Hair samples from naturally preserved child mummies discovered at the world's highest archaeological site in the Andes have provided a startling insight into the lives of the children chosen for sacrifice. Researchers funded by the Wellcome Trust used DNA and stable isotope analysis to show how children as young as six years old were 'fattened up' and taken on a pilgrimage to their death.

A team of scientists led by Dr Andrew Wilson at the University of Bradford analysed hair samples taken from the heads and from small accompanying bags of four mummies found in the Andes. These included the 15-year-old 'Llullaillaco Maiden' and the seven-year-old 'Llullaillaco Boy', whose frozen remains were found in 1999 at a shrine 25 m from the summit of Mount Llullaillaco, a 6739 m volcano on the border of Argentina and Chile. The Maiden, described as a 'perfect mummy', went on display for the first time last month in Salta, north-west Argentina.

Dr Wilson and colleagues studied DNA and stable light isotopes from the hair samples to offer insight into the lives of these children. Unlike samples of bone collagen and dental enamel, which give an average reading over time, hair growth allows scientists to capture a unique snapshot at different intervals over time, helping to build up a picture of how the children were prepared for sacrifice over a period of months. The results are published today in the journal 'Proceedings of the National Academy of Sciences of the USA'.

"By examining hair samples from these unfortunate children, a chilling story has started to emerge of how the children were 'fattened up' for sacrifice," says Dr Wilson, a Wellcome Trust Bioarchaeology Fellow.

It is believed that sons and daughters of local rulers and local communities were chosen for sacrifice, possibly as a way for the ruling Incas to use fear to govern their people. Some girls, known as acllas, were selected from around the age of four and placed under the guardianship of priestesses; some would later be offered as wives to local nobles, others consecrated as priestesses and others offered as human sacrifices.

By analysing stable isotopes found in the hair samples, Dr Wilson and colleagues were able to see that for much of the time prior to sacrifice, the children were fed a diet of vegetables such as potato, suggesting that they came from a peasant background. Stable isotopes of carbon, nitrogen, oxygen and hydrogen from an individual's diet are deposited in their hair where they can remain unchanged over thousands of years.

However, in the 12 months prior to sacrifice, the isotopic evidence shows that the Maiden's diet changed markedly to one that was enriched with plants such as maize, considered an 'elite' food, and protein, likely to have come from charki (dried llama meat).

"Given the surprising change in their diets and the symbolic cutting of their hair, it appears that various events were staged in which the status of the children was raised" says Dr Wilson. "In effect, their countdown to sacrifice had begun some considerable time prior to death."

Changes in the isotopes in the hair sample in the final three to four months suggest that the children then began their pilgrimage to the mountains, likely from Cuzco, the Inca capital. While scientists cannot be certain how the children died, it is believed that they were first given maize beer (chicha) and coca leaves, possibly to alleviate the symptoms of altitude sickness and also to inure them to their fate. This theory is supported by evidence of coca metabolites that the researchers found in the victims' hair and in particularly high concentrations in the Maiden's.

"It looks to us as though the children were led up to the summit shrine in the culmination of a year-long rite, drugged and then left to succumb to exposure," says co-author Dr Timothy Taylor, also of the University of Bradford. "Although some may wish to view these grim deaths within the context of indigenous belief systems, we should not forget that the Inca were imperialists too, and the treatment of such peasant children may have served to instil fear and facilitate social control over remote mountain areas."

Previous research has shown that Llullaillaco Boy appears to have met a particularly horrific end. His clothes were covered in vomit and diarrhoea features indicative of a state of terror. The vomit was stained red by the hallucinogenic drug achiote, traces of which were also found in his stomach and faeces. However, his death was likely caused by suffocation, his body apparently having been crushed by his textile wrapping having been drawn so tight that his ribs were crushed and his pelvis dislocated.

Wilson, AS et al. 'Stable isotope and DNA evidence for ritual sequences in Inca child sacrifice' PNAS, October 1, 2007