Agriculture, population expansion and mtDNA variation23 May 2011
Earlier this spring, I wrote about a paper by Brenna Henn and colleagues that presented new data on SNP variation in recent African hunter-gatherer populations
Another paper that came out this spring from the same research group is also very interesting. Christopher Gignoux, Henn and Joanna Mountain
The results are nonetheless useful because they provide some nice matches for the archaeology of early agriculture. For example, in Africa:
We find two periods of population expansion within our sample of lineages originating during the Holocene in western Africa. Although the majority of coalescent events occur during the Holocene, a number of lineages from this sample also coalesce during the Upper Paleolithic. The earliest growth begins at ?38,000 ya (CI: 33,50045,000 ya) (Table 1 and Fig. S1) and the second period begins at ?4,600 ya (CI: 3,00010,000 ya) (Table 1 and Fig. 1B). The correspondence between the timing of genetic evidence for a sharp increase in population size at 4,600 ya in our Holocene sample of sub-Saharan Africans and the archaeological evidence for origins of agriculture in western Africa is quite close (Fig. 1B and Table 1). In contrast, our southern African Upper Paleolithic sample representative of hunter-gatherers shows no growth over the past 20,000 y. We suggest Bantu-speaking farmers and other pastoralist groups migrated throughout southern Africa 2,000 ya (27) without impacting southern African mtDNA lineages (Fig. 1B).
We can’t really understand the pattern of genetic variation within Africa without understanding when the population grew. In Africa, Middle Stone Age genetic variation must have been more extensive than that in other regions of the world. But the survival of that MSA variation to the present day depends on the demography of populations over the past 50,000 years. In a growing population, fewer lineages will be lost by random genetic drift. So if Gignoux, Henn and Mountain are right about the growth of West African populations by 35,000 years ago, we might expect that region to preserve some extensive variation from MSA times. That might explain why that population preserves very deep Y chromosome lineages
I think this proposal of African population history matches very well the model that we assumed in our acceleration paper
The results also show agricultural population growth to have been late in Southeast Asia.
Direct archaeological evidence for rice agriculture in southeastern Asia dates to only ?4,400 ya in Thailand (28). Agriculture spread throughout Island Southeast Asia, with evidence of rice in Taiwan again dating to ?4,400 ya. Our Southeastern Asian Holocene population size curve indicates expansion beginning ?4,700 ya (CI: 3,0005,700 ya) (Fig. 1C and Table 1).
Again, useful. I think we need to exert some effort making sure that the initial dispersal of people into South/Southeast Asia can be differentiated from the post-agricultural history. But assuming that Gignoux and colleagues are correct, it makes sense in an overall picture of slowly adapting early crops to tropical climate regimes, or replacing early domesticates with different ones in those areas.
I am less sanguine about their results for Europe. They show a gradual period of growth associated in time with the Younger Dryas (around 12,000 years ago), which could make sense in the archaeology. But I am not convinced that the “European” haplogroups here are really European to that time depth. We know that the Neolithic and post-Neolithic saw some large-scale shifts in the frequencies of mtDNA haplogroups in Central and Western Europe. Some Upper Paleolithic Europeans probably contributed mtDNA to this later population, but I have no confidence that the proportion was great enough to accurately infer the demography of that pre-Neolithic population. (This is also a problem with the current paper in Current Anthropology by Peter Rowley-Conwy. I’ll discuss this sometime soon.)
The next frontier in reconstructing the population history of Europe will be ancient DNA. A good sample of Neolithic and pre-Neolithic whole mtDNA genomes would settle this question and allow inferences about the kind of demographic recovery Europe underwent after the Last Glacial Maximum.
An open question is to what extent the other populations have similar problems. The European population of today reflects West Asian population dynamics 10,000 years ago. The East African population today reflects West African population dynamics from before the Bantu expansion, possibly to a similar extent. The population of Southeast Asia reflects the population dynamics of early rice agriculturalists in South China. And so on.
Adding large-scale migration and partial population replacement to this kind of demographic analysis is not easy, but it will be essential if we want a better picture of how agriculture affected human populations. Considering these problems, I think it’s easy to see why I started working on Holocene population dynamics. Evidence about Late Pleistocene populations, like MSA Africans and Neandertals, still lies within our genomes. But we see it through a lens. Holocene population dynamics – movements and population growth – distort that lens. If we don’t account for those Holocene dynamics, we will conclude wrongly about the earlier dynamics.
I like this a lot, because this is what anthropology is really good for. We can bring a lot of archaeological and historical knowledge to bear on the question of post-agricultural population dynamics. But it’s a deep, deep field with a lot of specialized literature.