Ancient DNA has a way of uncovering complexity in seemingly simple stories of our past. Most famously, it has shown that modern humans didn't simply replace our archaic cousins as we spread across the world; we interbred with them along the way. Now ...and more »
Ancient DNA has a way of uncovering complexity in seemingly simple stories of our past. Most famously, it has shown that modern humans didnâ€™t simply replace our archaic cousins as we spread across the world; we interbred with them along the way. Now, this method is adding nuance to the story of farming, long known to have originated in the Fertile Crescent of the Middle East.Â
According to three teams who used new techniques to gain glimpses of the nuclear DNA of the worldâ€™s very first farmers, farming was adopted not by one group of people, but by genetically distinct groups scattered across the region. â€œIt was not one early population that sowed the seeds of farming in western Asia, but several adjacent populations that all had the good fortune to live in the zone where potential plant and animal domesticates were to be found and exploited,â€ says archaeologist Colin Renfrew of the University of Cambridge in the United Kingdom, who was not involved in the work.
The researchâ€”a paper published online in Science this week and two studies posted last month on the bioRxiv serverâ€”canâ€™t pin down whether agriculture spread quickly among diverse peoples or was independently invented more than once. But the diversity of the first farmers is â€œvery surprising,â€ says statistical geneticist Garrett Hellenthal of University College London, a co-author of the Science paper. â€œThese early farmers who lived pretty close to each other were â€¨completely different.â€
The earliest archaeological evidence for cultivating plants and herding animals dates back 10,000 to 12,000 years ago in the Fertile Crescent, which arcs from the Persian Gulf to Turkey and south to Egypt (see map, above). Excavations at Jericho in Jordan, Jarmo in Iraq, and Ã‡atalhÃ¶yÃ¼k in Turkey, for example, have found evidence of early grain farming and sheep and goat domestication in different areas at roughly the same time.Â
Geneticists have been trying to track whether one group of peopleâ€”or just their ideasâ€”spread farming early on. A single group did carry farming to Europe: DNA from ancient farmers in western Anatolia shows that they were the direct ancestors of Europeâ€™s first farmers, known as the Linear Pottery culture; present-day Sardinians share the most DNA with these ancient Anatolians.
But the trail of the first farmers went cold in the hot climate of the Middle East, which destroys DNA. Now, researchers are using new methods to prepare samples and extract them from the petrous bone of the ear, which is unusually rich in DNA. A team led by Joachim Burger of the Johannes Gutenberg University of Mainz in Germany and including Marjan Mashkour and â€¨Fereidoun Biglari of the National Museum of Iran in Tehran sequenced the complete genomes of four goat herders who lived in the Zagros Mountains of Iran. They include a 9000-year-old male from Wezmeh Cave, and three 10,000-year-old skeletons from a site called Tepe Abdul Hosein that are among the oldest remains of farmers in the world. Tests of isotopes in the teeth of all four farmers confirm they had diets rich in grains, a signature of an agricultural diet.
(Graphic) V. Altounian/Science; (Data) J. Burger
By using a new method that looks at patterns of inheritance of chunks of DNA, â€¨Hellenthal found that the early Zagros Mountain farmers have left a genetic legacy in Pakistanis, Afghans, and others, particularly in Zoroastrians in Iran.
But the ancient Iranian DNA was dramatically different from that of the western Anatolian farmers. The two groups of farmers, who lived about 2000 kilometers and 2000 years apart, must have descended from completely different groups of hunter-gatherers who separated 46,000 to 77,000 years ago, Burger says.
A similar genetic disjunction appears in a study led by Harvard Universityâ€™s David Reich and posted on bioRxiv. This study analyzed ancient DNA from 44 Middle Easterners who lived 14,000 to 3400 years ago, including Natufian hunter-gatherers in Israel, Zagros farmers, and Bronze Age pastoralists in the Eurasian steppe, and compared it with that of 2864 living and ancient people from around the world. By sequencing 1.2 million nucleotides from across each genome, the team found that early farmers of Israel and Jordan (known as the Levant) were genetically distinct from those in the Zagros Mountains, and that both populations were distinct from the western Anatolians who later spread their genes throughout Europe.
The third study, also published on bioRxiv, reported the same stark differences. That study analyzed the complete genome of a 10,000-year-old woman from Ganj Dareh, a site in the Zagros Mountains with the worldâ€™s oldest evidence of goat herding.
Burger and Reich also each used their data to peer even further back in time, to the ancestors of the Zagros Mountain farmers. They found that the Zagros people descend from a group of basal Eurasians who separated from the ancestors of all other people outside of Africa 50,000 to 60,000 years agoâ€”before other non-Africans interbred with Neandertals. So the Zagros Mountain farmers had less Neandertal DNA than the western Anatolian farmers, whose ancestors must have branched off later.
The descendants of these early farmers went separate ways. Whereas the western Anatolians later migrated to Europe, Reichâ€™s team proposes that the ancient farmers of the Levant migrated to East Africa, where living people carry some of their distinct DNA, and the Zagros Mountain farmers spread north into the Eurasian steppe and east into South Asia.
Did these early people learn farming from each other, or was it invented more than once? Here, opinion differs. Archaeologists have noted that early farmers in different regions used different tools and grains, supporting the idea of multiple origins, says archaeologist Roger Matthews of the University of Reading in the United Kingdom. â€œThe genetic and archaeological evidence suggest at least two separate pathways to agriculture, at distant ends of the Fertile Crescent, eventually merging into a unified package that then spreads outwards,â€ he says.
But these groups traded obsidian, suggesting to Renfrew and Harvard archaeoâ€¨logist Ofer Bar-Yosef that seeds and farming knowledge could have been shared, too. Because new kinds of food preparation tools turn up first in the Levant, Bar-Yosef thinks farming sprouted here: â€œZagros foothills people adopted agriculture from the Levant.â€
Burger suggests that farming was such an advantage that it spread both as an idea and by migration of people. â€œInitially, agriâ€¨culture was an idea that spread,â€ he proposes. â€œThen, when it reaches the borders of Europe, it becomes people spreading farming. We have an extremely complex agricultural revolution that was created by people who were extremely diverse.â€