Mysterious knotted strings used by the Incas to control their sprawling South American empire may have been ledgers for their accountants to keep track of payments to the king.
The strings, called khipu, have long fascinated archaeologists because they seemed to be designed to carry information in some hitherto unknown code rather than being made simply for decoration.
At its height, the Incas' empire extended almost the full length of the Andes from Colombia in the north to Chile in the south, but strangely for such a huge administrative area the Incas did not appear to have a written language.
Scientists believe the knots of the khipu strings may have been used as some form of non-verbal communication but archaeologists have come across few clues as to how this was done.
Khipu can be immensely elaborate affairs with many thousands of knotted strings dangling from a main horizontal string. Such complexity suggested that khipu may be a store of information held as a form of three-dimensional writing.
Gary Urton and Carrie Brezine, archaeologists at Harvard University, believe they have made a breakthrough in understanding at least some of the khipu code and have concluded in the journal Science that it has something to do with Inca accounts.
They used a computer database of nearly half of the 700 known khipu to analyse the knots and strings of 21 khipu found in an urn excavated in 1956 from the Inca administrative centre of Puruchuco, near present-day Lima in Peru.
The computer analysis was able to locate patterns in the arrays of the many thousands of knotted strings of varying colours and lengths that formed each khipu.
Seven of the 21 khipu that were analysed appeared to contain cumulative numerical data that seem to have been generated when successive layers of officials in the Inca hierarchy compiled their sums.
The values of the khipu appear to sum upward and subdivide downward, suggesting the addition or subtraction of values as the khipu moved up and down the ranks of Inca bureaucracy, said Professor Urton.
"Every major ancient civilisation - Inca, Chinese, Egyptian, Mayan, Aztec and Mesopotamian - developed sophisticated means of tracking its sprawling empire," Professor Urton said.
"This communication was used to record the information deemed most important to the state, which often included accounting and other data related to censuses, finance and the military," he said. "In this regard, the discovery that khipu were used as ledger books reveals a new consonance between the Inca and other ancient cultures."
Tributes to the Inca state were levied in the form of a labour tax with each "taxpayer" working for a specified number of days on a state project, Professor Urton said. Inca accountants appear to have used khipu to assess tributes made by local workers who were organised into successively larger groups of 10, 50, 100, 500 or more. "This work gives us some sense of how this complex information was compiled, manipulated, shared and archived," Professor Urton said. "Instructions of higher-level officials for lower-level ones would have moved, via khipu, from the top of the hierarchy down.
"In the reverse direction, local accountants would forward information on accomplished tasks upward through the hierarchy."