Tracing the First Americans

The idea that the Americas were originally settled by Asian people migrating to North, and eventually South America across the Bering Strait, has been conclusively established by more than a century of extensive archaeological work. But recent decades have provided scientists with powerful new investigative tools to confirm this hypothesis. For over 24 years, Associate Professor of Anthropology Theodore Schurr has been tracing the history of ancient peoples through genetic analysis of DNA. His current research establishes a link between Native Americans and an ethnic population from the Altai region who are native to a small area of south central Siberia. The work was published in the American Journal of Human Genetics.

“Genetic methods have been used to study indigenous Siberian history for several decades,” Schurr notes. The technique examines DNA samples from present-day populations in Siberia and zeroes in on specific genetic markers that can be compared with those in Native American genomes. It’s the latest example of groundbreaking anthropological research that has expanded the boundaries of the science well beyond the analysis of artifacts or linguistic patterns and provides direct links from people living today to their ancestors in the distant past.

The Bering Strait theory, or the Beringian incubation model, as it’s known among scientists, is a relatively recent idea that identifies Beringia as a staging ground for the diversification of ancestral Native Americans. The theory was originally based on archaeological and geological evidence showing that humans had settled in North America at least 15,000 years ago and spread to South America over the next several millennia. But by analyzing DNA patterns found only in certain indigenous peoples, common ancestry can be shown even between populations that are widely separated by geography. Schurr and his collaborators concentrated their work on the mitochondrial DNA (mtDNA), which is passed only through female lineages, and the Y chromosome, which is inherited strictly through men. Specific variants in mtDNAs and Y-chromosomes allow female and male lineages to be identified and traced. Schurr explains that high-resolution methods to characterize these particular genetic variations have become available to anthropologists only during the past decade.

Samples from about 750 Altai and 1,500 Native Americans were analyzed and compared, along with those from other parts of Asia. “To understand what our data are telling us, we compare them to other data sets obtained from extant groups living in Eurasia and the Americas, both those generated by us and those produced through studies by other research groups from Europe, Russia and the United States,” Schurr says. Samples from contemporary populations are also compared with ancient human DNA samples recovered from archaeological sites, which provides another window into the various factors that influence genetic variations.

Schurr and his colleagues found that both Native Americans and the Altai display a particular male lineage known as Q, as well as a specific sublineage named Q-M3. On the female side, the mtDNA lineages A, B, C and D are found in Altai people and Native Americans. While some other Asian groups display some of these genetic markers, only the Altai share all of them with Native Americans.

For Schurr and his research group, the evidence points to a common origin, with Native American populations beginning to diverge genetically from the Altai about 25,000 to 20,000 years ago. This agrees with the Beringian Incubation Model, in which Asian people first settled in the Bering Strait region and then moved onward throughout the Americas.

Schurr, who has been at Penn since July 2001, has spent his entire career studying the genetic diversity and evolution of human populations from various parts of the world, including Africa; Australia; the Caucasus; Melanesia; North, Central and South America; Russia; South and Southeast Asia; and Turkey. His efforts have been aimed at clarifying human history from an anthropological genetic perspective. He’s concentrated particularly on exploring the origins of Native American populations and their genetic links to native Siberian peoples, as in this most recent work.

Such research helps to elucidate the prehistoric migrations that have shaped genetic diversity in Siberia and the Americas and the historical interactions of indigenous groups with Europeans. Schurr explains that molecular genetic methods can answer many pressing questions of anthropological importance, including the effects of social organization on the genetic structure of human populations; the emergence and spread of agricultural populations; the initial expansion of modern humans out of Africa; how and when Australia was settled; and how humans adapted to different environments.

The next step for Schurr and his group is new fieldwork in Siberia with other indigenous populations to expand their knowledge of the genetic history of the Altai-Sayan region and other locations in Russia. They also plan to extend their genetic analysis to involve other parts of the human genome beyond the mtDNA and Y-chromosome, which will provide fresh details about how evolution and environment have shaped genetic variations. “We’re especially interested in learning more about how indigenous Siberian populations have adapted to their environments, particularly from metabolic and bioenergetic viewpoints,” Schurr adds. For anthropologists such as Schurr, our DNA is more than simply a statement of what we are now, but an unbroken line to what we were and our ultimate origins in humanity’s dim past.