DNA sequences tell the tale
By Daniel W Nebert, MD • January 11, 2009

The 1953 discovery of DNA as a linear string of bases, the ability to sequence these bases in the 1970s, and incredible advances in DNA sequencing techniques, combined with bioinformatics computer software during the past decade, have led to fantastic breakthroughs in our understanding of evolution and racial differences.

The DNA bases are adenine, guanine, cytosine and thymine; we will refer them simply as 1, 2, 3 and 4. Suppose that a particular stretch of DNA stretch was 123412341234123412341-234123412 in the original modern human (Homo sapiens sapiens) around 350,000 years ago in Africa. As this sequence gets passed from one generation to the next, very little change occurs; however, over very long periods of time these DNA bases get randomly changed. Therefore, the more time a group has existed, the more changes from the original sequence are likely.

Let’s say this 30-base sequence today is 33 33 114 444 32 11 31 1442 1322 114 444 in a sub-Saharan African, 1233 12341232 1231 14 32 12344 24 413 in an East Asian, 3 23412341232 123-4121 42 23412344 2 in a Caucasian, and 1233 1234123414 3412341234124 412 in a Native-American from the rainforest of Panamá.

In other words, the African DNA shows 20 changes, the East Asian shows eight, the Caucasian five, and the Amerindian three; this means that Africans are the oldest population, the East Asian is older than the Caucasian, and all three are older than the Amerindian.

Today, we can gather sequence information to include the 20 million or more changed sites among the 3 billion base pairs in the human genome in each of many people from various ethnic populations. This information, combined with archeology and radioisotope-dating of fossils (and rocks found with the fossils), gives us an increasingly accurate picture of what has transpired during the past 5-6 million years since we humans diverged from our primate ancestor. Not only do we know approximately when and where human groups have migrated, we can even make valid estimations as to the number of people in that migration!

It is now very clear that humans originated in Africa and that modern human “out-survived” more than a half-dozen other subspecies that started and then failed between about 5 million and 350,000 years ago. One or more early migrations out-of-Africa (100,000 to 300,000 years ago, to the Middle East, and to Southeast Asia) did not survive permanently. We know that modern human migration, leading virtually to all non-African genetic diversity, took place only 50,000 to 65,000 years ago. One migration eastward led to the northern (Japanese) and southern (Chinese) East Asian branches. Migration northward from the Mediterranean resulted in the Caucasian branch; about 1,500 individuals are estimated to have founded the ancestral Eurasian population around 40,000 years ago.

Major groups of Melanesia (which includes New Guinea) were settled between around 50,000 and 30,000 years ago, and people migrated to the Polynesian Islands soon thereafter. With regard to colonizing the Americas, current genetic evidence indicates dispersal from a single Siberian population, across the Bering Land Bridge (migration of less than 200 people!) to what is now Alaska, took place around 16,500 years ago. Migration of Amerindians continued from northern North America to Central and South America – mostly along coastal routes.

When a population settles down in a specific geographic locale and interbreeds for 10,000 years (around 400 generations) or longer, certain characteristics become selected in response to the pressures of climate, weather and/or diet (called “adaptation”). Thus, African pygmies exhibit short stature, big eyes and large nostrils which facilitate hunting in the humid jungle. Africans on grassy plains became tall, thin, and able to run fast – to be most successful in their hunting. Eskimos developed small sloped eyes and narrow nostrils, which allowed them to see and breathe better under harsh windy, snowy and cold conditions. Africans developed dark skin, which was needed to prevent low folic acid levels, because they lived in almost constant strong sunshine. When humans migrated northward to less sun-intensive regions, low vitamin D levels became a problem; the adaptation of light skin color allowed better vitamin D production and hence stronger bones.

“Race” can therefore be simply defined as “major geographically-isolated subgroups” of human populations worldwide. There are five such major subgroups: African, East Asian, Caucasian, Oceanian (Melanesia, Polynesia), and Amerindians (North, Central and South America). All additional ethnic groups (Japanese vs. Chinese; Jews vs. Arabs; Swedes vs. Italians) are “offshoots” from these five major subgroups. “Latino” is a great example of the recent amalgamation of Amerindian, African and Caucasian – which has occurred within the past 450 years (around 18 generations) following the invasion of Spaniards and Portuguese into South and Central America. Although these groups have diverged with different traits, and the evolution of these traits can now be understood in scientific terms, “racism” in its negative connotation is completely a cultural-context issue.

Daniel W. Nebert, M.D., is a professor at the University Cincinnati Medical Center, and adjunct professor of pediatrics at Children’s Hospital, division of human genetics. His career has involved molecular genetics, evolutionary genomics, and gene nomenclature. He has done research on the genetics of drug metabolism, and has written extensively on ethnic differences in how we handle drugs. Nebert spoke at a symposium on race and ethnicity for the American Association for the Advancement of Science in 2003.