February 13, 2014
Human admixture common in human history (Hellenthal et al. 2014)
The paper has a companion website in which you can look up the admixture history of individual populations.
While reading this study, it is important to remember its limitations. Two are immediately obvious: (i) admixture events can only be detected for the last few thousand years, as this method depends on pattern of linkage disequilibrium which decays exponentially with time due to recombination, and (ii) detection of admixture seems to depend on the presence of maximally differentiated populations from the edges of the human geographical range; for example, the Japanese appear unadmixed even though they are clearly of dual Jomon/Yayoi ancestry. On the other hand, the method does detect the admixture present in the San at a similar time scale.
The case of Northwestern Europe appears especially striking as none of the populations from the region show evidence of admixture. This may be because the mixtures taking place there (e.g., between "Celts" and "Anglo-Saxons" in Great Britain) involved populations that were not strongly differentiated. Alternatively, population admixture history may have preceded the last few thousand years and is thus beyond the temporal scope of this method.
An exception to the rule that populations at the edges of the human range appear to be unadmixed are the Armenians who appear to be the only * between the Atlantic and Pacific in Figure 2D (shown at the beginning of this post). The companion site lists their status as "uncertain".
Other results are more questionable; for example, the authors assert that Sardinians are an admixed population with one side being "Egyptian-like" and the other "French-like" whereas the ancient DNA evidence as it stands would rather indicate that Sardinians are the best approximation of Neolithic Europeans currently in existence and so are more likely to (mostly) possess a gene pool that traces back to ~8-9 thousand years in Europe. It will be quite the surprise if so many Europeans from 5kya or earlier look like modern Sardinians and ancient Sardinians don't!
The three-way admixture in much of eastern Europe is not particularly surprising as history furnishes ample evidence for groups of steppe origin in the region during historical times. Some bequeathed their both language and name (e.g., Magyars), others only their name (e.g., Bulgarians) on the local Europeans, but records indicate a widespread presence of "eastern" groups in Europe from the time of the Huns to that of the Ottomans. A study of late Antique eastern Europeans from the Baltic to the Aegean may help better document how the twin phenomena of the eastern invasions and the spread of the Slavs shaped the present-day genetic diversity of the region.
I suspect that a few ancient samples will be far more informative for understanding the recent history of our species than the most sophisticated modeling of modern populations. Nonetheless, it's great to have a new method that maximizes what can be learned about the past from the messy palimpsest of the present.
Science 14 February 2014: Vol. 343 no. 6172 pp. 747-751 DOI: 10.1126/science.1243518
A Genetic Atlas of Human Admixture History
Garrett Hellenthal et al.
Modern genetic data combined with appropriate statistical methods have the potential to contribute substantially to our understanding of human history. We have developed an approach that exploits the genomic structure of admixed populations to date and characterize historical mixture events at fine scales. We used this to produce an atlas of worldwide human admixture history, constructed by using genetic data alone and encompassing over 100 events occurring over the past 4000 years. We identified events whose dates and participants suggest they describe genetic impacts of the Mongol empire, Arab slave trade, Bantu expansion, first millennium CE migrations in Eastern Europe, and European colonialism, as well as unrecorded events, revealing admixture to be an almost universal force shaping human populations.