Survival of the Sickest

by

Sharon Moalem

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Interspecies Connectivity and Adaptation Theme Analysis

Themes and Colors
Evolution and Illness Theme Icon
Interspecies Connectivity and Adaptation Theme Icon
Environment, Ancestry, and Race Theme Icon
Genetic Expression, Acquired Traits, and Mutation Theme Icon
Interdisciplinary Science and Research Theme Icon
LitCharts assigns a color and icon to each theme in Survival of the Sickest, which you can use to track the themes throughout the work.
Interspecies Connectivity and Adaptation Theme Icon

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.

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Interspecies Connectivity and Adaptation ThemeTracker

The ThemeTracker below shows where, and to what degree, the theme of Interspecies Connectivity and Adaptation appears in each chapter of Survival of the Sickest. Click or tap on any chapter to read its Summary & Analysis.
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Interspecies Connectivity and Adaptation Quotes in Survival of the Sickest

Below you will find the important quotes in Survival of the Sickest related to the theme of Interspecies Connectivity and Adaptation.
Chapter 1 Quotes

Our relationship with iron is much more complex than it’s been considered traditionally. It’s essential—but it also provides a proverbial leg up to just about every biological threat to our lives. […] Parasites hunt us for our iron; cancer cells thrive on our iron. Finding, controlling, and using iron is the game of life. For bacteria, fungi, and protozoa, human blood and tissue are an iron gold mine. Add too much iron to the human system and you may just be loading up the buffet table.

Related Characters: Sharon Moalem (speaker)
Page Number: 6
Explanation and Analysis:
Chapter 4 Quotes

On the other hand, as much as plants want animals to eat their fruit, they don’t want animals to get much closer than that—when creatures start to nibble on their leaves or gnaw at their roots, things can get tricky. So plants have to be able to defend themselves. Just because they’re generally immobile doesn't mean they’re pushovers.

Related Characters: Sharon Moalem (speaker)
Page Number: 78
Explanation and Analysis:

The next time you’re looking for some convenient birth control, you don’t have to snack on a field of clover, of course. But if you take many forms of the famous “Pill,” you’re not doing something all that different. The gifted chemist Carl Djerassi based his development of the Pill on just this kind of botanical birth control. He wasn’t using clover, though; he was using sweet potatoes—the Mexican yam to be exact.

Related Characters: Sharon Moalem (speaker), Carl Djerassi
Page Number: 79
Explanation and Analysis:

By releasing free radicals and raising the level of oxidants, fava bean consumption makes the blood cells of non-G6PD deficient people a less hospitable place for malarial parasites. With all the free radicals, some red blood cells tend to break down. And when someone with a mild or partial deficiency in G6PD eats fava beans, the parasite is in deep trouble.

Related Characters: Sharon Moalem (speaker)
Page Number: 90
Explanation and Analysis:
Chapter 5 Quotes

Most of these microbes are found in the digestive system, where they play crucial roles. These intestinal bacteria, or gut flora, help to create energy by breaking down food products we otherwise couldn’t break down; they help to train our immune systems to identify and attack harmful organisms; they stimulate cell growth; and they even protect us against harmful bacteria.

Related Characters: Sharon Moalem (speaker)
Page Number: 98
Explanation and Analysis:

Ewald believes that we can use this understanding to influence the evolution of parasites away from virulence. The basic theory is this—shut down the modes of transmission that don’t require human participation and suddenly all the evolutionary pressure is directed at allowing the human host to get up and get out.

Related Characters: Sharon Moalem (speaker), Paul Ewald
Page Number: 120
Explanation and Analysis:
Chapter 6 Quotes

By introducing the harmless virus to our bodies, we stimulate our immune systems to produce antibodies specifically tailored to defend against that virus. Then, if we are exposed to the harmful version, our bodies are prepared to defend themselves immediately.

Related Characters: Sharon Moalem (speaker), Edward Jenner
Page Number: 126
Explanation and Analysis:

According to Villarreal, this capacity of African primates to support the persistent infection of other viruses may have put our evolution on “fast forward” by allowing more rapid mutation through exposure to other retroviruses. It’s possible that this capacity helped spur our evolution into humans.

Related Characters: Sharon Moalem (speaker)
Page Number: 152
Explanation and Analysis:
Conclusion Quotes

I hope that you’ll come away from this book with an appreciation of three things. First, that life is in a constant state of creation. Evolution isn’t over—it’s all around you, changing as we go. Second, that nothing in our world exists in isolation. We—meaning humans and animals and plants and microbes and everything else—are all evolving together. And third, that our relationship with disease is often much more complex than we may have previously realized.

Related Characters: Sharon Moalem (speaker)
Page Number: 207
Explanation and Analysis: