By Anna-Marie Lever
Science and nature reporter, BBC News
Chemical and archaeological evidence has pushed back the earliest known use of cacao, the key ingredient of chocolate, by 500 years.
The bottles were found in the area known as "the cradle of chocolate" (Image courtesy of PNAS)
The chemical compound, theobromine, which only occurs in the cacao plant, has been found on pottery vessels dating back to as early as 1000 BC.
Experts say the vessels were used to serve a fermented cacao drink that was made from the sweet pulp of the plant.
The vessels were unearthed at sites in Puerto Escondido, Honduras.
"The earliest use of cacao in Mesoamerica is likely to have been for a fermented drink," lead author Professor John Henderson wrote in the Proceedings of the National Academy of Sciences (PNAS).
"Such drinks may contain up to 5% alcohol in volume," the Cornell University, US, academic added.
As well as chemical evidence, a change in pottery vessel shape allowed scientists to pre-date the use of cocoa.
It had been known that the seeds were used to make a frothed chocolate drink which became central to social life throughout Mesoamerica. It was drunk at important ceremonies to mark weddings and births, especially by elites.
As the drink was frothy, it was served in a spouted bottle with a flaring neck. However, long-necked bottle samples that pre-date the spouted bottle were also found to contain cocoa residues.
The researchers suggested that this vessel type was inappropriate for frothing but better for pouring.
This led the authors conclude that "early cocoa was consumed as a fermented beverage made from pulp", rather than seeds.
During the time of the Aztec empire, chocolate seeds were used as an early form of money.