Just as the evolution of other organisms has had a major effect on human evolution, the environment in which people live also plays an important role in making certain genes more advantageous. Whether it is a population’s exposure to sun, the climate in which one’s ancestors lived 10,000 years ago, or different social customs that have developed in particular areas of the world even in the past few centuries, our ancestors’ environment has been a key factor in the development of our genetics. The environment dictates different physical traits and behaviors among different groups of people, and so Moalem argues that this relationship between environment and ancestry serves as one of the building blocks to our ideas of culture and race.
In order to illustrate the idea of environment impacting human evolution, Moalem considers how the climate that early humans experienced caused them to develop certain adaptive traits. Moalem first looks at how a period of extreme cold added evolutionary pressure to those who lived in cold environments, causing them to be more likely to develop diabetes. The previous ice age, Moalem recounts, was followed by a period of warmer weather, in which human populations migrated out of Africa and into Europe, which had previously been too cold to be habitable during the ice age. Then, around 12,000 years ago, there was a rapid shift (occurring over just a decade) back to colder weather. This resulted in a period called the Younger Dryas, which lasted more than 1,000 years. During this period, it is likely that thousands of humans froze or starved to death in those colder areas. However, those who survived adapted in ways both social and biological. It is believed that diabetes may have actually allowed people to live by increasing the sugar concentration in people’s blood, which provided them with a higher chance of not freezing to death. There were additional social developments from the Younger Dryas that changed parts of our genetic code as well. For instance, many scientists believe that this colder period led to the first development of agriculture in Northern Europe and the collapse of hunter-gatherer societies in the area. Moalem posits that because of the development of dairy farming over time in these Northern European areas, populations from those areas are more likely to have enzymes that can break down lactose in milk and are therefore less likely to be lactose intolerant than people from other ancestral lines—another illustration of how geographic and social conditions can affect one’s genes.
The idea that geography and our ancestor’s environment can affect our genetics becomes tied up in the complicated idea of what race actually represents on a biological level. While Moalem shies away from trying to define what race means, he acknowledges that “distinct populations do share distinct genetic heritages, which are almost certainly the result of different evolutionary pressures our various ancestors experienced as they settled and resettled across the globe.” He argues that the prevalence of genetic mutations in certain ethnic groups is largely based on common environments and social conditions. In our ancestors, skin color was determined largely by sun exposure. Those with more exposure produced more melanin (which causes darker skin), blocked more ultraviolet light, and therefore protected their folic acid, which is integral to cell growth. Those with less sun exposure produced less melanin because ultraviolet light absorption is crucial in one’s vitamin D production. These skin color adaptations, which were developed over thousands of years, play a large role in several key health issues: for instance, people who have little access to sunlight (like Northern Europeans) have trouble maintaining healthy levels of vitamin D. People with very dark skin are also unable to absorb the necessary ultraviolet light that allows them to create vitamin D. Due to this deficiency, both groups have evolved to produce a protein called apolipoprotein E (ApoE4). This protein increases the amount of cholesterol in the bloodstream, which can then be converted to vitamin D. But this comes with a tradeoff: the extra cholesterol also puts people at greater risk for heart disease, stroke, and Alzheimer’s disease. Our ancestors’ varied environments thus had a major effect on our skin color, which in turn can affect one’s health conditions for generations, even after leaving that environment.
Moalem also delves into how cultural factors that arise from different geographic locations can change certain people’s genes in ways that are distinct from other groups. For instance, people of Asian descent frequently have a genetic variation which makes it difficult to break down alcohol, causing them to appear and often feel more drunk. People of European descent, however, rarely have this variation. This is because, as humans settled into cities and towns, the dilemma of how to clean water first arose. 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. As a result, Moalem writes, “there was evolutionary pressure in Europe to have the ability to drink, break down, and detoxify alcohol, while the pressure in Asia was a lot less.” Geographic location and the cultural customs that accompany that location both clearly affect the genetic makeup of populations who live there.
Moalem sums up his point about environment and ancestry as such: “One thing is clear—there is mounting evidence that where our ancestors came from, how they adapted to manage their environment, and where we live today all combine to have a significant impact on our health.” His main project, as with exploring the other factors that help us understand our genetics, is to provide a greater understanding for why we have certain conditions and therefore to find more effective ways to treat ourselves if we move to new environments where those conditions are no longer beneficial.
Related Themes from Other Texts
Environment, Ancestry, and Race ThemeTracker
Environment, Ancestry, and Race Quotes in Survival of the Sickest
The Younger Dryas had arrived, and the world was changed.
Though humanity would survive, the short-term impact, especially for those populations that had moved north, was devastating. In less than a generation, virtually every learned method of survival—from the shelters they built to the hunting they practiced—was inadequate.
So when the grape dumps water at the first sign of frost, it’s actually protecting itself in two ways—first, by reducing water volume and second, by raising the sugar concentration of the water that remains. And that allows the grape to withstand colder temperature without freezing.
But what if a temporary diabetes-like condition occurred in a person who had significant brown fat living in an ice age environment? Food would probably be limited, so dietary blood-sugar load would already be low, and brown fat would convert most of that to heat, so the ice age “diabetic’s” blood sugar, even with less insulin, might never reach dangerous levels. Modern-day diabetics, on the other hand, with little or no brown fat, and little or no expo- sure to constant cold, have no use—and thus no outlet—for the sugar that accumulates in their blood.
There is one notable exception to Jablonski and Chaplin’s equation—and it’s the exception that proves the rule. The Inuit—the indigenous people of the subarctic—are dark-skinned, despite the limited sunlight of their home. If you think something fishy’s going on here, you’re right. But the reason they don’t need to evolve the lighter skin necessary to ensure sufficient vitamin D production is refreshingly simple. Their diet is full of fatty fish—which just happens to be one of the only foods in nature that is chock-full of vitamin D.
This made clean water a real challenge, and some theories suggest that different civilizations came up with different solutions. In Europe, they used fermentation—and the resulting alcohol killed microbes, even when, as was often the case, it was mixed with water. On the other side of the world, people purified their water by boiling it and making tea. As a result, there was evolutionary pressure in Europe to have the ability to drink, break down, and detoxify alcohol, while the pressure in Asia was a lot less.
Instead of worrying about whether or not there are distinct “races,” let's concentrate on what we do know and use that to advance medical science. What we do know is that distinct populations do share distinct genetic heritages, which are almost certainly the result of different evolutionary pressures our various ancestors experienced as they settled and resettled across the globe.
According to the thrifty phenotype hypothesis, fetuses that experience poor nutrition develop “thrifty” metabolisms that are much more efficient at hoarding energy. When a baby with a thrifty phenotype was born 10,000 years ago during a time of relative famine, its conservationist metabolism helped it survive. When a baby with a thrifty metabolism is born in the twenty-first century surrounded by abundant food (that is also often nutritionally poor but calorie rich), it gets fat.
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.
