Saturday, January 05, 2008

Historical science out of context: Neandertal extinction   posted by Razib @ 1/05/2008 12:29:00 AM

Evidence for declines in human population densities during the early Upper Paleolithic in western Europe:
In western Europe, the Middle to Upper Paleolithic (M/UP) transition, dated between ~35,000 and ~40,000 radiocarbon years, corresponded to a period of major human biological and cultural changes...New faunal data from the high-resolution record of Saint-Cesaire, France, indicate an episode of significant climatic deterioration during the early Upper Paleolithic (EUP), which also was associated with a reduction in mammalian species diversity. High correlations between ethnographic data and mammalian species diversity suggest that this shift decreased human population densities. Reliance on reindeer (Rangifer tarandus), a highly fluctuating resource, would also have promoted declines in human population densities. In this context, the possibility that a modern human expansion occurred in this region seems low. Instead, it is suggested that population bottlenecks, genetic drift, and gene flow prevailed over human population replacement as mechanisms of evolution in humans during the EUP.

I bolded words which I thought emphasized the provisional and tenuous nature of the contingent sequences of inferences being made in this paper. A summary in National Geographic where the first author is quoted is less equivocal:
Morin argues that Neandertal populations thinned out gradually as Europe's environment became harsher, with some groups going extinct.

But climate stresses may have wrought evolutionary adaptations in surviving Neandertals, leading them to develop characteristics like those of modern humans, Morin added.

"Neandertals adapted to this harsher climate by expanding their social networks, a process that allowed the diffusion of 'modern traits' into the Neandertal gene pool," he said.

Some modern humans may have migrated to Europe during this period, Morin added, "but I don't think it happened to the large scale implied by many scholars."

Such an influx probably didn't occur until about 10,000 years ago, with the spread of agriculture from the Middle East, he said.

I don't know much paleontology. Like history this is a field where theory can only take you so far, and you have a comparative advantage if you can cogitateoff an empirical distribution data that you've already internalized via years of close study. But because it is a historical science you also have to place your hypothesis is a bigger context, and the conditional and probabilistic nature of evolutionary processes makes a broader framework essential. Remember the Etruscan story? Archaeologists confronted with extremely strong genetic data (from multiple angles) simply shrugged and expressed ignorant skepticism, as opposed to changing their priors and shifting their paradigm.

When a palaeoanthroplogist makes assertions in a field where I'm not very clear about the details I pay attention to the things they say which I can evaluate. Concluding that 'modern' humans (or, specifically, the descendants of Africans of ~50,000 years B.P.) only arrived during the Neolithic seems to shed an unfavorable light on a scholar's credibility; genetic data suggests that Middle Eastern (Anatolian) Neolithic lineages (e.g., haplogroup J2) are extant across Europe on the order of ~25% penetration (peer reviewed range of 20-50%). In terms of the structure of scientific theories its seems that the researcher above is making an inference based upon their own data and model, which is disputable, and dismissing a far stronger body of work which contradicts said inference (or perhaps the author is ignorant of that body of work?).

There's also the large network of causal factors. In After the Ice Steven Mithen recounts how computer simulations show that the dynamic of a joint impact of both environmental stress (e.g., climate change) and modern human predation is the best explanation for the pattern of megafaunal extinctions we see around the world in the past few thousand years. Specific data points such as the extinction of Cuban ground sloths are highly persuasive to me. Evolutionary pressures are often interspecific, intraspecific and environmental; there is no need to assume the operation of one excludes the operation of another. And yet in natural history there is this constant tendency to present "silver bullet" models with one primary causal factor. Just because a condition is necessary does not mean that it is sufficient. I have no doubt that the correlations are highly striking, but the sequence of events need to be assessed in light of the full data set we have in terms of time and space. "Cold snaps" and extinctions are not sui generis, but rather are recurrent features of the history of our planet. Neandertals persisted for hundreds of thousands of years across at least half of Eurasia; during this period there were no doubt great fluctuations in temperature and ecological conditions. And yet they, and most other "archaic" types, disappeared in the space of several tens of thousands of years as "modern" morphologies spread across the world, while at the same time geneticists do conclude that the predominant (if not exclusive) element of our ancestry derives from the African continent within the last 100,000 years. Should we ditch this model because of a set of analyses of faunal remains? I'm skeptical, largely because I don't see the data above as falsifying the orthodoxy. Rather, it is compatible with a range of hypotheses.

As I tried to make clear in my post about cultural anthropology science that isn't physical or mathematical is hard. It's messy. There are lots of interlocking factors, statistical variation and historical contingency. I don't think that means it is impossible, but it does mean that it requires care, humility and an attention to detail. The logical structure of math is perfect, in theory you need to know only patches to infer enormous expanses. An ecologist I knew once joked about the shock that a physicist who was moon-lighting in a biostatistics class in graduate school experienced in terms of the noise that they had to confront in every experiment and analysis. In the historical and human sciences knowing a patch and generalizing from that locality seems to be a waste of time, and only justifies the critiques of extreme subjectivists. Rather, the structure of knowledge is by its nature a big picture with a riot of small details, and those details make no sense unless you familiarize yourself with the whole.

Note: The last sentence of the abstract, "it is suggested that population bottlenecks, genetic drift, and gene flow prevailed over human population replacement as mechanisms of evolution in humans during the EUP" is problematic for me...I kind of smell the tendency to use genetic drift as a deux ex machina here. Just like Judith Rich Harris pointing out the use of "interaction effects" as a get out of jail card in much of behavior genetics in No Two Alike.

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