Guillermo Cock, an Andean expert based in Lima who was not involved in the new study, said that taking yanacona from diverse regions probably helped Inca rulers break ties of allegiance between villagers and their local authorities.
"The greater the distance, the greater the rupture between the yanacona and their lords and the greater their dependence on their Inca authority," Cock said.
But evidence suggests the yanacona were treated with honor and privileges to help soften the blow and create new loyalties, he said.
For example, the retainers were given gifts such as textiles and agricultural lands, and their bones showed no signs of hard physical labor.
The servants likely performed agricultural work, administrative jobs, served in defense, and generally maintained the site, study author Turner said.
Machu Picchu seems to have been abandoned after the Spanish conquest, and it was apparently ignored by the invaders. Yale archaeologist Hiram Bingham brought Machu Picchu to worldwide attention after local Indians led him to the site in 1911.
During the initial excavations of the site in 1912 and 1913, archaeologists found three cemeteries containing 177 bodies.
Later analysis of the graves and objects found with the bodies suggests the people buried there were not elite, leading experts to theorize that they were yanacona.
(Related: "Rare Mass Tombs Discovered Near Machu Picchu" [September 15, 2008].)
For the new study, Turner and colleagues looked at ratios of oxygen, strontium, and lead isotopes in the teeth of 74 individuals from those graves.
The team looked for the isotopes in tooth layers that develop when a person is three to four years old.
Comparing those results with analyses of food and water sources near Machu Picchu helped determine whether the people were native to the area or were likely immigrants.
The analysis shows "widely different backgrounds in where [the people] lived and what their diets were," Turner said, although she cautions that her team's study is just an initial attempt at uncovering the yanacona's origins.
She and colleagues will publish their work in an upcoming issue of the Journal of Archaeological Science.
Cock, the Andean expert, said that as permanent servants, yanacona were extremely useful to the Inca Empire.
If the Inca needed a labor force, they could just pick from the yanacona instead of requesting new temporary workers from communities under their power.
"The yanacona gave them direct access to labor, making it a much more efficient system," Cock said.
In fact, the state's success owed a great deal to the yanacona, said Fernando Astete, director of the Machu Picchu Archaeological Park.
"Without the work of yanacona, the Inca state would never have developed," Astete said. "Their work was the foundation of Inca productivity."
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