Despite its relative proximity to the Pacific Ocean, South America’s Atacama Desert is so arid that scientists use it to simulate conditions on Mars. Yet a thousand years ago, farmers grew crops there and raised thriving communities.
“People created the most amazing irrigation and terrace systems to manage the precious water that they had,” says Jonathan Sandor, who studies agriculture and ancient civilizations at Iowa State University. “They developed crops that were adapted to these places. It’s a real testament to people and how they’ve developed incredible knowledge over long periods of time.”
Ingenuity made it possible, along with a surprising secret ingredient—seabird poop. Farmers knew they had to maximize every drop of precious water they had, but they also knew that the arid region's soils needed nutrients. A new study appearing in Nature Plants suggests that workers delivered guano to the region’s parched soils—transported overland to desert farmers for whom the fertilizer was worth its weight in gold.
“Because the arid regions of the Atacama are more than 90km (50 miles) from the coast, the use of guano instead of other animal dung, decomposed leaves and human night soil, also evidences the expansion of regional travel and trade networks,” says archaeologist Francisca Santana-Sagredo, a co-author of the study who works at the Pontifical Catholic University of Chile and the University of Oxford.
The Atacama Desert’s extreme aridity challenged its residents, but it was a huge advantage for Santana-Sagredo and colleagues as they searched for clues to the success of the ancient desert farmers. Human remains and scraps of ancient foods can be extraordinarily well preserved after drying out in the region’s ancient villages, burial mounds and cemeteries.
The team investigated mainly maize, but the prehistoric smorgasbord also included quinoa, chili pepper, gourds, squash, beans, potatoes, popcorn and other delicacies. Food remains were so prevalent and diverse that they begged a question: “How were all these crops produced in the desert?”
To uncover the secrets of this agricultural success, Santana-Sagredo and colleagues sampled 246 plant remains in the south central Andes of northern Chile. They found that crop nitrogen isotope values, an indicator of increased nutrients in the soil, began to soar dramatically in the region from 1000 A.D., reaching levels that they believe can’t be explained by natural variations or the more conventional fertilizers of the era.
One thing the scientists know can deliver nitrogen in large doses is seabird guano, a resource plentiful along the Pacific coast. The feces of pelicans, boobies and cormorants, splattered deep on rocky islets up and down the coast, is rich in nitrogen thanks to the birds feasting on schools of small fish.
Lab experiments show using this guano as fertilizer can supercharge plant nitrogen levels 20 to 40 percent. Experiments in Peruvian cornfields show that seabird fertilizer can boost nitrogen five times more than the use of lama dung.
“The only plausible explanation then for their high nitrogen values was the consumption of fertilized crops with seabird guano," says Santana-Sagredo. “There is no other fertilizer that could reach these values.”
At the study sites, Santana-Sagredo found remains of maize cobs and kernels were far more ubiquitous in places dated to after about 1000 A.D., when the crop likely transitioned from ritual object to everyday food. This crop boom wouldn’t have been possible in such arid locals without a fertilizer like guano.
Evidence of ancient people’s diet can not only be found in food; it can be uncovered in their remains through stable isotope analysis. Stable isotopes of elements like carbon and nitrogen never decay, so the enduring ratios between isotopes of different elements can act as signatures. When animals eat plants, the telltale ratios found in the food become part of the animals’ bodies and can be seen centuries later by analysis of their bones.
Previous studies have used stable isotope analysis to suggest the use of guano at pre-Inca agricultural sites. A 2013 paleodiet study of dental plaque from 28 ancient skeletons found extremely heavy stable isotope nitrogen concentrations at inland valley sites, suggesting that guano fertilizer was used in those places around 1000 A.D.
For this recent study, Santana-Sagredo and team further analyzed 846 published samples of human bone collagen and tooth enamel from across northern Chile from 500 to 1,450 A.D. Stable isotopes of carbon revealed the onset of a more corn-based diet around 1000 A.D. which matched the increase of cobs and kernels found on the ground.
More strikingly, those humans living in the inhospitable inland desert also showed a dramatic spike in nitrogen isotope values around 1000 A.D., mirroring the same trend seen in the remains of the crops they once ate. The same rise and extremely high nitrogen levels didn’t occur along the coast where marine diets would have been plentiful, suggesting guano fertilization made the production of inland crops so fruitful.
“The study is neat because it uses isotope analysis to connect diet, agriculture and the need to supply nutrients, a growing dependence on corn and how all this is kind of linked together with an increase of farming around 1000 A.D.” says Jonathan Sandor, who wasn’t involved in the research. “It means they realized, not only was water a really critical limiting resource but the nutrients were too. Otherwise the production would really fall off.”
But neither the plant nor human remains show a universal rise in nitrogen isotope values across the various study sites in the Atacama Desert. Some samples were much higher than others, which suggests that some farmers had to make do with ordinary lama dung and others nothing at all. “Seabird guano probably became a high-status resource,” says Santana-Sagredo, “only accessible to the local elites.”
When the Spanish arrived in the region some 500 years after guano fertilizing began in earnest, they chronicled the Incas’ elaborate efforts to collect the prized excrement. After Incas settled in the area around 1450 A.D., workers used specially constructed pontoon rafts, made with the skins of sea lions, to visit the tiny, rocky islets along the coasts of southern Peru and northern Chile and gather what later became known as ‘white gold.’ The Spanish also observed the Incas transporting guano from the coast to inland oasis settlements via lama caravans.
Pedro Rodrigues, a biologist studying the evolution and geographic ranges of birds at the Austral University of Chile, accidentally came across enduring evidence of just how important the guano production became to the Inca. He noticed that if one mapped the distribution of the Guanay cormorant, Peruvian pelican and Peruvian booby, their ranges nearly matched the boundaries of the 15th century Inca empire.
Last year he published a study detailing how the Inca created what were likely some of the first human conservation laws, protecting these birds and their precious poop. Other regulations dictated who could harvest the guano and how it would be distributed—including to arid farming areas like the Atacama. The Inca’s guano industry was highly regulated, and those violating poop protocols by using or hoarding more than their share faced execution.
“I think this practice was always important for the sustainability of all these old civilizations living in arid places where guano was available,” Rodrigues writes via email. “What the Incas did was to use the knowledge from these previous civilizations. With this knowledge and by applying strict laws and conservation measures for the protection of the guano birds and their natural habitats, the Incas achieved a surplus of food and [expanded their] empire as never seen before in America.