Article and Discussion: Four Key Migratory Events Shape the European Gene Pool

As you all know, I am intrigued by the stories genetics is adding to human history.  The journal Science has a very interesting article that I will summarize and contextualize for folks.

By about 600,000 years ago, the hominid ancestor of Neanderthals leaves Africa and eventually settles Europe.  This ancient relative of modern humans will evolve into Neanderthals.

~40,000 years ago:  The first modern humans, Cro-Magnons, enter Europe -probably via Greece and the Danube Valley.  They encounter Neanderthals, and genetics point to some limited interbreeding.

~30,000 to ~25,000 years ago:  The last full Neanderthals die out.  The last skeleton of Neanderthals found to date is from a cave in Gibraltar.  

~28,000 to ~14,000 years ago:  The last Ice Age drives human and Neanderthal populations south to four refuges in a) Iberia, b) Italy, c) the Balkans, and d) the Ukraine.  A mutation arises in the population living in Iberia.  This mutation gives rise to the Rhesus negative factor in human blood (A-, B-, O-, and AB-).  Today  about 50% of Basque people carry this mutation with the frequency of the mutation dwindling the farther you get from the Pyrenees.  

~8,000 years ago:  agriculture arrives in Europe -again with people moving up the Danube Valley- via farmers immigrating from Turkey (Anatolia) and the Fertile Crescent.  These first European farmers appear genetically to be similar to modern Sardinians, residents of a large, relatively isolated island that is now part of Italy.  The genetics of Ötzi, the Ice Man whose body was found in a glacier on the Austrian-Italian border, places him as belonging to this group of Neolithic farmers.  The T and J mitochondrial haplogroups are also associated with these first farmers.  Note:  Ötzi and I are both in the T haplogroup.

~7,500 years ago:  Among the new Neolithic farmers in the Danube Valley a mutation occurs that will allow most Europeans to have lactose persistence, or the ability to digest dairy products after childhood.  

Now for where the paper starts off:

Geneticists and historians wondered if the first farmers assimilated the original Cro-Magnon hunter-gatherers, killed them off, or diluted their genetic contributions by filling Europe with larger families supported by agriculture. Instead, the researchers -Brandt et al.-  found some really unexpected and fascinating things by comparing modern European populations' DNA with the DNA from skeletons found in central Germany.  These German skeletons cover 4,000 years of history from the first farmers to about 3,500 years ago during the Bronze Age.  What did they find?

a. For 2,000 years after the Neolithic farmers arrived, there appears to have been limited interbreeding between the farmers and the hunter-gatherers.  The farmers appear to have been successful with their numbers steadily growing.  It is interesting to ponder what social mechanisms kept two populations from intermarrying for two millennia.  

b. ~7000 years ago:  About a 1,000 years after the farmers arrived, their overall percentage in the gene pool appears to decline and the hunter-gatherers and their genes make a comeback.  This may have resulted from climate change or disease.  Adapting Near Eastern plants and farming to cooler Europe surely must have entailed crop failures and setbacks.  The rapid spread of the lactose persistence gene points to the importance of dairy.  The ability to digest it conferred survival when those without it starved.  Moreover, most infectious diseases in humans arise from pathogens that jump the species boundary and are more likely to infect people living with livestock.  

c. ~4,800 years ago:  A new migration of people from the Russian steppes arrives.  These people have horses and carts.  They decorate their pottery in a distinctive design giving them the name the Corded Ware people.  They likely are the first proto-Indo-European speakers, and thus are the linguistic ancestors of almost every European language spoken today except Basque, Hungarian and Finnish.

d. ~4,500 years ago:  A fourth migration starts in Portugal around modern Lisbon.  These people, the Bell-Beaker people, work copper and appear to have a culture built around trade.  They make pottery in the shape of wide-lipped, upside-down bells and build megalithic stone monuments.  At first they spread along the Atlantic and Mediterranean coasts.  These first maritime settlements include modern Brittany in northwest France.  From there the traders appear to follow existing, ancient trading routes up the valley of the Seine to the Rhine and then into the heart of Germany.  In Germany the Bell-Beaker traders mix with the a) hunter-gatherers, b) farmers, and c) Indo-European speakers.  They bring with them the H mtDNA haplogroup, the most common maternal lineage in Europe today.

The authors say Europeans' -and European-Americans'- DNA is a more complex story than Neolithic farmers simply replacing Paleolithic hunter-gatherers.  Instead, today's European gene pool appears to be a mix of these four migrations plus a little Neanderthal, Denisovan, and some later additions from outside Europe.

Guido Brandt, Wolfgang Haak, Christina J. Adler, Christina Roth, Anna Szécsényi-Nagy, Sarah Karimnia, Sabine Möller-Rieker, Harald Meller, Robert Ganslmeier, Susanne Friederich, Veit Dresely, Nicole Nicklisch, Joseph K. Pickrell, Frank Sirocko, David Reich, Alan Cooper, Kurt W. Alt, the Genographic Consortium. Ancient DNA Reveals Key Stages in the Formation of Central European Mitochondrial Genetic Diversity. Science, 2013 DOI: 10.1126/science.1241844