Sometimes in applied fields artistic license is constrained by the necessity of function to particular creative channels. Architecture comes to mind, at least before innovative technologies produced lighter and stronger materials, freeing up form from its straitjacket (whether this was a positive development is a matter of taste). But there’s only so much you can do with your palette when your palette is limited. This can be a bug, or it can be a feature. Science is not art, but in some ways at its heart it’s a story about the universe. The story can be in words or math, no matter, ultimately it’s the human attempt to map nature and make its subtle patterns comprehensible to us in plainer fashion. Some of the human biases in our quest are transparent. Why is there anthropology? A whole discipline devoted to the study of mankind and his nearest biological kin. We don’t peruse the patters with an objective and uninterested eye. We’re shaped by our presuppositions, as well as the constraints of the methods, and the results we have before us. The emergence of a theoretical evolutionary biology in the decades before the molecular revolution after World War II may have been in part simply a function of the fact that there were only so many results one could squeeze out of classical evolutionary genetic techniques, which relied on tracking only a limited set of phenotypes due to large effect mutations in breeding populations. With the rise of molecular evolution you saw the crystallization of theoretical frameworks, such as the neutral theory, to explain the burst of novel results.
Around the year 2000 something similar happened in historical population genetics. The analysis of mtDNA lineages, passed from mother to daughter, had matured, and techniques for typing the Y chromosome had started to catch up, so that a symmetry between the sexes could arise. “Mitochondrial Eve” was now paired with “Y chromosomal Adam.” Though mtDNA and Y lineages were only two direct lines of ancestry, because there was no recombination across much of their sequence it was easy to analyze them within the context of coalescent theory. In contrast, the genealogy of autosomal regions of the genome were confounded by recombination, which mixed & matched the variation in a manner which made reconstruction of past history far more difficult. So we had the technology to extract the genetic variation from mtDNA and the Y chromosome, and, we knew how to model their evolution. The two together produced a genetic time machine.
The result was a swelling of papers utilizing uniparental markers. You can see the chronology to some extent at the frequency of postings at Stanford’s Human Population Genetics Laboratory online repository. Another byproduct was the emergence of public intellectuals who filled the need which arose to interpret and communicate the findings to a lay audience. Four books are emblematic of the era, Bryan Sykes’ The Seven Daughters of Eve: The Science That Reveals Our Genetic Ancestry, Spencer Wells’ The Journey of Man: A Genetic Odyssey, Stephen Oppenheimer’s The Real Eve: Modern Man’s Journey Out of Africa, and Steven Olson’s Mapping Human History: Genes, Race, and Our Common Origins. As a professional journalist Olson’s treatment was the odd one out, as much reportage as a personal interpretation. In contrast, Sykes, Wells, and Oppenheimer were making scholarly cases from their own vantage point. Oppenheimer and Wells also paired their books with television documentaries. Wells continues to remain in the public eye, he’s become a new sort of intellectual entrepreneur with the The Genographic Project. The age of uniparental markers then spawned careers and truisms. For example, the patterns of variation of mtDNA and Y chromosomes resulted in the consensus that ~75% of the ancestors of modern Europeans are descended from Paleolithic hunter-gatherers. The proportion of the ancestry contributed by Neolithic farmers decreased from southeast to northwest, converging upon zero in the far reaches of the British Isles and Norden.
This inference was made in large part based upon the contemporary patterns of genetic variation, by assigning modern haplogroups to putative ancient populations. To the left is a map of the frequency of haplogroup R1b, which is the most common Y chromosomal lineage in western Europe. The frequency is highest among the Basques, who were presumed to be the most pristine reservoir of the genetic substratum of Paleolithic Europe. The conception here was that the Basques were clearly indigenous to Iberia, they were already there before the arrival of outsiders such as the Celts, Phoenicians, and finally Romans (this has influenced modern Basque nationalism to some extent). Their non-Indo-European language was assumed to be a relic of many dialects which once existed before Indo-European swept over them. Using R1b, and other haplogroups at high frequency among the “indigenous peoples” of Europe, historical geneticists pegged the ancestral quanta of hypothetical prehistoric groups using these putative indigenes as modern references. But the inferences rested on assumptions, assumptions which couldn’t be directly tested. Until that is another methodological revolution arrived on the scene: the extraction of ancient DNA! These new waves of results, which came to the fore in the latter 2000s, have unsettled our preconceptions. It now seems that the past was likely more complex than we’d presumed, and the palimpsest of human genetic variation over time may have obscured and clouded our understanding of the map of what once was.
More recently some researches have gone back and looked at the variation within the R1b haplogroup, specifically the subclade which is very common in Western Europe, R1b1b2, and concluded that in fact it was most diverse in the eastern Mediterranean. The most plausible inference to be made from this was that the R1b1b2 originated to the east, and spread to the west, rising in frequency due to genetic drift as populations went through bottlenecks and then rapidly expanded in size. Additionally, the last common ancestor of these lineages was on the order of ~10,000 years ago. This naturally upends the model which geneticists were confidently pushing forward in the early 2000s, shutting the door on debates as to the provenance of modern Europeans and their relationship to Ice Age hunter-gatherers. A follow up paper rebutted this new claim as to the origin and expansion of R1b1b2. What had been a stable and conventional area of historical population genetics has now been thrown into tumult, and researchers are looking more closely at the uniparental lineages which had had their time in the sun. Or so it seemed.
So with that background, a paper in The European Journal of Human Genetics steps into the “R1b controversy,” leaning to the side of those who argue for its origin more recently among Neolithic farmers. A major Y-chromosome haplogroup R1b Holocene era founder effect in Central and Western Europe:
The phylogenetic relationships of numerous branches within the core Y-chromosome haplogroup R-M207 support a West Asian origin of haplogroup R1b, its initial differentiation there followed by a rapid spread of one of its sub-clades carrying the M269 mutation to Europe. Here, we present phylogeographically resolved data for 2043 M269-derived Y-chromosomes from 118 West Asian and European populations assessed for the M412 SNP that largely separates the majority of Central and West European R1b lineages from those observed in Eastern Europe, the Circum-Uralic region, the Near East, the Caucasus and Pakistan. Within the M412 dichotomy, the major S116 sub-clade shows a frequency peak in the upper Danube basin and Paris area with declining frequency toward Italy, Iberia, Southern France and British Isles. Although this frequency pattern closely approximates the spread of the Linearbandkeramik (LBK), Neolithic culture, an advent leading to a number of pre-historic cultural developments during the past 10 thousand years, more complex pre-Neolithic scenarios remain possible for the L23(xM412) components in Southeast Europe and elsewhere.
There’s aren’t incredibly novel techniques of analyses here. Rather, the confusion around R1b1b2 has prompted researchers to expand their population coverage and resequence the markers around the haplogroup. These phylogenetic trees are constructed by genealogies which are separated by mutational steps, with steps of daughter mutations down a particular branch and distinguishing various derived clades. The terminology can kind of get confusing, but R1b1b2 is equivalent to the M269 branch in this study. What they did was analyze the phylogenetic relationships of the branches of R1b1b2 and it sister clades, and plot their frequencies as a function of geography. Below are a set of figures which show the frequencies of various clades across Europe. The last figure has several panels because they’re all subclades, and of somewhat less interest to the big picture. The first figure has the various branches, so you can see how they relate before browsing the maps.
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M269 is really the one to focus on. It and its daughter branches are at the heart of the Paleolithic vs. Neolithic controversy. Compare the phylogenetic tree in the first image, and the distributions of the allele frequencies in the subsequent images. The Western European variants seem to be daughter branches from an ancestral variant which is found in Anatolia or thereabouts. The authors also confirm the coalescence back to the last common ancestor ~10,000 years ago, though the methods have a bias toward inflating the value, so that’s an upper bound. They also used PCA analysis show how the haplogroup variation exhibited cluster patterns. The first panel has the haplogroups, with PC 1 separating the ancestral R1b variant from the daughters, and the second PC separating each daughter branch. The second panel inputs the various fractions of R1b haplogroups in populations. There’s an obvious recapitulation of the geographical map in the distribution of haplogroups.
What’s the moral of this story? I’m not going to get into the correlations they adduce between various archaeological groups and genetic lineages. That got us into trouble earlier as I implied. I don’t think the fine-grained results are solid enough that we should be taking that sort of interpretation too seriously. Rather, it’s telling us what we don’t know, and what we shouldn’t be clear on. I lean toward the proposition that R1b1b2 was brought by Neolithic farmers at this point, the paper which refuted that finding leaned strong on samples from Sardinia, which I suspect are more than not atypical and not representative (Sardinia tends to be an outlier on genetic plots because of its island isolation). But my confidence is hardly even modest at this point. There’s a lot we don’t know.
History begins in Sumeria with the written word ~5,000 years ago. But as history dawns agriculture was still new to Norden and the fringes of the Baltic and British Isles. By the time what the ancients called Thule came into some focus, after the fall of the Roman Empire, much had passed beyond our line of sight. The original geneticists and archaeologists who attempted to synthesize their disciplines and construct a plausible model of how Europeans and Europe in its linguistic, genetic, and cultural variation, came to be, followed the principle of parsimony. Cavalli-Sofrza, Ammerman, and Renfrew presented us with a model where Paleolithic Europeans, who hunted & gathered, and spoke non-Indo-European languages, were slowly replaced culturally, linguistically, and partially genetically, by Indo-Europeans who brought farming from the Middle East. This was the “demic diffusion” hypothesis. I don’t think anyone accepts this as likely at this point, at least in its total simplicity of explanatory power. We need to reconsider whether the Basques can even serve as models for Paleolithic European man anymore! It may be that the Basques themselves are culturally and genetically intrusive, bringing their language and folkways along Mediterranean shores with agriculture, eventually marginalizing the thin numbers of hunter-gatherers beyond the limes of their “civilization.” Additionally, we have to remember that there was history before history, that what we term prehistory is rich with many developments which are preserved only vaguely and in the mists of oral tradition (though that tradition rapidly decays in fidelity). The more recent expansion of the Bantu and Austronesian languages do not benefit from copious records, because they spread with preliterate societies. The expansion of Turkic and Indo-Iranian dialects can only be perceived in the outlines because these peoples were on the fringes of societies where writing was part of their culture. Europe’s shift to agriculture occurred over thousands of years, and those thousands of years were all preliterate. Stonehenge and the megaliths were constructed by societies which we can comprehend only through their most robust monuments. The stones speak to a complexity which genetics can not resolve. Sometimes admitting that you don’t know is an answer in and of itself.
Citation: Myres NM, Rootsi S, Lin AA, Järve M, King RJ, Kutuev I, Cabrera VM, Khusnutdinova EK, Pshenichnov A, Yunusbayev B, Balanovsky O, Balanovska E, Rudan P, Baldovic M, Herrera RJ, Chiaroni J, Di Cristofaro J, Villems R, Kivisild T, & Underhill PA (2010). A major Y-chromosome haplogroup R1b Holocene era founder effect in Central and Western Europe. European journal of human genetics : EJHG PMID: 20736979
Image Credit: Frédéric Vincent, Matthew Field, National Geographic, Wikimedia
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