Challenging a number of published statements based on Lewontin's research, Edwards emphatically concludes,
It is not true [as Lewontin claimed] that “racial classification is…of virtually no genetic or taxonomic significance.” It is not true, as Nature claimed, that “two random individuals from any one group are almost as different as any two random individuals from the entire world,” and it is not true, as the New Scientist claimed, that “two individuals are different because they are individuals, not because they belong to different races” and that “you can't predict someone's race by their genes.” Such statements might only be true if all the characters studied were independent, which they are not.21
So who is right? When Lewontin did his analysis in 1972, the number of people and the number of genes he examined were just a minuscule fraction of the number of people and genes that have been studied today. More extensive research has confirmed the conclusions of both Lewontin and Edwards—negating, however, many of the claims that others inferred from Lewontin's results. In 2004, human geneticists Lynn Jorde and Stephen Wooding of the University of Utah School of Medicine summarized the results from several large-scale studies. First, they confirmed that humans as a species are much less diverse than many other species. According to their estimates, people worldwide differ on average by about 0.1 percent, evidence that all humans are genetically quite similar to one another. They then confirmed Lewontin's major conclusion:
Of the 0.1% of DNA that varies among individuals, what proportion varies among the main populations? Consider an apportionment of Old World populations into three continents (Africa, Asia, and Europe), a grouping that corresponds to a common view of three of the “major races.” Approximately 85–90% of genetic variation is found within these continental groups, and only an additional 10–15% of the variation is found between them…. These estimates…tell us that humans vary only slightly at the DNA level and that only a small proportion of this variation separates continental populations.22
The accumulation of information from many individuals and their DNA also revealed correlations that correspond with the three major continents of geographic ancestry. In all cases, Jorde and Wooding were able to accurately assign native Europeans, east Asians, and sub-Saharan Africans (Africans whose ancestral origin is south of the Sahara) to their respective continents of origin even when the samples were examined for DNA variation alone without taking into account any other characteristic.
The authors were quick to point out, however, that their data do not support the traditional boundaries of race:
[I]t might be tempting to conclude that genetic data verify traditional concepts about races. But the individuals used in these analyses originated in three geographically discontinuous regions: Europe, sub-Saharan Africa, and east Asia. When a sample of South Indians, who occupy an intermediate geographic position, is added to the analysis, considerable overlap is seen among these individuals and both the east Asian and European samples, probably as a result of numerous migrations from various parts of Eurasia into India during the past 10,000 years. Thus the South Indian individuals do not fall neatly into one of the categories usually conceived as a “race.”…Ancestry, then, is a more subtle and complex description of an individual's genetic makeup than is race.23
This quotation makes a crucial point regarding sampling and interpretation of scientific data from human populations, one that often leads to misconceptions regarding racial classification. Our species is highly mobile; people have been migrating throughout the world for thousands of years, resulting in a distribution of genetic variation that is complex, overlapping, and more continuous than discrete. When people are sampled from discontinuous geographic extremes (such as northern Europe, east Asia, sub-Saharan Africa, Australia, and the Americas), without including samples from people whose ancestry originates between these extremes (such as the Middle East, central Asia, and south Asia), data from these discontinuous samples, not surprisingly, portray a discontinuous distribution and can be classified into discrete groups. Even so, about 85 percent of genetic variation is still shared among these so-called discrete groups. A more accurate representation of the worldwide human population, however, is a sampling of people whose ancestral origins are spread throughout the world. When DNA is analyzed from a more geographically broad sample of people, substantial overlap is evident, and the notion of discrete racial boundaries disappears.
The massive amount of scientific evidence we currently have reveals complex and interwoven histories of human diversity, which provide a far more compelling case against racism than ever before. In the upcoming chapters we explore this evidence and what it tells us about the history of our worldwide human family.
On the ceiling of the Sistine Chapel in the Vatican are Michelangelo's magnificent fresco paintings, one of the greatest triumphs in the history of art. They depict biblical stories, the three central panels portraying the creation of humankind. Perhaps the most famous is a powerful image of God surrounded by angels and reaching out to touch Adam's extended finger, granting him the spark of life. The center panel shows Eve emerging from Adam's rib cage—Adam having been divinely anesthetized into a deep sleep—while God beckons Eve to come forth. The third panel shows two stories, one with Lucifer as a humanized serpent tempting Adam and Eve, and the other, the expulsion of Adam and Eve from the Garden of Eden by a sword-wielding angel robed in crimson (figure 2.1).
Figure 2.1. Fresco paintings on the Sistine Chapel ceiling by Michelangelo. Image from Michelangelo: des meisters werke in 166 Abbildungen by Fritz Knapp, 1906.
Congruent with widely accepted beliefs in sixteenth-century Italy about the biblical account of human origins, Michelangelo depicted Adam, Eve, God, Lucifer, and the angels with Caucasian features. The same is true for innumerable depictions of Eve and Adam by European and American artists who lived before and after Michelangelo. The trend persists to the present. The Creation Museum in Petersburg, Kentucky—a suburb of Cincinnati, Ohio—displays life-size mannequins of Adam and Eve with Caucasian features, consistent with most contemporary American religious depictions of Eve and Adam.
Modern science paints a dramatically different picture of human origins and the genetic basis that underlies worldwide human diversity. The scientific picture is founded on abundant evidence from a wide range of sources, including anthropological excavations, research on how human genetic characteristics are distributed and inherited, and large-scale analysis of DNA from thousands of people representing the world's indigenous populations. This evidence reveals in abundant and exquisite detail where and when our species originated and how humans populated the world.
The evidence can be divided into two major categories. The first is anthropological evidence: skeletal remains of ancient humans unearthed at archaeological sites and relics of human activity such as tools, ornaments, potsherds, waste, and building remnants. The second is genetic, which includes the distribution of inherited characteristics among humans, coupled with a flood of recent evidence from DNA that is increasing at an exponential rate. Although these two major lines of evidence have been mostly independent, they converge to tell essentially the same story. The major conclusion, now supported by overwhelming evidence from both anthropology and genetics, is solid and no longer questioned by scientists: the human species originated in Africa.
The anthropological evidence alone is powerful. The oldest remains of what anthropologists call “anatomically modern humans” (skeletons with features that resemble those in modern humans) are exclusively from Africa, dating to about two hundred thousand years ago. By contrast, the earliest remains of anatomically modern humans outside of Africa thus far discovered are about one hundred thousand years old. These are from an ancient population that lived in what is now Israel, determined from human remains found in the Qafzeh and Es Skhul caves.1 This population apparently suffered extinction around seventy thousand years ago, leaving no modern descendants. Remains of anatomically modern humans d
ating to sixty thousand years ago or less are common both inside and outside of Africa, and the more recent they are, the more abundant and widespread they tend to be. The locations and dates of these remains are consistent with a scenario in which humans emigrated out of Africa about sixty thousand to seventy thousand years ago, and their descendants, through many generations, eventually populated the rest of the world.
There is, however, evidence that other humanlike species lived in Asia and Europe long before the dates assigned to remains of anatomically modern humans in these regions. Homo erectus, for example, emigrated out of Africa at least two million years ago, before anatomically modern humans existed, migrating across Asia into what is now China and Southeast Asia. Neanderthals evolved in Europe from ancient ancestors who apparently emigrated about seven hundred thousand years ago from Africa into Spain across what are now the Straits of Gibraltar. Neanderthals then spread throughout Europe and much of the Middle East.2
The prevalence of fossils outside of Africa from humanlike species that predate anatomically modern humans led some anthropologists to propose what is known as the multiple-origins hypothesis, the idea that modern humans arose in several places throughout the world from humanlike species already there. The alternative is the single-origin hypothesis—sometimes called the “Out of Africa” hypothesis because it proposes that anatomically modern humans evolved in sub-Saharan Africa, and then a subset of these humans later emigrated out of Africa and became the ancestors of everyone whose ancestry is not confined to Africa. Notably, both hypotheses propose a sub-Saharan African origin for the ancient ancestors of all modern humans.3
As anthropological evidence from numerous excavations accumulated over time, it tended to favor the single-origin hypothesis. For instance, anatomically modern humans and Neanderthals overlapped in Europe and the Middle East from about sixty thousand years ago, when people immigrated into the Middle East from Africa, to twenty-six thousand years ago, when Neanderthals suffered extinction. Had humans originated from Neanderthals in those regions, we would expect to observe fossils that gradually change from Neanderthal anatomy to modern human anatomy. Instead, both Neanderthal and modern-human fossils simultaneously date to this time period, consistent with a scenario in which modern humans immigrated into regions already occupied by Neanderthals.
Once Neanderthal DNA was extracted and examined, it became evident that for people throughout the world, except those whose ancestry is entirely African, a small amount of DNA (less than 4 percent) is derived from Neanderthals.4 For instance, I have 2.8 percent Neanderthal DNA, according to a DNA test. Some have argued that this observation supports the multiple-origins hypothesis. On the contrary, it supports the single-origin hypothesis with limited mating between modern humans and Neanderthals in the Middle East and Europe from about sixty thousand to twenty-six thousand years ago, when the geographic distribution of the two overlapped. If this is the case, we expect to see no Neanderthal DNA in people of entirely sub-Saharan African ancestry because, having evolved in Europe and the Middle East, Neanderthals were not present in Africa at the time when modern humans emerged in Africa as a distinct species. This is exactly what the data show: people entirely of sub-Saharan African ancestry have no Neanderthal DNA.
This discovery is just one among a deluge of discoveries in recent years from DNA analysis. The most abundant, detailed, and compelling evidence of our origin and history is genetic—from DNA—and it indisputably supports the single-origin hypothesis. This evidence reveals the ancient history of our species, including how and when humans populated different regions of the world, and, in doing so, it explains much about the concept of race in humans. It is here—in the patterns of diversity in our DNA—that we find the strongest evidence of an African origin for modern humans, evidence that, for the most part, has been collected and analyzed independent of the evidence from anthropology. Yet the DNA evidence is entirely congruent with the anthropological evidence.
Our genetic history is written in our DNA. The word written, of course, is metaphorical; no one took a pen and wrote out our genetic information. But it is an apt metaphor because DNA carries linear hereditary information, much like the linear arrangement of letters in written language. And scientists can now readily decipher and read that information.
Each DNA molecule is long and slender, composed of bases, analogous to the letters on the page of a book. The number of bases in each DNA molecule in humans ranges from the thousands to more than two hundred million. The English alphabet has twenty-six different letters. The DNA alphabet is much simpler, with only four different bases, represented as the letters T, C, A, and G. Because of the linear arrangement of DNA information, we can write the sequence of any DNA molecule with these four letters. For example, a very small part of the DNA responsible for determining the amount of pigment in human eyes, hair, and skin has this sequence:
DNA molecules are typically double stranded, which means there are two parallel strands of bases in each molecule. Each base in one strand is paired with a corresponding base in the other, and they pair according to strict rules: T pairs with A, and C with G. So we can write the sequence we just examined as a double-stranded sequence by matching the bases according to the T–A and C–G rules, with the bases in one strand directly below their paired bases in the other:
Notice that every T is paired with an A and every C with a G. This pairing of bases along the two strands makes possible one of DNA's essential functions: its ability to faithfully replicate. When a DNA molecule is replicated, it splits into single strands, and a process in your cells makes new strands paired to the old ones according to the T–A, C–G pairing rules. When replication is complete, the two resulting DNA molecules are identical in their sequences, each with an original old strand and a newly made strand:
This precise replication is the foundation for biological reproduction. Ultimately, the fact that children genetically and outwardly resemble their parents is, at its most fundamental level, a consequence of DNA replication and its fidelity.
Although DNA usually replicates faithfully, producing two identical double-stranded molecules from a single original double-stranded molecule, in rare instances the sequence changes ever so slightly, often by just one base pair. Once changed, the altered sequence is then replicated faithfully from that point forward; in other words, the change is inherited. When these changes initially occur, they are called mutations. However, scientists often instead refer to these inherited changes as variants because most of the variation in our DNA is considered normal—a consequence of mutations that happened long ago in our ancestors, variants that have since been inherited over many generations. All the variants in the DNA of all people constitute the genetic diversity of our species, and these variants originated as millions of mutations that arose at various times in our ancestors.
Many of these variants have no effect whatsoever. Others may influence the variation in our outward characteristics. For example, a variant associated with differences in skin, hair, and eye pigmentation in humans happens to be within the DNA sequence we just examined. Some people have the sequence
whereas others have the sequence
And some people have both of these sequences, one inherited from each parent. Notice that the boxed C–G pair in one sequence is a T–A pair in the other sequence. The C–G pair, as it turns out, is the original version, and the T–A variant arose from it long ago through mutation. We typically use the term ancestral variant to denote the original DNA sequence carried in ancient humans and the term derived variant for any variant that arose by mutation from the ancestral variant. In this example, the T–A variant is derived, and it is associated with less pigmentation in eyes, skin, and hair. It is highly prevalent in people with northern European ancestry, whereas the ancestral C–G pair is associated with a greater amount of pigment and is most common in people whose ancestries trace to parts of the world outside Europe.
Having briefly discussed the nature of DNA and it
s variants, we're ready to examine the evidence of our origins and see what it tells us about human diversity. Every human being has about six billion pairs of bases in the forty-six DNA molecules we carry in each of our cells. And we all are extremely similar in the base pairs we carry, about 99.9 percent identical on average. The tiny proportion of DNA that makes each of us genetically different from everyone else, however, is significant. Because 0.1 percent of six billion is six million, the number of variants one person carries relative to another can number in the millions, and the degree of difference depends to some extent on how closely related those two people are.
The differences we carry in our DNA encode the genetic diversity of humans throughout the world. And the measure of diversity is not just the number of variants but also their prevalence. A variant that is present in less than 1 percent of people contributes less diversity than one that is present in 10 percent of people. As scientists have studied variants and their prevalence, one major conclusion has emerged in essentially every large-scale worldwide study: the highest diversity by far is among people whose recent ancestry is African.
I use the word recent here because if we go back far enough, everyone's ancestry is African. In this case, recent means within the past several thousand years. And African, in this sense, means sub-Saharan African. It does not include most people who currently live in northern Africa (mostly in Egypt, Libya, Algeria, and Morocco), who descend, to a large extent, from immigrants who entered northern Africa from the Middle East, the Balkans, and Europe. Nor does it include Africans who descend predominantly from people who immigrated to Africa from Europe during the past several centuries, such as South Africans descended from Dutch and British immigrants.
Everyone Is African Page 3
