Themes
Key
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.
Evolution and Illness
Interspecies Connectivity and Adaptation
Environment, Ancestry, and Race
Genetic Expression, Acquired Traits, and Mutation
Interdisciplinary Science and Research
Summary
Analysis
While the sun is necessary for all life on Earth, the sun is just as important to humans on an individual, biochemical level. Sunlight helps your body create vitamin D but also destroys your body’s reserves of folic acid—both of which are essential to your health. Vitamin D ensures that we have sufficient levels of calcium and promotes bone growth. American milk is now fortified with vitamin D, but the vitamin can be made by the body itself. When exposed to the right kind of sunlight, ultraviolet B (UVB), we can convert cholesterol to vitamin D.
An essential part of the first two chapters of the book has been illustrating how evolutionary adaptations have built-in tradeoffs. Moalem shows here that this is true on a smaller scale as well: human evolution has had to find a balance between getting enough sunlight to produce vitamin D and not getting so much that the sun’s UVB rays destroy the body’s folic acid. Thus, not all adaptations necessarily result in diseases: rather, humans have evolved certain built-in tradeoffs that prevent disease.
Themes
While sunblock guards against sunburn and skin cancer, it also blocks the UVB rays we need for vitamin D. Conversely, researchers have discovered that tanning can help people with vitamin D deficiency, like people with Crohn’s disease, a disorder that involves inflammation of the small intestine and impairs absorption of nutrients. On the other hand, folic acid is just as important to human life, and it is destroyed by too much UVB light. Folic acid is an integral part of cell growth, helping to replicate DNA when cells divide. It is particularly important during pregnancy, as too little folic acid can result in serious birth defects.
Although the medical community often cautions against excess sun exposure because of the risk of skin cancer, here Moalem shows that tanning can actually be advantageous for people with diseases that cause vitamin D deficiency. On the other hand, this could result in folic acid-related complications, particularly for pregnant women. Thus, Moalem again demonstrates that many of humans’ biological mechanisms involve maintaining a delicate balance between different health problems on either end of a spectrum.
Themes
The skin both protects folic acid and serves as a key player in producing vitamin D. The wide range of human skin color is related to the amount of sun a population has been exposed to. It protects against sunburn and protects one’s folic acid, because the more melanin in the skin (the darker it is), the less UVB light you absorb. Skin color can change, to some extent, based on response to sun exposure. When a person is exposed to the sun, melanin production is stimulated in order to protect from UVB rays.
Skin color serves as the key adaptation for how to maintain the balance between getting enough vitamin D and keeping enough folic acid. It is in this way that these adaptations start to build upon our idea of race. While Moalem ultimately recognizes that skin color is primarily a factor of geography, it also gives certain populations distinct common traits.
Themes
Skin color is directly related to the amount of sun exposure in a population. When the first humans were evolving in Africa, they had dark skin designed to block UVB absorption. But when some populations migrated north to places like Europe, they received too few UVB rays and couldn’t produce vitamin D— therefore, they evolved to have lighter skin. Researchers posit that it would take 1,000 years for a population’s skin color to change based on their climate.
Just as the environment may have played a key role in determining whether people developed diabetes during the Younger Dryas, the environment is another key factor here in determining vitamin D production and folic acid retention because of the way environment sparks an adaptation in a population’s skin color.
Themes
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There is one exception to this relationship between sun exposure and skin color: the Inuit, who are dark-skinned despite living in a place with little sunlight. But their diet is composed of fatty fish, which happens to be full of vitamin D. Therefore, they do not need lighter skin in order to create vitamin D.
The exception of the Inuit proves the general rule: if the Inuit, who live in the Arctic, do not need lighter skin because they already have enough vitamin D, it only serves as further proof that people with light skin have developed that way because they do not get enough vitamin D.
Themes
People with darker skin have an additional problem, however, because their skin blocks most UVB rays and therefore they may not produce enough vitamin D. But these populations have evolved a protein called apolipoprotein E (ApoE4) which increases free-flowing cholesterol in the blood to maximize vitamin D production. People throughout Northern Europe also carry this protein, because even with light skin, they receive so minimal sunlight with which to produce vitamin D. But though it allows people to create vitamin D, the cholesterol resulting from ApoE4 puts people at greater risk for heart disease and stroke.
Here, Moalem makes the evolutionary tradeoff more explicit: people who do not get enough vitamin D increase their cholesterol production. But even while this allows them to mitigate their vitamin D deficiency, high cholesterol can lead to other problems. In addition, Moalem again emphasizes how environment can affect these adaptations and tradeoffs, because the amount of sunlight one gets (or that one can absorb) plays a key role in that overcompensation of cholesterol.
Themes
Moalem gives other examples of culture-specific gene development. People of Asian descent frequently have a genetic variation called ALDH2*2 that makes them less effective at breaking down the by-products of alcohol. This causes them to appear and feel more drunk. This variation is rare in Europeans, however. Moalem explains that this is because as humans settled into cities and towns, they had to figure out how to clean their water. In Asia, people purified water by boiling it and making tea. In Europe, they used fermentation, and the resulting alcohol from that fermentation killed microbes even when it was mixed with water. Thus, there was evolutionary pressure in Europe to be able to break down alcohol, but not in Asia.
Temperature and climate aren’t the only geographical factors that played a role in our ancestors’ adaptations. Here, Moalem illustrates that the culture of a population in a given geographical location can also have significant effects across generations. Different populations came to different solutions for cleaning water, which in turn affected their respective groups’ genetic development and thus created common geographically-specific adaptations. This is another factor in our ideas of race: these genes developed in a population that already had similar traits, and their common culture added further similarities to their DNA.
Themes
In additional to alcohol, this concept can also be found in the ability to digest milk. The majority of the world’s adults cannot digest lactose. People who can continue to digest lactose after infancy are likely descended from farmers who drank animal milk, who evolved to retain the ability to break down lactose.
Being lactose intolerant is another example of how both culture and geography combined into a racially-specific gene. People of European descent are less likely to be lactose intolerant because dairy farming was more prominent there, unlike in Asia or Africa.
Themes
Moalem admits that even though these differences seem “racial,” it is difficult to define what race means. Skin color isn’t reliably based on genetics, as moving to a new environment would change a population’s skin color. On the other hand, many Jewish people seem to share a distinct genetic heritage despite the fact that they can have very different appearances. Comparing the DNA of a large group of Jewish people called Cohanim (who trace their roots to Moses’s brother Aaron), despite the fact that they came from all over the world, led researchers to discover genetic markers so specific that they almost certainly descended from just a few male individuals.
Moalem concedes that it’s difficult to define what race is, as even though there are commonalities in the genes of given populations, it is difficult to know whether commonalities can be attributed to race. They’re clearly not linked solely to appearance, as the example of the Jewish people illustrates. And even genes that constitute appearance can be altered based on geography. Thus, Moalem recognizes that there is nuance in the interplay of geography, genetics, and our ideas of race.
Themes
These studies are controversial, Moalem writes, and he says that while it’s hard to define race, what researchers know is that distinct populations do share distinct genetic ancestry. This is most likely the result of the varied conditions that different groups of people experienced as they spread out across the world. As Moalem has already discussed, different circumstances that populations encountered (plagues, ice ages, and tropical diseases) all had an effect on how people evolved.
Even with Moalem’s concession that race is a complicated issue, he asserts that much of what we think of as race are actually adaptations based on a common geography and culture—both of which largely remained the same for generations up until our recent period of globalization.
Themes
Moalem examines how even conditions in the last 500 years can affect traits in certain sub-groups of people. For instance, high blood pressure is particularly common in African Americans compared to the rest of the American population; their blood pressure is also very reactive to salt. Though Moalem again admits the theory is controversial, he posits that when Africans were taken to America against their will by slave traders, they were transported under horrible conditions and were usually not fed or given sufficient amounts of water. It’s possible that those who naturally tended to retain more salt had a better chance of survival, as extra salt helped their bodies hold onto water and thus combat dehydration.
While the background of this adaptation is much more horrific and culturally motivated than the others Moalem has mentioned, the effects are similar to how the Younger Dryas affected people. In both cases, an immediate shift in environmental conditions and food availability made certain traits within the population immediately more advantageous, and therefore it made those people more likely to survive and reproduce. This effect, Moalem posits, continues to have ramifications for the descendants of those African slaves today.
Themes
Moalem returns to the adaptations of Africans in response to sun exposure: the evolution of darker skin to protect folic acid and increased cholesterol to produce vitamin D. But when populations with darker skin move to places like New England, they’re twice as likely to be deficient in vitamin D and to have high cholesterol. This may be why African Americans who live in colder climates are more prone to rickets, prostate cancer, and heart disease.
Moalem returns to the compromise between vitamin D and folic acid, but he illustrates how the problem is exacerbated when people move out of the environments which provided them with the adaptation—essentially relinquishing their evolutionary advantage and making the downsides even more problematic.
Themes
The pharmaceutical industry has begun to tailor its drugs based on genetics, as new research has shown how genetic variations can affect our body chemistry and how we respond to drugs. Because of the consensus that normal hypertension drugs don’t work as well on African Americans, there is a drug called BiDil for self-identified black patients who have heart failure.
Research on health conditions that arise from ancestral differences—particularly the relationship between vitamin D, skin color, and cholesterol—have, in turn, prompted scientists to create drugs that take these genetic differences into account. The fact that this had led to more effective medicine for African Americans further demonstrates the importance of understanding the root cause of a health condition.
Themes
Moalem says that one thing is clear: evidence suggests that when placed in the context of our current circumstances, the specific environmental adaptations that our ancestors passed onto us can significantly impact our health. We can use this information to tailor medical treatments—perhaps recommending tanning salons rather than lifetime prescriptions of drugs for excess cholesterol.
Rather than trying to compensate both for less vitamin D and for too much cholesterol, Moalem suggests a solution that will fix both issues at once. Thus, even though high cholesterol is an adaptation that has become problematic, understanding the condition’s underlying causes by examining patterns of genetics and migration can lead to more effective treatments.
Themes
