Somewhere in the high desert of western Argentina, on the eastern slope of the Andes near what is now the town of Uspallata, people have been burying their dead for more than two thousand years. The Uspallata Valley sits at roughly 1,800 meters elevation, flanked by the kind of terrain that makes farming precarious: cold, dry, thin-soiled, far from the domestication centers of the Central Andes where maize, quinoa, beans, and squash first became staples. Agriculture reached this place late, and on its own terms.
A study published in Nature1 this week has used ancient genomics, stable isotopes, and pathogen DNA from 46 individuals to reconstruct more than two millennia of population history in that valley. The picture it produces is not the one most models of agricultural spread would predict. And one corner of it — a cemetery filled almost entirely with sick, malnourished migrants who were already declining in number before they arrived — is difficult to account for by any simple story of expansion or collapse.
The central question in studies of how farming spread is deceptively simple: did farmers move, or did ideas move? In much of Europe and western Asia, the genomic evidence has come down firmly on the side of farmer movement. Incoming Anatolian agriculturalists largely replaced or absorbed local Mesolithic hunter-gatherers. But South America is not Europe, and the Uspallata Valley is not a typical case even within South America.
The team sequenced genome-wide ancient DNA from 46 individuals spanning the pre-farming period (roughly 2,200 years ago) through the centuries just before the Inka expansion. What they found was strong genetic continuity across that entire span. The people who were growing maize and herding camelids in the Uspallata Valley were, genetically, the same people who had been hunting and gathering there before farming arrived. No wave of incoming farmers displaced them. The agricultural transition here happened through cultural transmission — crops and practices spreading into an existing population — not through replacement.
That population also turns out to have a distinctive genetic signature. The Uspallata groups carried an ancestry component that appears to represent a unique regional population history within the broader diversity of Indigenous South Americans, something that had been partially detected before in present-day populations from Central Western Argentina but whose deep roots had not been traced. What this study adds is the time depth: that component was already present in the pre-farming groups, suggesting it reflects a long and relatively isolated local lineage rather than a recent admixture event.
