Blue Collar Scientist

Andrew Kitchen, Michael M. Miyamoto, and Connie J. Mulligan report in PLoS-ONE on their development of a three-stage model for the colonization of the Americas by Homo sapiens. This issue is of deep interest to anthropology outreach in Alaska, and I’m accordingly very interested in the paper. The attention that these ideas will likely receive in Alaska suggests several major avenues for effective public outreach:

• It provides an opportunity for “what is the nature of science and knowledge” education. The concepts of falsifiability and refinement of knowledge over time are particularly rich opportunities with these new results.
• It provides an opportunity to provide some “cutting edge” science to students. As noted below, many of the interpretive materials in greater Anchorage on these subjects reflect what was “cutting edge” twenty years ago, but which is now largely rejected in paleoanthropology.
• This paper is largely about analysis of genetic populations, and statistics. Therefore, it is an open door to talk about mutation rates and evolution, and some simple statistical exercises could easily be devised to give students an idea of what the authors are doing in their analysis.
• It provides an example of multidisciplinary work in science. The authors present a genetics analysis but subject it to controls imposed from other fields.
• Because some broadly similar studies of the past have not been subject to those controls, it provides an example of why there might be apparent disagreement about knowledge amongst scientists. For example, I’ve heard about genetic data that supports migration into the Americas both much earlier, and significantly later, than well-dated archaeological sites. By not imposing constraints from other fields of study, such findings result in apparent disagreement, without necessarily being valid disagreement. The distinction is worth teaching since organized antiscience uses such cases as a wedge.

The authors propose that the population of Amerind ancestors expanded out of east central Asia between 43,000 and 36,000 years ago, and occupied Beringia, the easternmost portion of Asia and the western part of Alaska, including the sea floor which was exposed at the time. A stable population of 8,000 to 10,000 people remained there from that time until around 16,000 years ago, at which time 1,000 to 5,400 of them rapidly expanded into the Americas. The study conforms to prior hypotheses that this expansion occurred either through an ice-free corridor in eastern Alaska and western Canada, or along the coast.

Consistent with other recent work, this paper proposes a single migration, as opposed to studies of the past that considered Amerinds, Na-Dene, and Eskimo-Aleuts to be the result of different migrations. This hypothesis gained popularity in the mid-1980’s, and is the model adopted by a number of interpretive materials in and around Anchorage. The model has been in disfavor for some time in the professional literature, and it seems likely that this new study would help to change these interpretive aids (assuming that scientific evidence trumps political expediency).

The authors point out that the genetic studies to date have strongly supported a single-migration model, but that they have varied significantly concerning the proposed date of the migration, with dates anywhere from about 13,000 years ago, to 40,000 years ago. As a result, that data has been interpreted by a variety of scenarios involving additional migrations, migrations of various ages, and so on. At least from the layman’s perspective, many of these seemed like clever possibilities that had the unfortunate air of being ad-hoc about them.

The new study accommodates some of the more puzzling aspects of the prior genetic studies, particularly ones that come up with very old dates of 30,000 years or more for the migration. A stable population in Beringia for some thousands of years would explain those results, and also explain why there are no American archaeological sites older than around 15,500 years old, while accommodating nicely the archaeological evidence that Homo sapiens was in northwest Beringia by about 30,000 years ago.

The study incorporates data from both nuclear and mitochondrial DNA of both Native Americans and Asians. Mitochondrial DNA evidence was cited in 2005 (with quite a bit of publicity, at least in Alaska) to support the idea that the population colonizing North America was extremely small, so it appears to me that the re-analysis of the mitochondrial data is of particular interest. Also of interest is that this study, unlike some in the discipline, uses archaeology, geology, and paleoecology as opportunities for imposing controls on the analysis of the genetic data. Some of the genetics studies of the past have given the appearance of being statistical analyses that avoided giving very much consideration to what is known from other disciplines. The study constrains divergence time to 15,000 years ago, and by trying out different migration rates between Asia and Beringia (and back), it is shown that the lower (and “more biologically realistic,” as the authors put it) the migration rate the larger the population of Amerind ancestors:

Our results demonstrate that smaller estimates of Ne depend upon a substantial level of migration from Asia to account for present-day levels of Amerind genetic diversity, e.g. Hey’s estimate of ≈70 founders is associated with a mAsia→NW > 9.0, which is twice the migration rate for contemporary Europe (m = 4.3).

Emphasis mine. I agree with the authors that the high migration rates assumed by other studies are implausible. Intuitively, I have a hard time accepting that the rate of migration on a modern industrial continent serviced by jets and trains is substantially lower than that found in east Asia in the Pleistocene, but I’m not an expert.

The authors also build into the paper a very nice opportunity for those doing outreach to talk about “what is science:”

Our goal is to provide a comprehensive model for the initial settlement of the Americas that generates new testable hypotheses and has high predictive power for the inclusion of new datasets. In light of our results, we propose a three-stage model in which a recent, rapid expansion into the Americas was preceded by a long period of population stability in greater Beringia by the Paleoindian population after divergence and expansion from their ancestral Asian population.

In other words, science produces conclusions that are testable. When you come to a conclusion, you are sticking your neck out a bit - because by definition a scientific finding is subject to being disproved at some point by someone who has better data, or is better at interpreting your data than you are.

One of the most interesting aspects of this paper, from an outreach perspective, is the opportunity to discuss how we know the dates. Here in a single paper are incorporated various methods for dating prehistoric events and materials (carbon dating, stratigraphy, genetic statistics, and surely a few others), and all of the methods agree that this recent event in world geological history still took place thousands of years before some believe the world was even created. The contrivances that are required to refute these vastly different, yet mutually-supporting dating techniques are awesome in their implausibility, and that’s where the teaching opportunity comes from.

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This blog article is about:

Kitchen, A., Miyamoto, M.M., Mulligan, C.J., Harpending, H. (2008). A Three-Stage Colonization Model for the Peopling of the Americas. PLoS ONE, 3(2), e1596. DOI: 10.1371/journal.pone.0001596

Tags: Alaska, America, Amerind, anthropology, Beringia, colonization, education, genetics, migration, outreach, paleoanthropology, population, science

This entry was posted on Thursday, February 14th, 2008 at 9:45 pm and is filed under blogging on peer-reviewed research, education and public outreach, paleo. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.