Carbon-dating of an ancient beam from a Guatemalan temple may help end a century-long debate about the Mayan calendar, anthropologists said on Thursday.
Experts have long wrangled over how the Mayan calendar -- which leapt to global prominence last year when the superstitious said it predicted the end of the world -- correlates to the European calendar.
Texts and carvings from this now-extinct culture describe rulers and great events and attribute the dates according to a complex system denoted by dots and bars, known as the Long Count.
The Long Count consists of five time units: Bak'tun (144,000 days); K'atun (7,200 days), Tun (360 days), Winal (20 days) and K'in (one day).
The time is counted from a mythical starting point.
But the date of this starting point is unknown. Spanish colonisers did their utmost to wipe out traces of the Mayan civilisation, destroying evidence that could have provided a clue.
An example of the confusion this has caused is the date of a decisive battle that shaped the course of Mayan civilisation.
It occurred at nine Bak'tuns, 13 K'atuns, three Tuns, seven Winals and 18 K'ins -- or 1,390,838 days from the start of the count. Attempts to transcribe this into the European calendar have given estimates that vary by hundreds of years.
Anthropologists led by Douglas Kennett at Pennsylvania State University hit on the idea of carbon-dating, which measures the age of organic material from residue of carbon 14, an isotope that decays at a steady rate.
They took a tiny sample from a carved wooden lintel found at a temple in the city of Tikal, the centre of the now-vanished Mayan civilisation.
The carvings recount the key event when Tikal's king, Jasaw Chan K'awiil I, defeated Yich'aak K'ahk, known as "Claw of Fire," who headed a rival kingdom at Calakmul, 90 kilometres (55 miles) away.
Using a technique called accelerator mass spectrometry, the team concluded the tree was cut down and carved around AD 658-696.
The estimate closely matches that of a decades-old benchmark for Mayan dating, the so-called Goodman-Martinez-Thompson method.
According to the Goodman-Martinez-Thompson estimate, the big victory occurred around AD 695-712.
That figure was bolstered by early use of carbon-14 dating on two other wooden beams from Tikal in the 1950s.
The small discrepancy between the two dates may find an explanation in the wood itself, Kennett's team believes.
The huge lintel was taken from a tree called the sapotilla (Latin name Manilkara zapota), which has a very hard wood and would have taken years to strip and carve using stone-age implements, says the new study.
Armed with two good fixes on the date of this decisive battle, historians should be able to build a more accurate chronology of the rise and fall of the Mayan civilisation, says the study, appearing in the Nature journal Scientific Reports.
"Jasaw Chan K'awiil's defeat of Calakmul followed a series of great wars between Tikal, Calakmul and Dos Pilas between AD 657 and 677 that resulted in the defeat of Nuun Ujol Chaak," it says.
"These events and those recorded at cities throughout the Maya lowlands can now be harmonised with greater assurance to other environmental, climatic and archaeological data."
The ancient Mayans reached the peak of power in Central America between AD 250 and 900.
The civilisation's sudden collapse has also been hotly debated. Mooted causes include disease, foreign invasion and deforestation or climate change that led to a prolonged, devastating drought.