A cave in Iran where the bones of ancient human farmers were found. Their DNA was sequenced to unravel the mystery of who the region’s first farmers were.
FEREIDOUN BIGLARI
The greatest technological revolution in human history occurred about 12,000 years ago, when humans first stopped living as hunter-gatherers and became farmers. This so-called Neolithic Revolution transformed human culture, our genomes and our ecosystems. But the origins of agriculture have remained a mystery. Was there a eureka moment when an early Neolithic person realized that the seeds they scattered in the fall had sprouted into grains two seasons later? Or, more intriguingly, have different groups of people started farming on their own?
Two new studies published this month in Science And Nature journals use DNA analysis of ancient human bones to conclude that agriculture originated in several regions at once. The Science study focused on four farmers who lived between 9,000 and 10,000 years ago in the mountainous Zagros region of Iran. The Nature study analyzed 44 individuals (both farmers and hunter-gatherers) from Armenia, Turkey, Israel, Jordan and Iran who lived between 14,000 and 3,500 years ago. By sequencing parts of the DNA of these ancient people, researchers were able to determine their likely ancestry, as well as which populations are descended from them today. The researchers conclude that there are at least two groups of ancient people who separately discovered agriculture in the Middle East and then exported the Neolithic Revolution to large parts of the continent.
The secrets of ancient DNA
Over the past decade, modern DNA sequencing techniques have enabled scientists to recover strands of genetic material from decayed bones that have been infested with microbes for thousands of years. These techniques are now widely accessible and highly refined. It starts with how researchers choose their bones. If possible, they extract DNA from the rocky bone in the inner ear, a gold mine for genetic material that can yield about 100 times more ancient DNA than other parts of the skeleton. Then researchers use a process called in-solution hybridization, which uses special probes made of DNA or RNA that attach to the desired ancient human DNA and fish it out of a soup of other genetic material from other organisms that accumulated. in the decomposing bone. Techniques like these make it easier than ever for us to sequence ancient DNA and reconstruct the human past.
The fertile crescent is highlighted in green.
Looking at ancient DNA from farmers, researchers discovered a clear genetic divide between the ancient peoples of the Fertile Crescent, a region stretching across the Middle East from present-day Egypt, through Jordan and southern Turkey, through Iraq and into the west of Iran. “Probably the biggest surprise news about this study is how genetically different the early farmers to the east and west of the Fertile Crescent were,” evolutionary geneticist Mark Thomas told BBC News. Agriculture arose simultaneously in these groups despite their genetic and geographic distance from each other. In other words, we have solid evidence that agriculture evolved twice, at about the same time in two communities that had almost no contact with each other.
The descendants of the two peasant groups also dispersed in different directions. The progeny of the western Fertile Crescent farmers can be found throughout the Middle East and Europe. Meanwhile, the Iranian farmers from Zagros spread north to the steppes and south to India and Pakistan. Some also stayed put. There are strong genetic links between the ancient Zagros farmers and a group of Zoroastrians living in Iran today. What is clear is that most people in both groups of early farmers were part of great migrations and mixed with many other peoples along the way.
These ancient farmers also seem to share a common ancestral group known as Basal Indo-Asians, an ancient lineage that split from other Eurasian groups about 40,000 to 50,000 years ago. Unlike other early human groups in Eurasia, the Basal Indos mated infrequently with Neanderthals – very little Neanderthal DNA made its way into their population. But the Basal Indos seem to have mated with everyone. We find traces of Basal Eurasian DNA in people across the continent. Still, we have yet to find the skeleton of a person whose DNA is clearly Basal Eurasian. For that reason basal Indo-Asians are called a “ghost population”. We can only see their genetic legacy in modern populations and have to guess where they came from and how they reached so many parts of Eurasia.
Why did agriculture develop in different places at the same time?
That agriculture originated in separate populations seems to suggest a kind of inevitability for the discovery of agriculture. A popular hypothesis is that when people reach a certain population size, they are destined to innovate more. Growing crops would be such an innovation, especially in a hospitable region. This would help explain why ancient people living in South America and East Asia developed agriculture thousands of years ago independently of the early Fertile Crescent farmers.
The idea that population size leads to cultural change stems from the idea that people strive to imitate the most talented among them at a given task. The more people there are in a group, the more likely there will be outliers who are exceptionally talented and drive social progress. While some population biologists embrace this idea, others in the field feel it is too deterministic because social change is stochastic. After all, agriculture did not arise in all major groups at the same time.
Researchers will continue to debate what exactly inspired disparate groups to take up farming and pet breeding, but at least one mystery has been solved. We now know that about 12,000 years ago, at least two groups of farmers emerged on opposite sides of the Fertile Crescent. Their progeny then spread across Eurasia, bringing Neolithic farming techniques as far as Western Europe and South Asia. Are we seeing a snapshot of the rise of agriculture or just a snapshot of many instances where groups independently came up with the idea of settling? Now that we live in an era where it’s increasingly easy to sequence ancient genomes, we may soon know the answers.