Everyone Is African (8 page)

At least among those individuals who participate in biomedical research, genetic estimates of biogeographical ancestry generally agree with self-assessed ancestry, but in an unknown percentage of cases they do not.

Despite its seemingly objective nature, ancestry also has limitations as a way of categorizing people. When asked about the ancestry of their parents and grandparents, many people cannot provide accurate answers…. Misattributed paternity or adoption can separate biogeographical ancestry from socially defined ancestry. Furthermore, the exponentially increasing number of our ancestors makes ancestry a quantitative rather than a qualitative trait—five centuries (or twenty generations) ago, each person had a maximum of >1 million ancestors. To complicate matters further, recent analyses suggest that everyone living today has exactly the same set of genealogical ancestors who lived as recently as a few thousand years in the past [about two hundred thousand years ago in Africa], although we have received our genetic inheritance in different proportions from those ancestors.

In the end, the terms “race,” “ethnicity,” and “ancestry” all describe just a small part of the complex web of biological and social connections that link individuals and groups to each other.
¹⁶

The time has come to abandon the notion of race as a presumed biological construct when referring to humans. It may be legitimately argued that terms such as
race
and
ethnicity
have value as social constructs. But when referring to a person's genetic constitution, we should turn our attention to
ancestry
, which is scientifically more informative, less burdened by political and historical baggage, and immensely more complex and fascinating.

A few years ago, I visited the Lincoln Memorial in Washington, DC, early on a cold January morning. Except for a guard sitting in his warm booth in a dark corner of the building, I was alone. I gazed at Daniel Chester French's monument of Abraham Lincoln carved in Georgia marble, admiring the rugged lines French had so skillfully captured in Lincoln's face. I then turned around, looking east toward the Washington Monument. At my feet, I noticed a small brass plaque embedded in the marble step, identifying that place as the site where Martin Luther King Jr. delivered his most stirring speech: “I Have a Dream.” His familiar words entered my mind, along with my childhood recollection of the penetrating sound of his voice:

Now is the time to rise from the dark and desolate valley of segregation to the sunlit path of racial justice…. I have a dream that my four children will one day live in a nation where they will not be judged by the color of their skin but by the content of their character.
¹

I chose his words, “the color of their skin,” as the title for this chapter because no other human characteristic is so strongly associated with the perception of race. The names of colors—black, white, red, and yellow, among others—have been used as legal definitions of race, as labels of supposed superiority or inferiority, and as common descriptions of people whose ancestry derives from various regions of the world. Since I was a child, I've always been puzzled by these color characterizations of people, in light of the obvious fact that actual variation for skin color in humans does not fall into discrete classes, nor is it actually white, red, yellow, or black. Instead, it ranges from intense to little pigmentation in continuously varying gradations.

The genetic basis for variation in human skin pigmentation, as well as
hair and eye pigmentation, is now quite well understood. Current scientific evidence paints a clear picture of how, when, and where DNA variants arose and why patterns of genetic variation that confer pigmentation are distributed as they are throughout the people of the world. That variation for pigmentation is largely inherited is beyond question, a fact well known for centuries, long before the scientific principles of inheritance in humans were understood. There is, of course, some influence from environment, such as the protective response of increased pigmentation in some people when their skin is repeatedly exposed to sunlight, popularly known as tanning. Yet even the ability to tan is itself an inherited characteristic. For the most part, variation for skin, eye, and hair pigmentation is a consequence of genetic ancestry.

Inheritance of skin color is complex, conferred by variants in a relatively large number of genes, a situation scientists call
polygenic inheritance
. The polygenic nature of variation for skin color in humans has been known for some time; I recall studying the evidence of it in the first college biology course I took, in 1978. However, until recently, most of the individual genes, and the specific variants in their DNA that influence variation for skin pigmentation, remained unknown. Now, many of those genes and their variants have been identified, and scientists have examined them in detail, discovering how they regulate pigmentation and how they are distributed geographically. Before examining these variants individually, let's review some of the major conclusions from this research.

First, high pigmentation in the skin, hair, and eyes is the ancestral state of all humans. Current evidence from DNA overwhelmingly confirms this conclusion, and it fits a common pattern in genetics. Most mutations in DNA that have any effect on a gene reduce or eliminate the function of that gene. The reduction of pigmentation in people whose postdiaspora origins lie outside of Africa is due to ancient mutations in these genes. The variants that arose from those mutations cause a reduction in gene function, which reduces pigmentation, resulting in the variation currently present in modern humans.

The second conclusion of this genetic research is that most of the mutations that became variants affecting skin, eye, and hair pigmentation happened outside Africa in the distant descendants of people who originally left Africa. Unlike variation for blood groups (such as A, B, AB, and O blood), which vary among people throughout the world, most variants that influence
pigmentation are not original African variation. Instead, the specific variants responsible for reduction in skin, hair, and eye pigmentation arose as mutations in individual people and then spread through their descendants within broad geographic regions. These variants constitute some of the most reliable ancestry informative DNA markers.

The third conclusion is that by examining the genetic background of any particular variant, scientists can detect patterns in DNA that indicate that Darwinian natural selection has influenced the prevalence and geographic distribution of each variant. Of the many contributions Darwin made to science, the one that most impacted human thought is the principle of
natural selection
. His definition of it in the
Origin of Species
is perhaps the best ever penned:

Any variation, however slight and from whatever cause proceeding, if it be in any degree profitable to an individual of any species…will tend to the preservation of that individual, and will generally be inherited by its offspring. The offspring, also, will thus have a better chance of surviving, for, of the many individuals of any species which are periodically born, but a small number can survive. I have called this principle, by which each slight variation, if useful, is preserved, by the term of Natural Selection.
²

There is clear evidence that variants in DNA conferring variation for skin pigmentation in humans, ranging from very dark to very light skin, have been subjected to intense natural selection. In some cases, the effect of selection was relatively rapid, taking place over a few hundred generations instead of a few thousand. Such rapid effects of natural selection are called
selective sweeps
. For example, several variants in humans conferring resistance to disease were subject to rapid selective sweeps. People resistant to the diseases survived and reproduced, whereas those susceptible to these diseases tended to succumb and die before reproducing. The prevalence of any variant conferring resistance to disease, therefore, rapidly increased from one generation to the next through natural selection. Recognizable patterns in human DNA provide evidence that some variants known to reduce skin pigmentation have also been subjected to rapid selective sweeps, resulting in an increase in their prevalence. In fact, some of the most powerful selective sweeps in ancient human history appear to be associated with variation for skin pigmentation.

The fourth conclusion is that the geographic distribution of skin pigmentation in humans is best explained through a pattern of natural selection superimposed on ancient human migrations, called the
vitamin D hypothesis
. When ancestral human populations are examined, pigmentation is greatest in equatorial Africa and tends to decrease with increased distance from the equator. The most likely environmental agent responsible for this pattern is the amount of winter sunlight to which ancient people were exposed. Pigmentation protects the skin from ultraviolet radiation when sunlight is intense; this is the principal biological function of skin pigments. In regions where sunlight is intense throughout the year, high pigmentation confers a selective advantage to people who have it because it protects against ultraviolet light-induced degradation of folate, a substance that is essential for fetal development. This is especially important in pregnant women because folate protects the fetus from disabling birth defects.
³

But, according to the vitamin D hypothesis, there is a natural tradeoff. The skin is the place in our bodies where vitamin D is produced, and exposure to sunlight stimulates vitamin D production. High pigmentation in the skin can inhibit vitamin D production when sun exposure is reduced. In regions where sunlight is intense throughout the year—regions near the equator—exposure to sunlight was sufficient to ensure more than adequate vitamin D production in ancient people, while the inhibitory effect of skin pigmentation also protected against folate degradation. However, when the descendants of people who left Africa immigrated into the more northern latitudes of Europe and east Asia, the world was in the midst of a major ice age, which reached maximum cooling and glaciation at about twenty-two thousand to twenty-four thousand years ago, about the time when people were colonizing these regions. Winters were colder than they are now, so people in these regions had to cover most of their bodies and seek shelter to protect themselves from the cold, depriving them of already meager winter sunlight. Even more then than now, short winter days in northern latitudes and overcast skies limited the amount of sunlight during the winter months. Insufficient exposure to sunlight resulted in vitamin D deficiency in these people, causing a disease known as rickets, because those with highly pigmented skin could not produce enough vitamin D during the winter season. The most serious
symptom of rickets is weak and easily fractured bones. Pregnant women were especially affected, often culminating in death for them and their unborn children. People with lower amounts of skin pigmentation, however, had a strong selective advantage because their bodies were better able to produce vitamin D with limited sunlight than people with higher skin pigmentation. And the lack of ultraviolet light exposure also protected against folate degradation. Rapid selective sweeps caused variants that reduced skin pigmentation to spread over a period of generations in people living in northern regions—in some cases, entirely displacing original ancestral variants in people who lived in the northernmost regions of Europe. And the farther north people migrated, the more variants conferring reduced pigmentation accumulated over generations through a combination of mutation and natural selection.

There are explainable exceptions to this general rule. For example, Native Americans in coastal regions of Alaska, northern Canada, Greenland, and northeastern Siberia—collectively known as Eskimo people—have lived in high-latitude, low-sunlight environments for thousands of years. Yet light skin complexions have not evolved there. In this case, the most probable reason also has to do with vitamin D. Diets in these regions since ancient times have been high in seal liver and fish, which naturally contain high amounts of vitamin D, compensating for reduced vitamin D production in the skin. With adequate vitamin D in the diet, reduced pigmentation did not confer a strong selective advantage.
⁴

Although rare, cases of rickets are still reported when people fail to consume a diet with sufficient vitamin D and when sunlight exposure is insufficient for adequate vitamin D production in the skin. This is especially true for infants, children, and adolescents, whose bodies are growing and require proportionally more vitamin D than adult bodies. The American Academy of Pediatrics recommends vitamin D supplementation in the diet to prevent rickets.
⁵

Skin damage resulting from overexposure to sunlight, however, remains a serious health issue. Sunlight-induced skin cancer does not explain why natural selection favored reduced skin pigmentation in northern regions (this type of cancer typically appears well after people have reproduced), but repeated exposure to sunlight can be a serious danger to individuals. Over time, it increases the probability of lethal skin cancer and other long-term skin disorders,
especially for people whose skin complexions are light. Although skin pigmentation protects against sun damage, even high skin pigmentation is not an absolute protection. All people should protect themselves from overexposure to the sun. Nonetheless, light skin complexion is the highest genetic risk factor for malignant melanoma, the most serious and potentially lethal form of skin cancer.

Having reviewed the main conclusions of research on how and why variation for pigmentation in humans originated, let's dig a bit deeper, examining some of the direct evidence of how this variation, and its geographic distribution, evolved throughout human history. To understand this variation, we need first to understand what skin pigmentation is.

Human skin, hair, and eyes contain a group of related pigments called
melanins
, the prefix
melan-
meaning “dark” or “black.” The word
melancholy
, meaning a dark and somber mood, has the same root. Melanins ultimately arise from the food we eat—specifically, from the protein in our food. Each individual protein molecule is structured like a linked chain that, when stretched out, becomes linear. The links that make up each protein chain are called
amino acids
. When you eat anything with protein (which is in almost everything you eat), your digestive system breaks down each protein chain into the individual amino acids, which your digestive system then absorbs into the bloodstream. Your cells extract the amino acids from the blood and reassemble them into new chains in new combinations to form your own proteins.