Everyone Is African (7 page)

This deletion completely obliterated any function for the O variant; it could no longer encode production of the substance that causes type A blood. The letter O is from the German word
ohne
, which means “without.” More than four billion people throughout the world have type O blood, so, obviously, the lack of function conferred by the O variant is not detrimental for survival. In fact, as we'll see momentarily, there is some evidence that it anciently conferred an
advantage
for survival in some parts of the world.

Although all four blood types—A, B, AB, and O—are distributed among people worldwide, the relative proportions of these types differ geographically. Type O blood is the most common type worldwide, present in about 63 percent of the world's people, even though the O variant is derived from an ancient African mutation. Its predominance is particularly high in people whose ancestry is African, northeast Asian, or Native American, especially in Central and South America, where it approaches 100 percent among indigenous people. Type A is most common in people whose ancestry is Aboriginal Australian, northern European, or from the northern regions of North America. Type B is most frequent in people whose ancestry is from central Asia ranging from northern Russia to the southern tip of India. Type AB is the most rare type everywhere, and it happens only when a person inherits the A variant from one parent and the B variant from the other.

A complex pattern of ancient human emigrations, natural selection, and random fluctuations in the proportions of different variants accounts for the uneven distribution of ABO blood types. Natural selection has apparently
been underway for all of human history, with different ABO blood variants conferring resistance or susceptibility to several diseases—in most cases, favoring type O blood and thereby explaining its high worldwide prevalence. The most important is protection against severe malaria. People with type O blood may contract malaria, but, once they have it, they are less likely to develop symptoms as severe as those who have other blood types. This observation may explain why type O blood is the most common type in Africa and other tropical regions, where malaria may have acted as a selective agent favoring survival of those people with type O blood who contracted malaria both before and since the out-of-Africa diasporas.
¹⁰
By contrast, people with type O blood may be more susceptible to cholera than those with type A, B, or AB, which explains the higher prevalence of the A and B variants in regions where cholera was historically common but malaria was not.
¹¹

The uneven yet worldwide distribution of A, B, and O variants is typical of ancient African variation in humans. The variants originated anciently in Africa and have spread throughout the world through complex human migrations. A combination of factors, including migration, natural selection, and random fluctuations, resulted in different proportions of the variants among people in different geographic regions.

Variants in DNA can generally be classified as three different types as a result of ancient human migrations: 1) ancient African variants present in people throughout the world, such as the ABO blood variants and
NANOG
variants we just discussed; 2) ancient African variants that remained predominantly or exclusively in people who were native to Africa until recent times; and 3) more recent variants that trace to a particular region, found in people with ancestry from that region. A subclass of this third class is very recent variants that are extremely rare and highly localized, and they are exceptionally numerous because of the rapid expansion of the world's population in modern times.
¹²

Although most of the worldwide variants are the legacy of ancient African diversity, the recent localized variants tell us much about how recent diversity originated and what it means. The scientific literature is replete with examples, and my colleagues and I have discovered some of them firsthand in research we conducted with DNA from geographically diverse human populations.
¹³

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: