African American Lives
Analyzing the Evidence
The Science and the Investigators
Who am I? A Genealogy Guide
Sharing Stories
For Educators
About the Series

The Science and the Investigators
Intro Race and Science The Tests Learning from DNA
Race and Science
Image of a 19th-century illustration of racial differences
Most scientists today agree that Homo sapiens -- modern human beings --originated in Africa some 200,000 years ago, and that between 60,000 and 100,000 years ago, small groups began migrating out of Africa and spreading out across the continents.

Over time, these groups of migratory humans became isolated from one another, and eventually mutation and other evolutionary pressures produced slight genetic changes, some of which are responsible for the differences in appearance we've commonly used to divide up human beings into racial groups. But periods of isolation were followed, as trade and transportation were developed, by periods of admixture -- the mixing of populations that had previously been separate.

Map of human migratory patterns over the past 100,000 years.
click to enlarge

A map of the migrations of early humans, as reconstructed via DNA studies.
By tracing slight changes in DNA and by combining that data with a calculation of the rate at which genetic material changes over time, researchers have been able to reconstruct the general pattern of the historical migrations of peoples across the globe, starting in East or South Africa and extending north into Europe and Asia and eastward to the Pacific Islands and through the Americas, to the tip of South America. The research indicates that all humans alive today share a couple of common ancestors, a woman who lived in Africa around 150,000 years ago, and a man who lived in Africa around 60,000 years ago, at around the time human beings began to leave Africa to explore the rest of the world.

Considering that commonality, it's no surprise the amount of variation between even the most disparate groups of human beings is very small. It's estimated that people share some 99.9% of their DNA. Even looking at that .1% that does vary, 85% of the variation that has been observed is unconnected to membership in any particular group of people; only 15% of the variation is between defined groups. In fact, because as a species we spent the most time in Africa, the greatest amount of genetic variations are currently found among people living on the African continent -- on a genetic level, an individual of East African ancestry may have more in common with a European than with a West African.

Since the differences that separate groups of humans are truly miniscule -- amounting to a little more than one-tenth of one-tenth of a percent of the entire human genome -- researchers into the genetics of populations have to work with extremely minute variations in the code of life. What they look for are genetic markers known as "single nucleotide polymorphisms" or SNPs (pronounced "snips"). These are single-"letter" variations, where one of the four bases that make up DNA -- adenine (A), guanine (G), cytosine (C), or thymine (T) -- is substituted for another at a specific point within a known gene. In particular, scientists look for and document SNPs whose frequency varies widely between different population groups. These SNPs are referred to by scientists as being highly "informative," that is, they correspond to differences in ancestry, though they may not be within the genes that code for skin color, nose shape, hair texture, or any of the other physically observable characteristics we think of as indicating race.

While the actual science is quite complex, and depends heavily on statistics and information theory, one can think of a group of SNPs that occur together -- that is, they seem to be inherited together as a group -- as making up a "haplotype," and a group of haplotypes as a "haplogroup."

When the SNPs in question are highly informative -- that is, they have been observed to occur noticeably more often in one or more population groups than simply among human beings in general, they can be used to make deductions about the history of populations and the heritage of individuals. Basically, these informative markers (and groups of informative markers linked in haplotypes and haplogroups) can be used to infer ancestry and linked to particular regions of the world or estabilished lines of descent, then simple DNA testing can be used to make surprisingly clear deductions about what sort of genetic heritage a particular person might have -- the very premise behind AFRICAN AMERICAN LIVES.

Map of distribution of human genetic markers worldwide.
click to enlarge

A map of the general distribution of genetic markers informative of ancestry among populations worldwide.
While haplogroups are not directly analogous to races or ethnicities, certain haplotypes and haplogroups are observed more frequently in some groups of people than others. It's possible, in some sense, to think of these haplogroups and haplotypes as branches of the human family tree. But it's a far more complex and varied type of tree than 19th century theorists of race had in mind -- a better image to keep in mind might be a dense, intertwined network of roots, with new shoots springing up from a number of intersections. Remember, over the course of human history populations have gone through periods of isolation and intermixing, so the difference between groups is largely a matter of different statistical frequencies of variations from an agreed-on standard, not a radical difference.

There is a biological dimension to what we understand as race, but it differs from the simplistic, linear understanding of racists who have sought to ground their ideologies in science. Looking at the human genome, race is nearly as complex a biological concept as it is a social one.

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