Everyone Is African (5 page)

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 its 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.

The reason people with entirely African ancestry have the highest diversity in the world is straightforward and can be illustrated with a simple
analogy. When I was in grade school, my friends and I used to play games with marbles, and I had a large collection of them. They were highly diverse, with a multitude of colors and patterns. Some marbles in my collection were identical to others, especially those with solid colors—and I had numerous copies of those, lots of solid red, green, blue, orange, yellow, black, and white marbles. Several were much more varied in their coloration but also more rare, represented just once or a few times in my collection. Now, imagine thousands of these marbles, both common and rare types, are mixed randomly in a large container. Imagine reaching into this large container with a cup and scooping out about fifty marbles. The overall diversity of marbles in the cup is not likely to be the same as in the container. Some of the rare types will almost certainly be absent from the cup, remaining only in the large container. Most, and perhaps all, of the more common types are likely to be in the cup, albeit in somewhat different proportions than in the container. In any case, the collection of marbles in the cup is likely to be less diverse than the one in the large container, and the reason has to do with sampling a less diverse subset of individuals from a much larger and more diverse collection.

This same sampling phenomenon happens genetically when a group of people emigrates from a region. Those who leave the original population constitute a subset of that population, and they carry a subset of the overall genetic diversity of the original population. They become the founders of a new population containing the more limited genetic diversity of the emigrants. Now, imagine that several generations later, another group of people emigrates away from the descendants of the first group of emigrants. The diversity diminishes even further in these secondary emigrants. Each group of subsequent emigrants carries a less diverse subset of the diversity that was present in the population from which they originated. Thus, the greatest diversity should be among people whose ancestors constituted the original population, and the region where they live typically represents the region of origin. For humans, that region unquestionably is sub-Saharan Africa.

Studies of human genetic diversity consistently show the diversity in people of African ancestry is the highest in the world. And the evidence from both anthropology and DNA strongly supports a scenario in which people emigrated out of Africa about sixty thousand to seventy thousand years ago
and founded what ultimately became the rest of the world's human population, carrying with them a subset of the diversity in Africa. Although far more people in the world's current human population are descended from these out-of-Africa emigrants than from people who remained in Africa, the majority of the world's genetic diversity is still indigenous African.

This observation explains and augments the major conclusion of Lewontin's 1972 study, discussed in the
previous chapter
. His observation that there is more diversity within major geographic groups than among them is largely a result of the original diversity that was present in Africa more than one hundred thousand years ago, when all humans lived there. The emigrants who left Africa carried a subset of that original diversity in their DNA, and, as a result, many of the same variants are present in people throughout the world, both African and non-African. Thus, much of the variation within major groupings of people is original African variation predating the out-of-Africa diaspora.
⁵
More recent variants—those that originated after the dispersal of humans throughout the rest of the world—should be more rare and concentrated in geographically localized populations. The more recent these variants are, the more rare and geographically localized they should be. And, consistent with Edwards' description of correlation, these more recent variants tend to be correlated with one another according to the region of more recent geographic origin.
⁶

Patterns of genetic diversity are evident in all types of DNA, but they have been most extensively documented in what is called
mitochondrial DNA
, and for some very good reasons. In general, each of us inherits about half our DNA from our mother and half from our father. But mitochondrial DNA is a very important exception. It resides in different compartments of our cells than the rest of our DNA, compartments called mitochondria, and each of us inherits our mitochondrial DNA
exclusively from our mothers
. Thus, variants in mitochondrial DNA are inherited purely through the maternal lineage, from a mother to all her children, but transmitted to the following generation only through females—from mother to daughter. Although males have mitochondrial DNA, it is a hereditary dead end; they do not pass it on to their offspring.
⁷

Mitochondrial DNA is relatively small compared to the rest of our DNA—only 16,569 base pairs, compared to slightly more than six billion
base pairs in the rest of our DNA (slightly more than three billion inherited from each parent). Thus, it is relatively easy for scientists to track and sequence it. However, it has one feature that makes it especially useful for studying diversity: mitochondrial DNA does not recombine.

For most of your DNA, you inherited half from your mother and half from your father. Go back a generation, and each of them inherited essentially half of their DNA from each of their parents, so about one quarter of your DNA is from each of your four grandparents. During the formation of an egg cell in your mother, the DNA molecules from her parents came together and exchanged segments, shuffling the information they carried. This same type of shuffling also happens during the development of sperm cells in males. And this shuffling recombines maternal and paternal DNA in every generation.

Mitochondrial DNA, however, does not recombine. It is replicated faithfully each generation and inherited through multiple generations exclusively through maternal lineages. If a mutation happens in mitochondrial DNA, it may end up being transmitted as a variant from mother to daughter through subsequent generations. Then, in a later generation, a new variant may originate against the background of the first variant. Some people inherit just the first variant, and others inherit the second variant on the background of the first. This pattern then repeats itself for additional variants that arise at various times and places through many generations. Because there is no recombination, new variants are superimposed on backgrounds of previous variants.

A simple analogy illustrates how these various layers of variants that originated as mutations at different times in mitochondrial DNA allow scientists to reconstruct ancient human genetic history. Before the printing press became available, scribes made handwritten copies of valuable manuscripts. In most cases, the original manuscript had been lost or was not available, so scribes made copies from other copies. Occasionally, a scribe made an error—perhaps a word copied incorrectly or left out—and other scribes subsequently copied the change. Then, later, another scribe made yet another error, adding it to a manuscript with the previous error that had persisted through several rounds of copying, so now two errors were present. Later, another scribe added yet another error to these two. Over time, errors accumulated, more recent ones added to earlier ones. The earlier errors tend to be more widespread,
whereas the more recent errors are localized among fewer copies. Modern literary scholars can compare all existing copies of a particular work and hierarchically group them, ultimately extrapolating back to determine much of the original wording.

Scientists who examine the sequences from mitochondrial DNA can reconstruct the same sorts of hierarchical groupings based on the variants they find. Widespread variants in large numbers of people from diverse geographic origins must be the most ancient. Rarer variants in smaller groups of people with a more limited geographic origin must be more recent. And, in each case, newer sets of variants are superimposed on identifiable sets of older variants, allowing scientists to classify different sequences of human mitochondrial DNA hierarchically into numerous small groups clustered within sets of larger groups, and yet again within sets of even larger groups. Ultimately, the variants coalesce into a single group of the most ancient variants, which is the trunk of the maternal human family tree. Furthermore, many of the mitochondrial DNAs examined are from people who belong to indigenous populations, groups of people who have been geographically and reproductively isolated for many generations. By comparing mitochondrial DNA sequences with the geographic regions where these indigenous people live, scientists can reconstruct the geographic migration patterns of ancient humans.