A Mesolithic genome from Spain

Nature today published a paper on the complete genome of La Brana 1, a Mesolithic hunter-gatherer from Iberia: Olalde et al. 2014. Based on genetic variants associated with pigmentation traits, it's likely that this individual had blue eyes, dark hair and deep brown skin.

Moreover, he was probably lactose intolerant (in other words, unlike most Europeans today, he couldn't drink milk as an adult), and his Y-chromosome belonged to the European-specific, but today extremely rare, haplogroup C6 (aka. C-V20), and mtDNA to haplogroup U5b2c1, which again is a European-specific marker. Below is an artist's impression of his mug (courtesy of CSIC), and below that the paper abstract.

Ancient genomic sequences have started to reveal the origin and the demographic impact of farmers from the Neolithic period spreading into Europe1, 2, 3. The adoption of farming, stock breeding and sedentary societies during the Neolithic may have resulted in adaptive changes in genes associated with immunity and diet4. However, the limited data available from earlier hunter-gatherers preclude an understanding of the selective processes associated with this crucial transition to agriculture in recent human evolution. Here we sequence an approximately 7,000-year-old Mesolithic skeleton discovered at the La Braña-Arintero site in León, Spain, to retrieve a complete pre-agricultural European human genome. Analysis of this genome in the context of other ancient samples suggests the existence of a common ancient genomic signature across western and central Eurasia from the Upper Paleolithic to the Mesolithic. The La Braña individual carries ancestral alleles in several skin pigmentation genes, suggesting that the light skin of modern Europeans was not yet ubiquitous in Mesolithic times. Moreover, we provide evidence that a significant number of derived, putatively adaptive variants associated with pathogen resistance in modern Europeans were already present in this hunter-gatherer.

Indeed, the pigmentation traits are basically the same as those of Loschbour, a Mesolithic genome from Luxembourg, featured recently in the groundbreaking Lazaridis et al. preprint (see here). So we can already speculate with some confidence that this was a common, and perhaps dominant, trait combination among European hunter-gatherers.

However, early European farmers, whose ancestors almost certainly migrated to Europe from the Near East during the Neolithic, probably had somewhat different pigmentation traits. We know this because a 7,500 year-old Linearbandkeramik (LBK) farmer genome from Stuttgart, Germany, also featured in Lazaridis et al., showed markers for brown eyes, dark hair, and relatively light skin.

So as things stand, it appears that Europeans only acquired their present coloring, including pale skin and a high incidence of light eyes, relatively recently, well after the hunter-gatherers and farmers began mixing, and their hybrid DNA had time to go through some really powerful selective sweeps. These sweeps were possibly in part a reaction to the Neolithic diet, rich in carbohydrates but poor in vitamin D, amongst other things. Vitamin D doesn't have to be acquired from food because the body can synthesize it from the sun, but this is done more effectively by people with fair skin, giving them an advantage, especially in places like Europe, which has fairly long winters and lots of cloud cover.

But perhaps this isn't the full story, and present-day European pigmentation traits are also sourced from a late migration into Europe of a prevailingly blond people from somewhere in what is now Russia?

This might sound far fetched, but during the middle Bronze Age the Eurasian steppe was home to the Andronovo culture, with archeological links to earlier cultures in what is now southern Russia. Based on the DNA of Andronovo nomads from Kurgans in South Siberia, it seems they had fair skin and a lot of blue eyes and blond hair (see here). They also overwhelmingly belonged to Y-chromosome haplogroup R1a1a, which is very common today in Central and Eastern Europe and also parts of Scandinavia. So it'll be interesting to see the pigmentation markers of Mesolithic Eastern Europeans and Central Asians when their genomes become available, probably in the not too distant future, and if they contributed any ancestry to present-day Europeans. Early indications are that they did, and I discussed that in my previous blog entry here.

La Brana 1 and Loschbour were both classified as part of the West European Hunter-Gatherer (WHG) mata-population by Lazaridis et al., even though only a partial sequence from La Brana 1 was available at the time. As far as I can see, the results in Olalde et al. based on the complete genome don't contradict this classification, because they show that La Brana 1 is most similar to present-day Europeans from around the Baltic Sea, just like Loschbour. Note, for instance, the position of Swedes (SE) and Poles (PL) on the far right of these graphs, indicating inflated allele sharing between them and La Brana 1 relative to other Europeans.

Unfortunately, I have to say that the main Principal Component Analysis (PCA) from the paper isn't as informative as it could have been, due to the large number of Finnish individuals included in the analysis. It's mostly a reflection of the recent population growth, founder effect and genetic drift among Finns, particularly those from eastern Finland.

Nevertheless, note that all of the non-Finnish Europeans more or less fall along the cline that runs from La Brana 1 to present-day Cypriots. This suggests that Europeans today are mostly the product of mixture, in varying degrees, between indigenous European hunter-gatherers, like La Brana 1 and Loschbour, and immigrant Neolithic farmers from the East Mediterranean. So it's a result that basically agrees with the findings of Lazaridis et al.

Interestingly, Loschbour and four other Mesolithic samples from Lazaridis et al. belonged to Y-chromosome haplogroup I, which is not at all closely related to C6. This hints at the presence of a diverse Y-chromosome gene pool in pre-Neolithic Europe, and indeed I'm still confident of seeing R1 and/or R1a among Mesolithic remains from Eastern Europe.

Even though the vast majority of haplogroup C clades are today specific to Eastern Asia, Oceania and the Americas, C6 has only been found among a handful of individuals from across Southern, Western and Central Europe, many of whom are listed at the FTDNA haplogroup C project (look for the V20+ results here). It's difficult to say when this marker or its ancestral lineage migrated to Europe, but C is one of the most basal human Y-chromosome clades, so it could represent the very first Anatomically Modern Human (AMH) wave into Europe, which actually isn't a new concept (see Scozzari et al. 2012).

The Olalde et al. paper includes a lot more information than I'm willing to cover in this blog entry. If you don't have access to the main report, please note that the extended and supplementary data are very detailed and open access.

Citation...

Olalde et al., Derived immune and ancestral pigmentation alleles in a 7,000-year-old Mesolithic European, Nature (2014), doi:10.1038/nature12960