Evolution and Illness Theme Analysis

Sharon Moalem’s primary goal in Survival of the Sickest is to explore the evolutionary history behind certain illnesses, diseases, and genetic disorders. Many of the chapters center around the question of why, if these diseases and disorders are harmful, evolution has not simply selected for individuals who do not have them. Moalem uses the book to illustrate how many inherited conditions still exist because they historically helped fight against more dangerous and life-threatening conditions. Thus, Moalem argues that evolution sometimes selected for traits that helped people survive in the short term and allowed them to reproduce and pass on those traits—even if, in the long term, those inherited conditions might end up being deadly.

Being able to survive deadly diseases in the short term is a necessary tradeoff to having a condition that might eventually cause harm, because those conditions allow enough time for humans to reproduce and pass on those inherited traits. Moalem’s first example of a disease that once provided short-term benefits is called hemochromatosis, a condition that causes iron to build up in the body. Hemochromatosis is inherited, particularly in people of Western European descent; if unchecked, it can lead to liver failure, heart failure, diabetes, arthritis, and cancer. However, in those with hemochromatosis, iron doesn’t build up in white blood cells called macrophages, which fight disease. In people without hemochromatosis, infectious agents feed on the iron in our macrophages, which actually makes the infectious agents more deadly. Thus, when the Bubonic Plague began in 1347, people who had hemochromatosis were more likely to survive it, because the bacteria were unable to access their iron and grow stronger. Thus, people who have inherited the gene for hemochromatosis may have done so because it allowed their ancestors to survive in the short term, even if it would harm them in the long term. There is a similar tradeoff with people who have a condition called favism, which is a condition that prevents people from clearing free radicals in their bloodstream. Free radicals are molecules or atoms with unpaired electrons that can wreak chemical havoc in cells in the body, and they often cause red blood cells to break down. This results in severe anemia, which can sometimes be deadly. However, Moalem posits that favism (which is found mostly in people around the Mediterranean) helps to combat malaria, because the parasitic protozoa that cause malaria can only infect normal, healthy red blood cells. Thus, those with favism, who do not have healthy red blood cells, are more likely to survive in their malaria-prevalent environments. So while anemia can be deadly, favism and the resulting anemia actually prevents people from dying from malaria and allows them to pass on those genes.

These genetic tradeoffs are not limited to safeguards against deadly diseases: in some cases, harmful adaptations have sprung up as a result of harsh nutritional conditions that existed thousands of years ago, which enabled people to survive even if it led to disease. Type 1 diabetes is thought to be an autoimmune disease wherein the body’s defense system mistakenly attacks the cells in the pancreas responsible for insulin production, and it is much more common in people of Northern European descent. For those with diabetes, the process through which insulin helps the body breaks down sugar doesn’t function properly, and the sugar in the blood builds up. Unmanaged, these abnormal blood sugar levels can lead to rapid dehydration, coma, and death. Though the theory is controversial, Moalem posits that flooding the bloodstream with sugar once acted as a kind of natural antifreeze that helped people survive in colder climates. Additionally, the extra sugar in the blood stream was not as problematic as it is today, because food and sugar were less available in general. During the Younger Dryas (a rapid period of climate change that occurred around 12,000 years ago, in which temperatures dropped significantly), people in Northern climates were particularly at risk of freezing—and those who were diabetic were more likely to survive because of their diabetes and the resulting sugar in their bloodstream. Thus, although diabetes is  an unhelpful adaptation in the industrialized world, it was extremely helpful for primitive humans amid their harsh climate. Obesity may be another adaptation to harsh conditions. In 1987, British medical professor David Barker developed a theory that fetuses who experience poor nutrition develop “thrifty metabolisms that are much more efficient at hoarding energy.” Ten thousand years ago, the theory explains, this metabolism helped a baby survive when it was born into relative famine. But now, when a pregnant mother eats junk food, the embryo “receive[s] signals that it’s going to be born into a harsh environment where critical types of food are scarce.” As a result, when that baby is born and is surrounded by calorie-rich but nutritionally poor food, the “conservationist metabolism” that would once have helped the baby survive now leads to obesity.

In Moalem’s final example, he argues that everyone experiences these kinds of evolutionary tradeoffs, as even aging is an example of our bodies protecting us from something more deadly: cancer. Normally, our cells are only able to reproduce a specific number of times before genetic information begins to be lost. This prevents cells from becoming cancerous and reproducing an infinite number of times, unchecked. But as a result of these built-in controls, our cells gradually hit the limit of the number of times they can replicate, and our bodies begin to break down as a result. Thus, the aging process is the body’s way of protecting us from cancer in the short term, even if it causes significant damage later in life.

As the book’s title suggests, the common phrase “survival of the fittest” does not tell the whole story—Moalem illustrates in numerous examples how illness can sometimes be advantageous. In understanding why certain conditions developed and what they were fighting against, Moalem argues, humans can understand how to better treat those conditions without offsetting the advantages that the diseases proffered.