It has long been assumed that the initial colonization of the Americas was a direct result of a human migration over the Bering Sea ice roughly 15,000 years ago.  Though alternate explanations have been presented, particularly with regard to the discovery of pre-Clovis sites which appear to be much older than this date, evidence of such a migration has been accepted for many years, and has become an important focal point for our understanding of the colonization of both continents.  This week, researchers from an international team led by Harvard Medical School have presented some interesting information regarding the legitimacy of this theory, by using DNA samples from a large number of native groups to determine whether this colonization took place in a single step, or through a series of smaller migrations over time. 

Single nucleotide polymorphisms (SNPs) are locations in the genome where variations in DNA sequence occur between either two individuals of the same species or two chromosomes in the same individual.   At most of these locations, only two potential variations (alleles) exist, though more have been observed at a large number of different locations.  These differences in SNP composition between individuals have long been used in the context of DNA fingerprinting, where the distributions of SNP variation can help distinguish between DNA samples from specific individuals.  In the current study, which was published in the July 11th edition of Nature, over 360,000 of these SNPs were analyzed from samples which included 52 Native American and 17 Siberian groups, the former from groups ranging from northern Canada to the tip of South America.  By using tree-building methods and cluster analyses to interpret the data from these SNP-variation data sets, a picture was revealed which comprises the most far-reaching, extensive explanation for the genetic diversity of these groups (and thus the supposed history of the movement of these peoples) ever examined.

"For years it has been contentious whether the settlement of the Americas occurred by means of a single or multiple migrations from Siberia," said Professor Andres Ruiz-Linares (UCL Genetics, Evolution and Environment), who coordinated the study. "But our research settles this debate: Native Americans do not stem from a single migration. Our study also begins to cast light on patterns of human dispersal within the Americas."

In the recent paper, this map was used to designate the areas where Native American DNA samples came from.  Samples were taken from 52 American groups, and 17 Siberian groups.  The tree on the right shows the supposed relationship between native groups which was determined from the large-scale analysis.

Instead, the authors argue, there must have been at least three different migrations which occurred over time, as evidenced by differences in the genetic makeup of the various cultural groups.  As expected, the vast majority of these native groups appear to have diverged from an initial (~15,000 years ago) “First American” migration over the Bering Sea bridge; a previously unconfirmed piece of the puzzle appears to be the fact that at least two additional streams of Asian gene flow appear to have also occurred at some point following that first journey. While these second and third migrations have been previously suggested based on analyses of tooth morphology and linguistic variations, this genome-wide study provides the first unmistakable evidence that these suggestions were indeed correct.  

"There are at least three deep lineages in Native American populations," said co-author David Reich, Professor of genetics at Harvard Medical School. "The Asian lineage leading to First Americans is the most anciently diverged, whereas the Asian lineages that contributed some of the DNA to Eskimo–Aleut speakers and the Na-Dene-speaking Chipewyan from Canada are more closely related to present-day East Asian populations."

Following these migrations from the Asian continent, understanding the history of movement and gene flow between various native American groups is fairly straightforward.  In the late 1490s, however, the introduction of both African and European peoples to the continents makes the interpretation of further analysis a bit murky.  Cutting through a great deal of this “noise” to arrive at the conclusions that they did was no simple task for the research team, but they were eventually able to arrive at a concise, relatively simple explanation for the blood origins of the native groups of the Americas.    

"The study of Native American populations is technically very challenging because of the widespread occurrence of European and African mixture in Native American groups," said Professor Ruiz-Linares.

"We developed a method to peel back this mixture to learn about the relationships among Native Americans before Europeans and Africans arrived," Professor Reich said, "allowing us to study the history of many more Native American populations than we could have done otherwise."

**All quotes taken from the original press release from the Harvard Medical School website:

http://hms.harvard.edu/research

For more information, see the original research paper:

http://www.nature.com/nature/journal/vaop/ncurrent/full/nature11258.html