Everyone Is African
Page 6
Another very important point is that most recent variants are not entirely confined to a limited geographic region. Instead, scientists usually look at prevalence: what proportion of people in any particular region carry a particular variant. The pattern that typically emerges is called a clinal pattern, in which the prevalence of a variant is highest in one particular region, then declines with increasing distance from that region, as illustrated in figure 3.2.
Figure 3.2. Clinal pattern for the prevalence of a variant in the gene SLC45A2, which influences skin pigmentation. This variant is most prevalent in Europe and declines gradually moving south and east.
The principal reason for clinal patterns is the combined effect of mutation and migration, sometimes coupled with natural selection. A variant originally arises by mutation in one person and is transmitted to at least some of that person's descendants. Over many generations, large numbers of that person's distant descendants may inherit the variant. And if it is favored through natural selection, its prevalence increases over multiple generations. Natural selection is not required, however; a variant's prevalence may also increase from one generation to the next purely by random fluctuations. If people who inherit that variant emigrate away from the region where their original ancestor lived, they carry the variant with them, and it becomes established elsewhere. In terms of probability, however, many variants (though not all) are most prevalent in the region of their origin.
Variants that appeared recently in human history tend to be localized geographically and often display clinal patterns. Although they are found in relatively small proportions of people, there are millions of these types of variants, and, thus, they offer plenty of opportunities for DNA analysis. When subjected to collective statistical analysis, they can identify geographic ancestries with high levels of certainty, revealing some fascinating information about our genetic histories. Some of these variants are so closely tied to ancestral geography that they are called ancestry informative markers. Each individual ancestry informative marker typically provides only meager evidence on its own about a person's ancestry. As one group of scientists who examined variants sampled from 383 indigenous people from various places in the world put it: “We found not a single [variant], out of nearly 250,000, at which a fixed difference would distinguish any pair of continental populations.”14 Rather, the cumulative information from thousands of variants provides reliable statistical probabilities for ancestry, be it from a single region or diverse lines of ancestry from different parts of the world.
For example, I've had my DNA tested for thousands of ancestry informative markers, and my recent ancestry (meaning the past ten thousand or so years) is almost entirely European. This conclusion, based purely on analysis of DNA variants, is fully consistent with my recorded genealogy, which is replete with British and Dutch surnames. And it has helped address some questions about my ancestry. For example, there is a long-held tradition in my extended paternal family that the surname Fairbanks was anglicized in fifteenth-century England in the region of York from the French surname Beaumont (beau means “beautiful” or “fair,” and mont means “hillside” or “bank”). The People who immigrated to England with the Norman Conquest during the eleventh century presumably carried the name Beaumont, and their descendants later Anglicized it to Fairbanks (actually Fayerbanke at the time).
Potentially, I might confirm or refute this proposed history with Y chromosome DNA analysis. My Y chromosome haplogroup is one of the most common and widespread Y haplogroups in Europe. The subtype I have is most prevalent in northern France and Germany, the Netherlands, Belgium, Denmark, and southern England, concentrated near the shores of the North Sea. There is little doubt that men who were part of the Norman Conquest from France into England carried it, although it might have already been present in England prior to that time. Thus, the evidence is fully consistent with the tradition for how my surname originated, but does not prove it.
Although they constitute a minority of the millions of variants that define overall genetic diversity among humans, ancestry informative markers are useful for revealing a person's ancestry, for forensic purposes, and for identifying genetic variants associated with human health.15 Variants associated with geography, such as those that cause variation in skin, hair, and eye pigmentation, are scientifically, socially, and politically important, in part because they dispel historic notions of racial superiority and help us to better understand our origins and diversity. Moreover, an understanding of how variants influence genetic conditions and disease, in the context of geography and human genetic diversity, are becoming increasingly useful for the prevention and treatment of disease through modern medicine.
The uneven distribution of genetic diversity throughout the world has important implications for the meaning of “race” in humans. The notion of three major races—African, European, and Asian—makes little biological sense because those three groupings are far from equal in terms of diversity, and there are no distinct genetic lines that separate them. If genetic diversity were the sole basis of racial classification, different geographic and ethnic groups in sub-Saharan Africa should be divided into a much larger number of races than geographic and ethnic groups throughout the rest of the world, due to the higher degree of ancient genetic diversity in Africa.
Doing so, however, would also make little biological sense because so-called ethnic groups represent complex two-dimensional continua of genetic overlap for literally millions of variants. When geneticists sample DNA from indigenous people living in discontinuous geographic extremes, such as northern Europe, central Africa, and east Asia, they can use ancestry informative markers to readily define these groups. However, this simple definition is an artifact of discontinuous sampling, not a true representation of the world's genetic diversity. When sampled over a wider range of geography, DNA variants that are common in people from a certain region may be found in lower proportions of prevalence elsewhere, with much overlap for different variants. Human migrations have distributed and mixed the world's genetic variation, and mutations have added to that variation throughout the course of human history. Geographic ancestry for any particular person is best defined statistically by the combination of numerous ancestry informative markers each person carries.
What this means scientifically is that classification by race is an oversimplified and inaccurate way to biologically define people. Instead, some people have lines of ancestry that may be highly diverse, tracing to different parts of the world, whereas others have more narrow ancestry, localized to a few nearby regions or, more rarely, to a single one. For instance, my DNA-based ancestry is narrowly European, whereas a friend's DNA analysis revealed a combination of significant proportions of European, Native American, north African, and west Asian ancestry. Both of us would be classified as “white” and “not Hispanic” under the US Census classification scheme. Moreover, because current classification schemes are based entirely on self-reporting, each person's classification is determined by her or his self-perception of race rather than by any sort of scientific analysis.
Ancestry, rather than race, is what defines each of us biologically. And a statistical analysis of the combination of DNA variants each person carries makes it possible to identify with a high degree of confidence the biogeographical origins of her or his lines of ancestry. Even so, social or cultural ancestry often means much more to people than biogeographical origins, and it may not necessarily be the same as biological ancestry. As stated by the Race, Ethnicity, and Genetics Working Group of the National Human Genome Institute:
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 whi
ch 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.