Waves of Migration

At some point within the last 35,000 years, prehistoric humans walked from the tip of Asia across a land bridge to Alaska, from where they spread through the unpeopled continents of North and South America. What else do we know about the arrival of these earliest Native Americans? Not much. The scant evidence available from archaeological sites suggests it could have been anywhere from 15,000 to 33,000 years ago. One widely held theory, based in part on linguistic analysis, holds that the migration occurred in three distinct waves, related to three well defined groupings of Native American languages.

Until fairly recently, these and similar questions of human origin — are we more like gorillas or chimps? — were the almost exclusive province of archaeologists and fossil-hunter paleontologists like the famous Leakeys. But since about 15 years ago, molecular biologists have been stirring the pot with a trove of new information provided by their ability to deduce evolutionary history from DNA.

University of Pittsburgh geneticist Andrew Merriwether is one among this new breed of genetic detective. Merriwether leads a team of researchers carrying out the most extensive survey yet undertaken of DNA from Native American populations, both living and ancient. Using the Pittsburgh Supercomputing Center's CRAY C90, he has analyzed DNA sequences, to identify and map genetic differences, on a scale that would be unthinkable without supercomputing. His recent findings, published in collaboration with colleague Robert Ferrell, are reshaping ideas of how the New World was peopled. "The distribution of genetic patterns," says Merriwether, "best fits a single wave of migration."

Mitochrondrial Eve

Merriwether's research is based in methods pioneered about 10 years ago by scientists at the University of California, Berkeley, who realized the advantages of using mitochondrial DNA to study evolution. Although most of our DNA resides in cell nuclei, another part of the cell, the mitochondria, also contains relatively short DNA strands. Research has shown that slight changes from generation to generation accumulate steadily in this DNA, like a fast-ticking evolutionary clock — making it possible to extrapolate backward in time.

Mitochondrial DNA, furthermore, is inherited from the mother alone. Nuclear DNA, by contrast, bears the genetic imprint of two parents, four grandparents, eight great-grandparents, etc. Because only one parent contributes to mitochondrial DNA, genetic changes trace backward in a single line, one person in each generation, making it feasible in theory to trace all humans to a single ancestor — so-called "mitochondrial Eve." Several major studies have used these methods to arrive at the still controversial conclusion that all humans are descended from people living in Africa about 200,000 years ago.

Results: A Single Wave

Early research with human mitochondrial DNA showed that genetic patterns cluster by geographical regions and that relationships between populations — evolutionary trees — can be established from these patterns. Some of this research shows genetic relations between Native Americans and Asians, supporting the theory of Asian origin. Merriwether's work is in the same vein, using Native American mitochondrial DNA to shed light on the migration of humans to the New World. "Using these molecular genetic techniques," he says, "we're attempting to trace the ancient migratory routes and determine how long ago contemporary populations separated from each other."

The three waves of migration proposed by some researchers correspond to three main Native American language groups — Amerind, Nadene and Eskaleut. According to this theory, Amerind people arrived about 30,000 years ago, followed 20,000 years later by the Nadene and then the Eskaleuts, who came to Alaska and northern Canada within the last 7,000 years. Some earlier mitochondrial DNA research supports this theory. This work found that all Native Americans come from four genetic lineages, labeled A through D, and that, significantly, Amerinds have all four lineages, Nadene only A, and Eskaleuts A and D — suggesting different migrations at different times.

Distribution of Native American Populations: This map shows populations included in Andrew Merriwether's study of evolutionary relationships among Native Americans. Many populations (red dots) were not included in prior studies. Other data (black dots) reported in the literature was also analyzed along with the new populations. The underlined populations indicate burial sites where DNA was collected from teeth, bone fragments and mummified tissue.
Working with a much larger dataset than the earlier studies, Merriwether found that the four lineages divide into nine distinct genetic subtypes. Furthermore, directly challenging the three-wave theory, all four lineages showed up in all three language groups. "The key finding," says Merriwether, "is that many of the populations have all four lineages and a number even have all the subtypes. And all types can be found in North, Central and South America. It isn't realistic to believe that the same lineages ended up in all these populations across two continents by separate migrations."

Merriwether's DNA sample includes 1,300 Native Americans representing more than 40 populations throughout the Americas, along with 300 samples from teeth, bone fragments and mummified tissue at three burial sites, one each in North, Central and South America. By including a broader range of populations and large sample sizes, 50 to 100 individuals per population compared to 10 to 20 in other studies, this research offers the most reliable results yet tracing evolutionary relationships among Native Americans. The large dataset also presents a demanding analytical task, which is where the CRAY C90 comes in. "When you look at thousands of individuals for long DNA sequences," says Merriwether, "you can't do it without the supercomputer. It makes problems feasible that weren't possible to address before. This methodology's been around awhile, but we didn't have the computing power. It's as simple as that."

As the dust settles and researchers in the field contemplate the likely demise of three-wave theory, Merriwether's team has taken up the task of building an evolutionary tree of the Yanomami Indians. This tribe, who live in the tropical rain forest between Brazil and Venezuela, were isolated from modern contact until the 1960s. Results from this unique living population, unmixed with post-Colombian influence, suggest at least three more genetic lineages for the original Native Americans.

Researchers: D. Andrew Merriwether & Robert E. Ferrell, University of Pittsburgh.
Hardware: CRAY C90
Software: User-developed code.
Keywords: land bridge, Native Americans, evolutionary history, DNA sequences, mitochondrial DNA, mitochondrial Eve, waves of migration.

Related Material on the Web:
University of Pittsburgh, Department of Human Genetics
Projects in Scientific Computing, PSC's annual research report.

References, Acknowledgements & Credits