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rfmcdonaldWill Baird linked to a very interesting bit of research introduced by one Cecil Lewis, examining patterns of genetic diversity among indigenous populations in the Americas.
This raises the fascinating question of how the Americas were settled. The traditional hypothesis was that the continents were settled by migrants crossng the Beringian land bridge between Siberia and North America some eleven thousand years ago, these migrants then constituting the Clovis technological culture. While it does seem as if the settlers of the Americans constituted a genetically homogeneous population reflecting a single migration, other sites have been discovered predating Clovis. The Monte Verde site in coastal Chile dates back some thirteen thousand years, while Pedra Furada sites in Brazil provide evidence of human habitation as early as 12 000 years ago, and--very controversially--as long ago as 48 thousand years. The fact that many of these sites predate signs of settlement in North America is perplexing, to say the least. Is the Pacific migration theory, suggesting that humans travelled quickly down the coast of the two continents, correct? But then, if that theory is correct, how is the greater genetic diversity of eastern South America to be explained?
Lewis studies genetic variation in populations to learn about the peopling of the Americas, but his studies also have an impact on genetic-related disease research. Some 15,000-18,000 years ago, people came from Asia through the Bering Strait and began to fill the American continents. The Americas were the last continents to be populated, so Lewis wants to understand how this process happened. His recent study focuses on South America and asks what part of the subcontinent has the most genetic diversity.
A complete understanding of this research depends on a very important population genetic process called the "founder effect." The geographic region with the most genetic diversity is characteristic of the initial or "parent" population. For example, a group of people leave a parent population and become founders of a new daughter population in an uninhabited geographic region. They typically take with them only a small set of the parent population's genetic diversity. This is called a founder effect.
The world pattern of founder effects in human populations begins in Africa. The genetic diversity in the Middle East is largely a subset of the genetic diversity in Africa. Similarly, the genetic diversity in Europe and Asia is largely a subset of the genetic diversity in Africa and the Middle East. The genetic diversity of the Americas is largely a subset of that in Asia. As a result, DNA tells a story about human origins, which began in Africa and spread throughout the world
Lewis is interested in the founder effects within the Americas with a particular focus on South America. At the outset, Lewis expected western South America to have a more diverse population than eastern South America because most anthropologists believe South America to have been peopled from west to east. Unexpectedly, the genetic data from the Lewis study was not consistent with this idea.
In this new study, Lewis looked at more than 600 independent genetic markers called short-tandem repeats. These markers were dispersed throughout the human genome. They were initially published by Lewis and his colleagues in large scale collaboration; the dataset is the largest survey of Native American genetic diversity today. Surprisingly, genetic analysis of these data estimated more genetic diversity in eastern than western South America. This was not the first time Lewis observed this pattern.
Lewis first observed this pattern in 2007 with his post-doctoral advisor using a more limited genetic dataset. The fact that the new genome-wide dataset provided a similar result was surprising; this result questions the most widely accepted scenario for the peopling of South America. While the focus of this study was South America, a similar and interesting pattern of genetic diversity emerged in North America. The pattern suggests another major founder effect in North America, but after the initial founder effect from Asia.
This raises the fascinating question of how the Americas were settled. The traditional hypothesis was that the continents were settled by migrants crossng the Beringian land bridge between Siberia and North America some eleven thousand years ago, these migrants then constituting the Clovis technological culture. While it does seem as if the settlers of the Americans constituted a genetically homogeneous population reflecting a single migration, other sites have been discovered predating Clovis. The Monte Verde site in coastal Chile dates back some thirteen thousand years, while Pedra Furada sites in Brazil provide evidence of human habitation as early as 12 000 years ago, and--very controversially--as long ago as 48 thousand years. The fact that many of these sites predate signs of settlement in North America is perplexing, to say the least. Is the Pacific migration theory, suggesting that humans travelled quickly down the coast of the two continents, correct? But then, if that theory is correct, how is the greater genetic diversity of eastern South America to be explained?
