Submitted by Sammy Smith (sammy@thesga.org)
Global Genetic History of Homo sapiens is the title of a new special issue of Current Biology (volume 20, issue 4, dated 23 February 2010), with eight papers available free online.
The first article is a guest editorial by Colin Renfrew, a British archaeologist who has worked for decades in the Middle East. His books often focus on the emergence of civilization, the emergence of Indo-European languages, and a relatively new subdiscipline in archaeology called archaeogenetics. Renfrew has defined archaeogenetics as research at the confluence of archaeology, linguistics, and genetics. Archaeogenetics, therefore, uses molecular genetics to expand archaeological data about early human populations. This is particularly useful in reconstructions—or models—of early human migrations and the populating of the globe.
Renfrew’s editorial, “Archaeogenetic—Towards a ‘New Synthesis’?” sets up the other articles in this special issue. The next six papers address human migration in specific geographic areas: Africa, Europe, South Asia, East Asia, Oceania, and the Americas. Each provides considerable detail and summarizes genetic, linguistic, and archaeological data for the region.
Renfrew closes his introductory editorial with these observations:
Perhaps the most important general point that can be drawn from the reviews assembled in this special issue might be that we have not yet learnt how to interpret the data very effectively. A number of contributors have commented upon the need for simulation studies, based upon explicit models which might allow the testing of specific scenarios…, and this is likely to be one of the most important future research directions. Above all, the pace of research is now so fast that new insights are soon likely to become available. These are early days in the field of archaeogenetic research, and I predict that over the next twenty years or so a more coherent synthesis of the data from genetics, archaeology and linguistics is likely to emerge than we can yet envisage. [page R165]
Figure 1* from O’Rourke and Raff’s article.
As an example of the geographically defined articles, consider the one on the Americas, “The Human Genetic History of the Americas: The Final Frontier” by Dennis H. O’Rourke and Jennifer A. Raff. O’Rourke and Raff marshal mitochondrial DNA data (which is passed down through the maternal line) that shows five major genetic groups among New World peoples. They conclude that these data, along with other genetic data, suggest New World peoples coalesced “just prior to or immediately after the LGM”—the Late Glacial Maximum. The LGM refers to the last time ice sheets extended far south/north from the Earth’s poles, when mid-latitude locales were cooler and drier than today. As the ice formed, it lowered sea levels, when the ice captured sea water. Thus, more land was exposed on the margins of continents, and the Bering Sea land bridge could have provided a “highway” for Asian peoples traveling eastward, either on land or in small boats following the coast. Given the very early dates for human occupations from southern South America, O’Rourke and Raff note:
…the archaeological data in the Americas continue to raise questions regarding the timing and mode of colonization. The resolution afforded by the newer molecular data assists in evaluating alternative migration scenarios. [page R202]
Yet, even with all the data they bring together, O’Rourke and Raff conclude, as do many researchers regarding a wide variety of topics, that “more work is needed.” They write:
“Complete agreement between mtDNA, Y-chromosomal DNA and autosomal genetic systems has not yet been realized with respect to colonization models….” [page R206]
This is consistent with observations in the final article, “The Genetics of Human Adaptation: Hard Sweeps, Soft Sweeps, and Polygenic Adaptation” by Jonathan K. Pritchard, Joseph K. Pickrell, and Graham Coop. They agree with the last quote above:
Ultimately, a comprehensive model of the nature of selection would tell us how much adaptation occurs by any of a variety of different models and mechanisms. … To make real progress on these problems will require much greater integration of selection studies with biological information. [page R213]
These articles that summarize the current understanding of human archaeogenetics are insightful and informative, although some of the data they discuss is rather technical. Perhaps after you take a look at one or two of them, you will have some comments you’d like to note here—please login and do so!
The link to the special issue is here; you can download any of the eight papers individually.
* The original figure caption reads: “Hypothesized routes for original migration into the Americas. The Beringian and Pacific coastal routes (blue and yellow, respectively) may have been roughly contemporaneous following the Last Glacial Maximum (LGM), although contemporaneity is not certain. The more hypothetical northern migration path (red) implies a pre-LGM population movement. These migration paths need not be considered mutually exclusive.”
Posted online on Friday, February 26th, 2010