A recently discovered stalagmite in the Zagros Mountains has unlocked a 18,000-year climate record, shedding new light on why civilization began in the Fertile Crescent. This find offers unprecedented detail on local climatic conditions from 18,000 to 7,500 years ago, a pivotal period when Earth was leaving the last glacial period. Lying close to the valleys where agriculture and civilization were born, the stalagmite provides crucial insights into the conditions that drove their rise. Moreover, the shifts it reveals match those occurring in Greenland, showcasing the global influence on the birthplace of civilization.
The question of why agriculture started when and where it did remains a significant mystery in human history. The fact that the first evidence for agriculture appeared in many unconnected places relatively soon after the end of the last ice age suggests climate played a significant role. However, in most of these locations, we lack good records of the local conditions at the time.
The Fertile Crescent, often considered the first center of civilization, makes limestone caves in the Zagros Mountains a prime location to search for answers. Stalagmites and stalactites, collectively known as speleothems, can record climatic conditions over their formation period through changes in isotopes. In the case of a stalagmite from Kurdish Iraq, this period overlaps with one of the most important developments in human history: the birth of agriculture and the development of villages and cities.
The stalagmite reveals that around 14,560 years ago, rainfall increased in the area, leading to faster deposition of limestone. Around 12,700 years ago, precipitation reduced, and conditions became dustier, indicated by increased concentrations of trace elements like barium, strontium, zinc, and sodium in the limestone layers.
Hsārok Cave, located within the Fertile Crescent, currently receives sufficient rainfall for agriculture, and tributaries of the Tigris River, along whose banks some of the earliest civilizations flourished, flow nearby. Archaeological evidence suggests that Palegawra Cave, 140 kilometers from Hsārok Cave, was frequently occupied in the initial warmth as the glaciers retreated but was largely abandoned around the time the stalagmite indicates the region dried out. Occupation became common again just as Hsārok Cave was recording evidence of renewed warmth.
The authors propose that until the Holocene era began, the foothills of the Zagros Mountains created a mosaic of spatially restricted yet resource-rich environments. While these were not suited to supporting large, year-round settlements, they encouraged mobility, allowing people to exploit seasonally available resources across different elevations and ecotones, such as open-woodland, grassland, and riparian habitats. The flexibility of living under such conditions promoted the building of a culture that, when the climate became warmer and more stable, was well-suited to taking advantage of new opportunities, including agriculture.
The ratios of carbon-13 to carbon-12 in the stalagmite reveal faster local plant growth during times when oxygen-16 and -18 ratios suggest were warmer and wetter, as expected. The local wet period recorded by the stalagmite coincides with the Bølling–Allerød interstadial, when Greenland first warmed significantly from the depths of the last glacial maximum. The subsequent drying matches the Younger Dryas Period, when the Earth, and particularly the North Atlantic Basin, experienced a still unexplained cooling.
The study is published in the Proceedings of the National Academy of Sciences.
