Houyuan Lu

Ancient Asian cultures may have risen and fallen on the strength of monsoons

About 3900 years ago, one of the biggest cradles of Bronze Age civilization fell apart. For unknown reasons, people fled the Indus Valley in what is today Pakistan and northwestern India to establish smaller villages and farms farther to the east. Now, analyses of cave formations from the region bolster the notion that changes in the strength of that semiarid region’s summer monsoon may have played a major role in this cultural upheaval, and others like it. Researchers looked at the ratios of two forms of oxygen atoms in the minerals deposited in layers in two stalagmites that had formed in a cave in the Himalayan foothills about 200 kilometers north of New Delhi. Combining new data with those garnered previously from another stalagmite from the same cave provides a continuous climate chronicle that stretches back more than 5700 years, the researchers report today in Science Advances. Notably, the rise and expansion of both the Indus Valley civilization (from about 5350 years to about 4600 years ago) and the Vedic civilization (from about 3450 years to about 3100 years ago) occurred during periods when climate was relatively warm, wet, and stable. On the other hand, these civilizations apparently started falling apart when climate cooled and the summer monsoons failed to deliver as much water. Also of major interest, the team notes, is that the sudden collapse of the nearby Guge kingdom of western Tibet (ruins shown) around 1620 C.E.—which had previously been ascribed to military conflicts—occurred near the end of the region’s driest 3 decades in the past 5700 years.