By leapfrogging from island to island across the northern Mediterranean, Neolithic people were able to quickly spread their farming lifestyle across southern Europe some 9,000 years ago, a new genetic study suggests.
Archaeological investigations have shown that individuals in the Near East first developed farming and herding around 12,000 years ago. Agriculture then quickly replaced the more mobile hunter-gatherer lifestyle—in what's called the "Neolithic transition"—as farmers migrated into Europe and other parts of the world.
"The establishment of agriculture provided the possibility for population growth, and that growth led people to expand to new horizons," said University of Washington geneticist George Stamatoyannopoulos.
In a new study, published today in the Proceedings of the National Academy of Sciences, Stamatoyannopoulos and his colleagues analyzed the DNA of individuals from modern Mediterranean populations to reconstruct the migration patterns of their ancient ancestors.
The genetic data showed that the people from the Near East migrated into Anatolia-modern—day Turkey—and then rapidly west through the islands of Greece and Sicily, before making their way north into the center of the continent.
"The gene flow was from the Near East to Anatolia, and from Anatolia to the islands," Stamatoyannopoulos said. "How well the genes mirror geography is really striking."
Two Routes Around the Mediterranean
The researchers catalogued and compared more than 75,000 tiny genetic differences—called single nucleotide polymorphisms, or SNPs—from thousands of individuals. They focused on people living in 32 different locations around the Mediterranean, from Yemen and Palestine to Morocco, Italy, and Spain, as well as several small island populations in Greece, to identify signature patterns for different populations.
By measuring which SNPs (geneticists call them "snips") populations have in common, it's possible to reconstruct how they're related. People living in central Turkey today, for example, share SNPs with Sicilians and Palestinians, but Sicilians and Palestinians have mutations that the two populations don't have in common. That suggests the people have common ancestral roots in Anatolia but the populations have not had much contact since.
The new data show that people living around the Mediterranean today have common ancestors in Anatolia. But then the genes diverge, with Greek islands like the Dodecanese archipelago and Crete forming a sort of genetic bridge to the rest of Greece, Sicily, Italy, and north into Europe. In the southern Mediterranean, the genetic signatures of modern-day Egyptians, Libyans, Tunisians, and Moroccans form a separate genetic branch.
The two distinct paths suggest that there wasn't much genetic mingling once the populations went their separate ways north and south from Anatolia. The sea helped migration in the northern Mediterranean, where people traveled by water from island to island as they spread west. But "there's very little gene flow" between the northern and southern Mediterranean, and "the sea served as a barrier," Stamatoyannopoulos said.
Those populations may not have been completely separate, however, said archaeologist Helen Dawson, a research fellow at the Topoi Excellence Cluster in Berlin who has excavated Neolithic sites on Mediterranean islands. There is evidence of contact across the Mediterranean: For example, archaeologists have found blades in Neolithic settlements in Tunisia made from volcanic glass that comes from isolated islands near Sicily.
"There could have been all sorts of networks across the Mediterranean that haven't left traces" in the genetic record, she said. "Maybe there was no genetic mixing, but there was definitely contact ... It's not like the sea posed an insurmountable barrier."
The new results come exclusively from genetic samples of modern individuals living in regions around the Mediterranean. Pontus Skoglund, a geneticist at Harvard Medical School, said more data are needed to confirm that these people accurately represent those who lived there 9,000 or more years ago. "This is a great initiative to look at these things more closely," he said. "The question is, what time depth does it have?"
Ideally, researchers would be able to compare ancient DNA recovered from the bones of Neolithic settlers to that of modern populations. "It's always difficult to make inferences mostly based on contemporary DNA," Skoglund cautioned. "Modern patterns are not necessarily representative of ancient patterns."
Also, the geographic spread of individuals included in the study may have given preference to Greece over other routes, including the possibility that early migrants hopped along the Adriatic coast through what is now Albania and Croatia and then moved south into Italy, Dawson noted. "I don't think there was just one route," she said. "I know you can't sample everything, but it would be nice to have more along the Adriatic."
Regardless of the exact routes ancient farmers took as they migrated west, it seems island hopping may have helped the settled, farming-oriented lifestyle spread faster.
Archaeologists estimate that agriculture spread across land through central and northern Europe at a rate of about half a mile each year, taking millennia to journey from the Balkans to Germany. But Stamatoyannopoulos said carbon dating of sites around the Mediterranean shows that by farmers jumping from island to island, farming was able to rocket from Italy all the way to Portugal in the space of just six generations.
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