2016-10-17 20:44:14
Where Did the First Farmers Live? Looking for Answers in DNA

20:44, October 17 186 0

Beneath a rocky slope in central Jordan lie the remains of a 10,000-year-old village called Ain Ghazal, whose inhabitants lived in stone houses with timber roof beams, the walls and floors gleaming with white plaster.

Hundreds of people living there worshiped in circular shrines and made haunting, wide-eyed sculptures that stood three feet high. They buried their cherished dead under the floors of their houses, decapitating the bodies in order to decorate the skulls.

But as fascinating as this culture was, something else about Ain Ghazal intrigues archaeologists more: It was one of the first farming villages to have emerged after the dawn of agriculture.

Around the settlement, Ain Ghazal farmers raised barley, wheat, chickpeas and lentils. Other villagers would leave for months at a time to herd sheep and goats in the surrounding hills.

Sites like Ain Ghazal provide a glimpse of one of the most important transitions in human history: the moment that people domesticated plants and animals, settled down, and began to produce the kind of society in which most of us live today.

But for all that sites like Ain Ghazal have taught archaeologists, they are still grappling with enormous questions. Who exactly were the first farmers? How did agriculture, a cornerstone of civilization itself, spread to other parts of the world?

Some answers are now emerging from a surprising source: DNA extracted from skeletons at Ain Ghazal and other early settlements in the Near East. These findings have already challenged long-held ideas about how agriculture and domestication arose.

What’s more, the new data are showing that early farmers would leave a tremendous mark. People from Ireland to India trace some of their ancestry to people who began growing barley and wheat in the Near East thousands of years ago.

“It’s a part of the story of civilization that we’re just beginning to understand,” said Iosif Lazaridis, a postdoctoral researcher at Harvard Medical School.

The agricultural revolution changed our species and our planet. As bands of hunter-gatherers began domesticating plants and animals, they quit the nomadic life, building villages and towns that endured for thousands of years.

A stable food supply enabled their populations to explode, and small egalitarian groups turned into kingdoms sprawling across hundreds of miles.

Agriculture originated in a few small hubs around the world, but probably first in the Fertile Crescent, a region of the Near East including parts of modern-day Iraq, Syria, Lebanon, Israel and Jordan. The evidence for full-blown agriculture there — crops, livestock, tools for food preparation, and villages — dates back about 11,000 years.

In the 1990s, archaeologists largely concluded that farming in the Fertile Crescent began in Jordan and Israel, a region known as the southern Levant. “The model was that everything started there, and then everything spread out from there, including maybe the people,” said Melinda A. Zeder, a senior research scientist at the Smithsonian National Museum of Natural History.

But in recent years, Dr. Zeder and other archaeologists have overturned that consensus. Their research suggests that people were inventing farming at several sites in the Fertile Crescent at roughly the same time. In the Zagros Mountains of Iran, for example, Dr. Zeder and her colleagues have found evidence of the gradual domestication of wild goats over many centuries around 10,000 years ago.

People may have been cultivating plants earlier than believed, too.

In the late 1980s, Ofer Bar-Yosef of Harvard and his colleagues began excavating a 23,000-year-old site on the shores of the Sea of Galilee known as Ohalo II. It consisted of half a dozen brush huts. Last year, Dr. Bar-Yosef and his colleagues reported that one of the huts contained 150,000 charred seeds and fruits, including many types, such as almonds, grapes and olives, that would later become crops. A stone blade found at Ohalo II seemed to have been used as a sickle to harvest cereals. A stone slab was used to grind the seeds. It seems clear the inhabitants were cultivating wild plants long before farming was thought to have begun.

“We got fixated on the very few things we just happened to see preserved in the archaeological record, and we got this false impression that this was an abrupt change,” Dr. Zeder said. “Now we really understand there was this long period where they’re playing around with resources.”

Many scientists have suggested that humans turned to agriculture under duress. Perhaps the climate of the Near East grew harsh, or perhaps the hunter-gatherer population outstripped the supply of wild foods.

But “playing around with resources” is not the sort of thing people do in times of desperation. Instead, Dr. Zeder argues, agriculture came about as climatic changes shifted the ranges of some wild species of plants and animals into the Near East.

Many different groups began experimenting with ways of producing extra food, which eventually enabled them to start a new way of life: settling down in more stable social groups.

Enter the geneticists, who have long wondered if they could help solve the riddle of agriculture’s origins with DNA from human remains discovered in places like Ain Ghazal.

Ancient genetic material can survive in skeletons for thousands of years, sometimes even hundreds of thousands of years. Scientists have been able to reconstruct entire genomes of ancient humans and extinct relatives like Neanderthals.

But a number of attempts to get DNA out of skeletons in the Near East failed. It looked as if the conditions in the region were too harsh for ancient DNA to survive.

“Genetically, the Near East was terra incognita,” said David Reich, a geneticist at Harvard Medical School.

It isn’t any longer. In two recent studies, geneticists including Dr. Reich used new methods to fish out enough DNA from the bones of the first farmers to figure out their relationship to other people. A team of researchers based at Johannes Gutenberg University in Mainz, Germany, reconstructed the genomes of four early farmers from the Zagros Mountains whose bones date back as much as 10,000 years.

Dr. Reich and his colleagues — including Ron Pinhasi, an archaeologist at University College Dublin, and Dr. Lazaridis of Harvard — recovered genetic material from 44 sets of remains around the Near East. Their haul included DNA from early farmers in Iran, as well as from bones from another site in the Southern Levant like Ain Ghazal. Dr. Reich’s group discovered even older genetic material from hunter-gatherers in the region, from as far back as 14,000 years ago.

The new results all point to the same overall conclusion: The first farmers in each region were the descendants of the earlier hunter-gatherers. What’s more, each population had its own distinct ancestry, going back tens of thousands of years.

They were as different from one another genetically as the Europeans and Chinese. And these groups remained distinct through the agricultural revolution as they changed from hunter-gatherers to full-blown farmers. “It was quite surprising to see how different these groups were from each other,” Dr. Lazaridis said. “It was more extreme than anything you could have imagined was going on.”

Dr. Reich and others argue that the findings show that people around the fertile crescent became farmers independently. “It’s not like you had one Near Eastern population that developed farming that expands and overruns all the others,” he said.

Archaeologists have welcomed the new results from the geneticists. But for now, they are interpreting the data in different ways.

Dr. Zeder said that ancient DNA supports a scenario where farmers across the Fertile Crescent independently invented agriculture, perhaps repeatedly. But Dr. Bar-Yosef says he thinks full-blown agriculture evolved only once, and then quickly spread from one group to another.

He points to the increasingly precise dating of archaeological sites in the Fertile Crescent. Instead of the southern Levant, the oldest sites with evidence of full-blown agriculture are in northern Syria and southern Turkey. That’s where Dr. Bar-Yosef thinks agriculture began.

In other parts of the Fertile Crescent, he argues, people were just toying with farming. Only when they came in contact with the combination of crops and livestock, and the technology to manage them — what scientists call the Neolithic package — did they permanently adopt the practices.

“You just map the dates” of the sites at which the evidence for farming is found, he said, “and you see it’s always later as you get away from the core area.” The new genetic results simply show that this farming technology spread through the Fertile Crescent, but that the populations sharing it did not interbreed.

The new research also shows that even after agriculture was established across the Fertile Crescent, people remained genetically isolated for thousands of years.

“If they were talking to each other, they weren’t intermarrying,” said Garrett Hellenthal, a geneticist at University College London who collaborated with the Gutenberg University researchers.

But the DNA research also shows that this long period of isolation came to a sudden and spectacular end.

About 8,000 years ago, the barriers between peoples in the Fertile Crescent fell away, and genes began to flow across the entire region. The Near East became one homogeneous mix of people.

Why? Dr. Reich speculated that growing populations of farmers began linking to one another via trade networks. People moved along those routes and began to intermarry and have children together. Genes did not just flow across the Fertile Crescent — they also rippled outward. The scientists have detected DNA from the first farmers in living people on three continents.

“There seem to be expansions out in all directions,” Dr. Lazaridis said.

Early farmers in Turkey moved across the western part of the country, crossed the Bosporus and traveled into Europe about 8,000 years ago. They encountered no farmers there. Europe had been home to groups of hunter-gatherers for more than 30,000 years. The farmers seized much of their territory and converted it to farmland, without interbreeding with them.

The hunter-gatherers clung to existence for centuries, and were eventually absorbed by bigger farming communities. Europeans today can trace much of their ancestry to both groups.

The early farmers in what is now Iran expanded eastward. Eventually, their descendants ended up in present-day India, and their DNA makes up a substantial portion of the genomes of Indians.

And the people of Ain Ghazal? Their population expanded into East Africa, bringing crops and animals with them. East Africans retain ancestry from the first farmers of the southern Levant — in Somalia, a third of people’s DNA comes from there.

Dr. Reich hopes to learn more about the early farmers by obtaining samples more systematically from across the Fertile Crescent. “It’s not easy to come by these unique and special specimens,” he said.

But he is pessimistic about filling in some of the most glaring gaps in the genetic map of the Fertile Crescent. No one has yet recovered DNA from the people who lived in the oldest known farming settlements. And it’s unlikely they’ll be trying again anytime soon. To do so, they would have to venture into the heart of Syria’s civil war.