Maritime route of colonization of Europe

[Tacitus]

http://www.pnas.org/content/111/25/9211.abstract

Significance

The question of colonization of Europe by Neolithic people of the Near East and their contribution to the farming economy of Europe has been addressed with extensive archaeological studies and many genetic investigations of extant European and Near Eastern populations. Here, we use DNA polymorphisms of extant populations to investigate the patterns of gene flow from the Near East to Europe. Our data support the hypothesis that Near Eastern migrants reached Europe from Anatolia. A maritime route and island hopping was mainly used by these Near Eastern migrants to reach Southern Europe.

Abstract
The Neolithic populations, which colonized Europe approximately 9,000 y ago, presumably migrated from Near East to Anatolia and from there to Central Europe through Thrace and the Balkans. An alternative route would have been island hopping across the Southern European coast. To test this hypothesis, we analyzed genome-wide DNA polymorphisms on populations bordering the Mediterranean coast and from Anatolia and mainland Europe. We observe a striking structure correlating genes with geography around the Mediterranean Sea with characteristic east to west clines of gene flow. Using population network analysis, we also find that the gene flow from Anatolia to Europe was through Dodecanese, Crete, and the Southern European coast, compatible with the hypothesis that a maritime coastal route was mainly used for the migration of Neolithic farmers to Europe.

http://dienekes.blogspot.com/2014/06/the-mediterranean-route-into-europe.html

An interesting new (open access) paper in PNAS includes some new data from Crete, the Dodecanese, Cappadocia, and several other Greek (and a few non-Greek) populations, and proposes that the Neolithic followed an island-hopping migration into Europe. This is a study on modern populations that nicely complements the recent ancient mtDNA paper from PPNB which found an affinity to Neolithic Near Eastern populations among the modern inhabitants of Cyprus and Crete.

It is hard to imagine that there were ever any major impediments to gene flow between Anatolia and the Balkans as the Aegean islands and Hellespont are not formidable barriers to any culture with even rudimentary technology. Hopefully in the future it will become possible to look at ancient DNA from Greece and Anatolia and directly determine how the transfer of the Neolithic package into Europe took place and how much of the ancestry of modern populations stems from the Neolithic inhabitants vs. more recent shuffling of genes in either direction.

The authors also computed f3-statistics to see if populations were admixed, but found no significant evidence for it. If, for example, Dodecanesians were intermediate between mainland Greece and Anatolia they might have a negative f3(Dodecanesian; Cappadocia, Peloponnese) statistic. A negative statistic proves admixture but a positive one does not disprove it, but, in any case, there is no signal of admixture here so the results are compatible with the authors' model and probably incompatible with a recent admixture that would leave a significant negative signal (i.e., Dodecanesians/Cretans would have intermediate allele frequencies between Cappadocians and mainland Greeks).

[Tacitus]

http://www.pnas.org/content/111/25/9211/F1.large.jpg

Fig. 1.
Genes mirror geography around the Mediterranean coast. (A) Geographic distribution of the populations included in this study. (B) Projection on top two principal components of samples from 25 populations genotyped on ∼75,000 genome-wide autosomal SNPs. A clear cline is observed with Anatolia (Cappadocia) connected to Southern Europe through the bridge of the islands of the Dodecanese and Crete. Bedouins and Yemenites drift toward Central-South Asia. No apparent gene flow between Northern Africa and the Southern coast of Europe is observed. (C) Projection on top two principal components of samples from 30 populations genotyped on the same set of SNPs presented in B. Northern European populations have now been added; Bedouins and Yemenites were removed. The cline now continues through Central and Northern Europe.

http://www.pnas.org/content/111/25/9211/F2.large.jpg

Fig. 2.
Genetic structure of populations along the Southern European Coast in relation to Anatolia. (A) PCA plot of 10 populations from the Southern European coast and Cappadocia. The first principal component reveals the East to West cline in genetic variation along the Southern Coast of Europe and Mediterranean islands. Basques and Sardinians appear isolated relatively to the remaining studied populations. (B) Structure of the Southern European populations and Cappadocia excluding the more remote Basques and Sardinians.

http://www.pnas.org/content/111/25/9211/F3.large.jpg

Fig. 3.
Population structure around the Mediterranean basin. A model-based, unsupervised ancestry analysis approach (ADMIXTURE) was used to analyze populations on 72,951 (LD-pruned) genome-wide autosomal SNPs (K = 2–8). Two separate East to West clines are observed along the Northern African and Southern European Mediterranean coasts.

http://www.pnas.org/content/111/25/9211/F4.large.jpg

Fig. 4.
A coastal route of colonization of Europe. (A) A network analysis and visualization of the connections among 30 populations as revealed by the top five principal components. The network was formed by identifying nearest neighbors of each individual outside its populations of origin. Thicker edges represent stronger genetic relationships between pairs of populations, whereas warmer colors indicate high centrality of the respective nodes. The route connecting North Africa, Middle East, and Anatolia via the islands of the Dodecanese, and Crete to the rest of Europe, is apparent. (B) A network analysis and visualization of connections among 30 populations as revealed by ADMIXTURE with K = 5. Results are very similar to those in Fig. 4A, despite the fact that Admixture is a very different technique to extract ancestry information. Our networks are robust to the use of additional principal components or larger values of the ADMIXTURE parameter K for their formation (SI Appendix, Figs. S7 and S8).