Ancient South American populations declined sharply as rainfall became increasingly unpredictable starting around 8,600 years ago, researchers say.

But hunter-gatherer groups from the Andes and the Amazon to the continent’s southern tip bounced back quickly once rain returned to a relatively stable pattern about 6,000 years ago, report archaeologists Philip Riris and Manuel Arroyo-Kalin, both of University College London.

During that roughly 2,600-year intervening period, bouts of unusually wet or dry conditions that disrupted local food sources occurred frequently, every five years or so on average, the scientists report online May 9 in Scientific Reports. Foragers would have been unable to predict whether extreme rainfall or drought was next up, or precisely when those conditions would hit. Previously, average rainfall patterns had included an abnormally wet or dry year only every 16 to 20 years, Riris and Arroyo-Kalin estimate from rainfall records gleaned from ancient sediments and other sources.

To estimate population changes from around 12,000 to 2,000 years ago, the researchers analyzed 5,450 radiocarbon dates from nearly 1,400 South American archaeological sites. Statistical estimates of when substantial population ups and downs took place, based on changes over time in numbers of archaeological sites, could not assess absolute numbers of people living in South America at various times. Climate records compared to ancient population patterns were divided into 100-year stretches.

The greatest rainfall fluctuations and largest human population declines were seen in northern, tropical parts of South America.

The new findings build on researchers’ previous observations that people abandoned many South American sites around 8,200 years ago. Unpredictable and extreme rainfall patterns might have been one reason why ancient South American hunter-gatherers started to domesticate and cultivate plants, perhaps as backup food sources, the archaeologists suggest.