the fruit that the plants provided, animals also began to attack other parts of the plants that were more vital to them. Subsequently, plants found defense mechanisms so that other parts of them were not eaten. Thorns are the most obvious defense mechanism plants developed, but certain species like clover, sweet potato, and soy all contain phytoestrogens, which restrict animals’ reproductive capability. Thus, as animal behaviors adapted to the new food source, plants in turn evolved defense mechanisms against those animals. Other plants developed toxins and poisons, even evolving adaptations that are specific to species that will harm them. For example, chili peppers have developed a chemical called capsaicin, which stimulates the nerve fibers that sense pain and heat in mammals, but not in birds. This adaptation likely developed because mammals’ digestive systems break down the chili pepper seeds, but birds’ digestive systems do not. Chili plants that developed this mutation were more likely to survive, and thus that adaptation was selected for. These adaptations illustrate the ways in which plant and animal life are connected, and how their behaviors and biology adjust based on each new adaptation.

Parasites and viruses serve as other examples of organisms that adapt to changing conditions in humans and other species. Yet unlike the adaptations between the plant and animal kingdoms, which can frequently lead to mutual benefit or simply self-preservation, these organisms frequently operate only on a parasitic level, using human adaptation to their own benefit. In the first chapter, Moalem explains how dependent humans—and most species—are on iron. He writes, “Humans need iron for nearly every function of our metabolism. Iron carries oxygen from our lungs through the bloodstream and releases it in the body where it's needed. Iron is built into the enzymes that do most of the chemical heavy lifting in our bodies, where it helps us to detoxify poisons and to convert sugars into energy.” We also store iron in our macrophages (white blood cells) to help fight disease. Yet parasites, bacteria, and viruses also use our iron, and because macrophages try to fight against these kinds of organisms, those pathogens have adapted to use the iron in our bodies and grow stronger as a result. For every resource and leg up that humans have tried to gain over potential threats, infectious agents have evolved to try and combat those advantages. Parasites also try to hijack our natural responses to threats to their own advantage. For instance, when a person is infected with a Guinea worm (usually by drinking unclean water in remote tropical areas), the worm develops and makes its way to the surface of the skin. Once there, it secretes acid, burning its way through the skin to make an exit tunnel. The burning caused by the acid prompts humans to seek relief with cooling water—but the worm has evolved in such a way that as soon as it senses water, it emits a milky fluid full of thousands of larvae. Thus, it uses humans’ natural adaptive response (the desire to relieve burns through cooling) to its own advantage, as a signal to reproduce. In this way, our own behaviors—which have been driven by evolution—end up changing the behaviors and adaptations of other organisms, sometimes even to our detriment.

Moalem makes it clear throughout his book that changes in any organism can have a ramification on the adaptations of many others, but his analysis does not stop there. The goal of the book is to increase knowledge about these conditions and to highlight how this knowledge can be useful. He writes, “[infectious agents] are faster, but we’re smarter.” Understanding how parasites use our behavior to our disadvantage can allow us to curb that behavior. For instance, President Jimmy Carter led a two-decade effort to spread information about the Guinea worm, which led to a drop in infections from 3.5 million in 1986 to 10,674 in 2005. Likewise, recognizing how we have impacted plants, causing them to develop phytoestrogens, helped us develop the birth control pill in turn. The more information we gain on the interconnectedness of all life, the more we can use that those adaptations to our advantage.