Survival of the Sickest Summary

Sharon Moalem states that Survival of the Sickest is about medical mysteries, and he shares a personal anecdote of one such mystery: when Moalem was 15, his beloved grandfather was diagnosed with Alzheimer’s. His grandfather also loved to donate blood because it inexplicably relieved his aches and pains. Distraught by his grandfather’s memory loss and confused by this strange healing effect of blood donation, young Moalem decides to do some research at a medical library. Through reading about iron, he discovers that his grandfather has hemochromatosis, a hereditary disease in which excess iron builds up in the body and can cause major organ damage. One of the most effective way of treating this condition is through donating blood to reduce iron levels, which explains the relief his grandfather feels after doing so.

Despite others’ doubts, Moalem intuits that hemochromatosis and Alzheimer’s are somehow related, since the former condition must have a negative effect on the brain. He pursues biology in college and dedicates the early part of his medical education and research to proving that this is, in fact, the case. Moalem reveals that he, along with 30 percent of people with Western European ancestry, also have hemochromatosis. This is what motivated him to write Survival of the Sickest: he wants to answer the question of why seemingly harmful inherited diseases like his persist in the gene pool. The book will also delve into the interconnectedness of all life and all evolution on Earth, and it will show that genetics is much more nuanced and fluid than people tend to assume. In doing so, Moalem hopes to instill a sense of curiosity in the reader and to help them better understand and take control of their own health.

Moalem begins the first chapter with another case study on hemochromatosis. Moalem then explains that iron is necessary for life—but that bacteria, cancer cells, and other parasites also need iron and have evolved to use the iron in our blood and tissue to survive. In particular, pathogens use the iron in our macrophages (white blood cells responsible for fighting biological invaders). However, in a person with hemochromatosis, the iron builds up everywhere except the macrophages. Thus, when facing something like the Black Plague (which is caused by a bacterium), hemochromatosis could provide an advantage because there is no iron in the macrophages of a hemochromatic person, and therefore the bacterium is unable to grow stronger using that iron. This theory is supported by the fact that hemochromatosis is particularly common in people of Western European descent—exactly the population that faced the plague.

In the second chapter, Moalem focuses on diabetes, which is a chronic disease that over 171 million people had when the Survival of the Sickest was written in 2007. For diabetics, the process through which insulin helps the body use glucose is broken, and sugar builds up to dangerously high levels in the blood. Studies of other plants and animals show how this might have been helpful in the past. Grapes, when experiencing a frost, offload water and increase their sugar content in order to lower their freezing point. The wood frog, which has the unique ability to completely freeze in winter, then spark back to life in the spring, also increases the glucose in its blood and decreases the amount of water in its bloodstream. These adaptations to the cold lead Moalem to argue that humans with diabetes had an advantage during the Younger Dryas, Earth’s last ice age. With a higher glucose content in their blood, diabetics were less likely to freeze to death. As further proof of the connection, Moalem points out that Type 1 diabetes is much more common in people of Northern European descent—the population that would have faced the harshest conditions during the ice age.

The third chapter examines humans’ relationship to the sun, and how the sun affects our health and skin color. Too little sun prevents people from producing vitamin D (which is essential to bone health), but too much sun destroys humans’ reserves of folic acid (which is important in cell growth and DNA replication). Thus, human skin color adapted to achieve a balance: people who lived in places with large amounts of sun exposure produced more melanin, resulting in a darker skin tone. This prevents ultraviolet light from being absorbed and destroying folic acid. People who have less sun exposure produce less melanin and have lighter skin tones. But for people who have developed very dark skin (and therefore can’t absorb enough ultraviolet light), or for people who don’t receive enough sunlight, they have adapted in another way. These two groups increase the amount of cholesterol in their bloodstream, which can then be converted to vitamin D. As a result, people of African descent and Northern European descent are both at risk for heart disease and stroke from their high cholesterol.

The fourth chapter examines the relationship between fava beans, anemia, malaria, and a condition called favism. Moalem explains that favism causes a deficiency in the G6PD enzyme, which helps to rid the body of chemical elements (like free radicals) that destroy red blood cells. Without this enzyme, people experience anemia, particularly when they eat fava beans, which have free radicals in them. But, Moalem notes, favism is most common in places where malaria is also prevalent, like Northern Africa and Southern Europe. This is because anemia offers some protection against the protozoa that causes malaria. Moalem then shifts gears to explore how plants and animals frequently affect one another’s evolution. Plants, for example, develop chemical toxins to ward off predators, while mammals in turn develop mechanisms to detect those toxins and to avoid them, focusing on eating plants’ fruit or other edible parts instead.

Next, Moalem focuses on organisms like the Guinea worm, bacteria, and other parasites. He recounts how many parasites have adapted “host manipulation,” in which parasites evolve behaviors that provokes their hosts to behave in a way that helps the parasites to survive and reproduce. The Guinea worm, for example, leaves the human digestive tract when it is ready to reproduce and secretes acid to burn its way out of the skin. This causes humans to seek relief through water, which in turn serves as a signal to the Guinea worm to secrete a milky fluid with thousands of larvae into the skin. But Moalem notes that by spreading understanding, we can prevent these parasites from harming humans. Former president Jimmy Carter led an effort to spread information about the Guinea worm’s transition methods, and infections dropped from 3.5 million in 1986 to 10,674 in 2005. Moalem also examines how we can use parasites’ and diseases’ need to survive and reproduce to our advantage. Cholera, for example, can be transmitted by physical contact, or through infected water. Cholera that is transmitted through water is much more virulent (deadly to the host) than that which is transmitted through humans, because cholera that is transmitted through humans needs those humans to be mobile. Thus, by developing ways to protect the water supply, countries can put evolutionary pressure for the bacteria to evolve to be less virulent.

The sixth chapter examines an area of research that looks at “jumping genes,” which were first discovered by Barbara McClintock as she researched corn genetics. McClintock found that in times of stress, sequences of DNA would copy themselves and insert themselves into other genes in order to trigger mutations that might be beneficial. Moalem writes that jumping genes have been found in other organisms like bacteria, fleas, and even humans. He also posits that jumping genes may have been descendants of retroviruses, which are a subset of viruses that can insert themselves into our DNA. Because retroviruses are “master mutators,” they can help spur adaptations at a much greater rate than we would be able to achieve without their help. Retroviruses and jumping genes prove that our DNA is not always set in stone.

The seventh chapter explores another way in which DNA can change, or at least how the expression of our DNA can change. The relatively new field of epigenetics suggests that certain compounds can “turn genes on or off,” changing the expression of those genes (a process called methylation). This was the focus of a study of agouti mice, whose genetic expression for coat color and size was completely changed when their mothers were fed vitamins during pregnancy. Many species have been observed to do this naturally: for example, one species of lizard is born with a large body and long tail or a small body and short tail, depending on whether the mother smells a lizard-eating snake while pregnant. Epigenetics may even account for rising obesity in children: when a mother eats junk food during pregnancy, the embryo may receive signals that it’s going to be born in a harsh environment with scarce nutrients. It would then develop a “thrifty” metabolism and would be more efficient at hoarding energy. Then, when the child is surrounded by calorie-rich food after being born, they become overweight. Studies have also suggested that our genetic expression can be affected throughout life, not only in early stages of development.

In the final chapter, Moalem argues that even aging may be preprogrammed as a safeguard against cancer. Cells can only reproduce a certain amount of times before losing crucial genetic information, which causes aging. This goes some way to prevent cancer cells (which grow and reproduce uncontrollably) from developing, though cancer cells have developed some mechanisms to circumvent this limit. He also likens aging to planned obsolescence in technology (like iPods): it helps us “upgrade” faster, which ultimately helps our species adapt and evolve. Moalem then shifts to exploring two different hypotheses as to how humans might have evolved: first, the widely-held “savannah hypothesis,” which suggests that humans adapted due to conditions found in the savannah and the development of hunting techniques. Second, marine biologist Alister Hardy proposed a different theory called the “aquatic ape hypothesis” which argues that humans evolved by living in and around water—this, Hardy believed, is why we lost our hair and have fat on our skin, like other marine mammals. Moalem posits that water births offer additional evidence that water played into our evolution, citing statistics that water births are just as safe as conventional births and are often easier on mothers.

In his conclusion, Moalem asks readers to take away three ideas from his book: first, that life is undergoing a constant process of creation. Second, that nothing on Earth is isolated from other things. And third, that human disease is highly complex. He emphasizes that we should keep asking questions and that we should appreciate “the miracle of evolution.”