The first peoples of the Atacama Desert lived among the trees: A 11,600- to 11,200-year-old grove and congregation site.

In deserts, water has been singled out as the most important factor for choosing where to settle, but trees were likely an important part of the landscape for hunter-gatherers beyond merely constituting an economic resource. Yet, this critical aspect has not been considered archaeologically. Here, we present the results of mapping and radiocarbon dating of a truly unique archaeological record. Over 150 preserved stumps around five Late Pleistocene/Early Holocene archaeological campsites (12,800 to 11,200 cal BP) show that trees were key features in the creation of everyday habitats for the first inhabitants of the Atacama Desert. At two of these sites, QM12 and QM35, the spatial and chronological correlation between trees and hearths reveals that people located their homes under the tree canopy. At residential site QM35, artifact distribution coincides with a grove dated to ~11,600 to 11,200 cal BP. A third residential area (QM32) occurred along the grove margins ~12,000 to 11,200 cal BP. Based on the distinct cultural material of these two camps, we propose that two different groups intermittently shared this rich wetland-grove environment. The tree taxa suggest a preference for the native Schinus molle, a tree scarcely present on the landscape today, over the endemic, nitrogen-fixing Strombocarpa tamarugo, both for toolmaking and firewood and even though the S. tamarugo was locally more abundant. Together with the spatial and chronological coincidence of campsites, hearths, and trees, we propose that people spared the most abundant and resilient species to create their homes, in turn promoting fertility oases amid the Atacama's hyperaridity.

Genetic studies of various Prosopis species (Leguminosae, Section Algarobia) co-occurring in oases of the Atacama Desert (northern Chile).

In the Atacama Desert from northern Chile (19-24°S), Prosopis (Leguminosae) individuals are restricted to oases that are unevenly distributed and isolated from each other by large stretches of barren landscape constituting an interesting study model as the degree of connectivity between natural populations depends on their dispersal capacity and the barriers imposed by the landscape. Our goal was to assess the genetic diversity and the degree of differentiation among groups of Prosopis individuals of different species from Section Algarobia and putative hybrids (hereafter populations) co-occurring in these isolated oases from the Atacama Desert and determine whether genetic patterns are associated with dispersal barriers. Thirteen populations were sampled from oases located on three hydrographic basins (Pampa del Tamarugal, Rio Loa, and Salar de Atacama; northern, central, and southern basins, respectively). Individuals genotyped by eight SSRs show high levels of genetic diversity (H O = 0.61, A r = 3.5) and low but significant genetic differentiation among populations (F ST = 0.128, F ST-ENA = 0.129, D JOST = 0.238). The AMOVA indicates that most of the variation occurs within individuals (79%) and from the variance among individuals (21%); almost, the same variation can be found between basins and between populations within basins. Differentiation and structure results were not associated with the basins, retrieving up to four genetic clusters and certain admixture in the central populations. Pairwise differentiation comparisons among populations showed inconsistencies considering their distribution throughout the basins. Genetic and geographic distances were significantly correlated at global and within the basins considered (p < .02), but low correlation indices were obtained (r < .37). These results are discussed in relation to the fragmented landscape, considering both natural and non-natural (humans) dispersal agents that may be moving Prosopis in the Atacama Desert.

'White gold' guano fertilizer drove agricultural intensification in the Atacama Desert from AD 1000.

The archaeological record shows that large pre-Inca agricultural systems supported settlements for centuries around the ravines and oases of northern Chile's hyperarid Atacama Desert. This raises questions about how such productivity was achieved and sustained, and its social implications. Using isotopic data of well-preserved ancient plant remains from Atacama sites, we show a dramatic increase in crop nitrogen isotope values (δ15N) from around AD 1000. Maize was most affected, with δ15N values as high as +30‰, and human bone collagen following a similar trend; moreover, their carbon isotope values (δ13C) indicate a considerable increase in the consumption of maize at the same time. We attribute the shift to extremely high δ15N values-the highest in the world for archaeological plants-to the use of seabird guano to fertilize crops. Guano-'white gold' as it came to be called-thus sustained agricultural intensification, supporting a substantial population in an otherwise extreme environment.

The pre-Columbian introduction and dispersal of Algarrobo (Prosopis, Section Algarobia) in the Atacama Desert of northern Chile.

Archaeological and palaeoecological studies throughout the Americas have documented widespread landscape and environmental transformation during the pre-Columbian era. The highly dynamic Formative (or Neolithic) period in northern Chile (ca. 3700-1550 yr BP) brought about the local establishment of agriculture, introduction of new crops (maize, quinoa, manioc, beans, etc.) along with a major population increase, new emergent villages and technological innovations. Even trees such as the Algarrobos (Prosopis section Algarobia) may have been part of this transformation. Here, we provide evidence that these species were not native to the Atacama Desert of Chile (18-27°S), appearing only in the late Holocene and most likely due to human actions. We assembled a database composed of 41 taxon specific AMS radiocarbon dates from archaeobotanical and palaeoecological records (rodent middens, leaf litter deposits), as well an extensive bibliographical review comprising archaeobotanical, paleoecological, phylogenetic and taxonomic data to evaluate the chronology of introduction and dispersal of these trees. Although Algarrobos could have appeared as early as 4200 yr BP in northernmost Chile, they only became common throughout the Atacama over a thousand years later, during and after the Formative period. Cultural and natural factors likely contributed to its spread and consolidation as a major silvicultural resource.