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The above model of the settlement of the Americas is from a new paper which utilized ancient mtDNA, Ancient mitochondrial DNA provides high-resolution time scale of the peopling of the Americas (open access):

The exact timing, route, and process of the initial peopling of the Americas remains uncertain despite much research. Archaeological evidence indicates the presence of humans as far as southern Chile by 14.6 thousand years ago (ka), shortly after the Pleistocene ice sheets blocking access from eastern Beringia began to retreat. Genetic estimates of the timing and route of entry have been constrained by the lack of suitable calibration points and low genetic diversity of Native Americans. We sequenced 92 whole mitochondrial genomes from pre-Columbian South American skeletons dating from 8.6 to 0.5 ka, allowing a detailed, temporally calibrated reconstruction of the peopling of the Americas in a Bayesian coalescent analysis. The data suggest that a small population entered the Americas via a coastal route around 16.0 ka, following previous isolation in eastern Beringia for ~2.4 to 9 thousand years after separation from eastern Siberian populations. Following a rapid movement throughout the Americas, limited gene flow in South America resulted in a marked phylogeographic structure of populations, which persisted through time. All of the ancient mitochondrial lineages detected in this study were absent from modern data sets, suggesting a high extinction rate. To investigate this further, we applied a novel principal components multiple logistic regression test to Bayesian serial coalescent simulations. The analysis supported a scenario in which European colonization caused a substantial loss of pre-Columbian lineages.

The key here is that looked at whole mitochondrial genomes, which gives them more information to work with. Earlier work often focused on a particular variable region of the mitochondrial genome. And, mtDNA is copious, so they got good quality data from all of their samples (really 5x is decent for population genomic work, and that was the worst). Combined with the fact that they had ancient genomes, which allow them to investigate the phylogeny in a more precise manner temporally, and have you the potential to make some really strong inferences.

F3.large Figure 3 in the paper makes everything really clear. The last common ancestors between Native American mtDNA lineages and those of Siberians is >20,000 years before the present. That is, before the Last Glacial Maximum. The next major feature you see is an explosion of lineages aroun ~15-16 thousand years ago. This is the hallmark of a rapid population expansion. But after the initial period of diversification you see the persistence of a lot of deeply divergent lineages. Additionally, further population genomic modeling indicate that there was a major extinction event ~500 years ago, no doubt due to the Columbian Exchange and the arrival of Old World populations and their diseases.

This paper is fundamentally about Native American historical genetics. It is another nail in the coffin of the “Clovis first” model of Amerindian origins. Basically, that the Clovis group of megafaunal hunters were the First Americans. No, it does seem likely now that modern humans were present in portions of the New World thousands of years before Clovis. The Monte Verde site’s occupation on the Chilean coast less than two thousand years after the opening of a coastal route from Beringia indicates that perhaps there was a strong focus on marine environments for a significant period of time. Once the New World was settled there seems to have been a lot of persistent population structure, until the arrival of Europeans, at least in comparison to what ancient DNA has told us about Europe. Additionally, the long isolation of the Beringians is also significant in my opinion.

In a world of billions of humans it may be that we lack proper intuition for how little gene flow may have occurred between populations in a sparsely populated globe. The Beringians were separated from Siberians for on the order of ~5,000 years. It only takes ~1 migrant between two populations per generation to prevent them from drifting apart in allele frequencies, so the gene flow was very low (this is mtDNA, so not strictly applicable, but the same logic holds). But it is possible that in much of northern Eurasia during the Last Glacial Maximum humans retreated to zones of survival, and vast swaths of territory became empty. This would result in islands of human habitation diverging and become very different over several thousands of years. In sharp contrast, the world over the past 4,000 years or so has been characterized by the ability of humans to travel long distances over inclement territory, and settle amongst strangers, usually through conquest. Partially this is due to the domestication of the horse, but partially it is probably due to the emergence of high density complex societies which can incubate specialist castes whose role arose initially as defense, but who often engage in offense whenever the opportunity arises.

 
• Category: Science • Tags: Genetics, New World 
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For obvious reasons I don’t usually post about material I haven’t read, but Tyler Cowen points me to the fact that Charles C. Mann has a new book coming out this summer. If you haven’t, I would highly recommend his previous book, 1491: New Revelations of the Americas Before Columbus. I think it is correct that Mann probably skewed his narrative a bit too much to the revisionist side, but it is a genuinely revelatory work.* I was aware of the broad outlines already, so it wasn’t surprising, but he marshals the data in a fascinating and engaging manner. The new book is apparently a sequel of sorts, 1493: Uncovering the New World Columbus Created. Cowen says “I am spellbound reading it, it will be one of the best books of this year, and, although I know this area somewhat, I am learning fascinating information on literally every page.” I suspect that what occurred in the New World after 1492 is actually an amplified version of what occurred across much of the Old World over the past 10,000 years, with the rise of massive agglomerations of humanity due to agriculture.

In other news, malnourished children apparently had mortality rates of 40% due to measles!.

* There is a lot of politics you have to navigate on this topic, from all directions.

(Republished from Discover/GNXP by permission of author or representative)
 
• Category: Science • Tags: Anthropology, Charles C. Mann, New World 
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One aspect of human demographic expansions seems to be the fact that we often model them as a constant diffusion process, when in reality there were likely pulses (economic historians can conceive of this as the periodic gaps between land and labor factor inputs). I don’t know much about the human movements prior to H. sapiens sapiens, and from what I can gather the fossil remains are too sparse to be too wedded to a specific model, but it seems clear that anatomically modern human expansion occurred through a series of rapid outward sweeps which would periodically reach a “natural barrier.” Modern humans reached the Solomon Islands ~30,000 years ago, after which there was stasis for ~25,000 years. Only with the Austronesian expansion did humanity push past the Solomons. And this was no baby-step, ultimately the Austronesians went as far as the Hawaiian islands and Easter Island.


The New World is similar. The initial migration out of Africa by modern humans resulted in the range expansion of the human lineage into a region which had been untouched by earlier hominins, Australasia. But after that point tens of thousands of years passed before our species pushed into virgin territory, in this case North America. The when and the how of this though is still up for debate. A new paper PLoS One attempts to construct a plausible scenario by taking archaeological data points and inputing them into a diffusion model. Archaeological Support for the Three-Stage Expansion of Modern Humans across Northeastern Eurasia and into the Americas:

We use diffusion models…to quantify these dynamics. Our results show the expansion originated in the Altai region of southern Siberia ~46kBP , and from there expanded across northern Eurasia at an average velocity of 0.16 km per year. However, the movement of the colonizing wave was not continuous but underwent three distinct phases: 1) an initial expansion from 47-32k calBP; 2) a hiatus from ~32-16k calBP, and 3) a second expansion after the LGM ~16k calBP. These results provide archaeological support for the recently proposed three-stage model of the colonization of the Americas….Our results falsify the hypothesis of a pre-LGM terrestrial colonization of the Americas and we discuss the importance of these empirical results in the light of alternative models.

It’s an interesting paper because it seems to have been triggered in part by inferences made from the genetic data. I don’t know how confident archaeologists are about their radiometric dates, but I think some of the molecular clock results from the genetics of Amerindians need to be taken with a grain of salt (I don’t see many people repeating some of the really ancient coalescence dates for Amerindian Y lineages at this point).

These data seem to indicate that modern humans made it no further than previous hominin groups for several tens of thousands of years. But something happened within the last 20,000 years, and our species made the leap across Beringia. The bottleneck here is certainly not the Bering Strait, which was spanned by land much of the time in any case. Rather, our species didn’t have the biological or cultural capacity to survive in extremely frigid environments. I’ve read modern humans pushed the boundaries of their range in northern Europe further than Neandertals ever did, indicating our flexibility and plasticity. Since the human lineage had been resident in Eurasia for at least one million years that suggests to me that it was behavioral modernity that was key. In particular, how quickly our cultures evolve and shift. Though that flexibility itself may be a function of our biological competencies.

(Republished from Discover/GNXP by permission of author or representative)
 
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Razib Khan
About Razib Khan

"I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. If you want to know more, see the links at http://www.razib.com"