Humans grew 'chocolate trees' over 3,600 years ago: Study

Humans may have started growing cacao trees -- the source of chocolate -- about 3,600 years ago in Central America, a study has found.

Researchers analysed the genomes of cultivated cacao and traced their origin to a "single domestication event".

The discovery opens a new front in a long-running argument regarding when and where humans started growing the source of chocolate.

"This evidence increases our understanding of how humans moved and established in America," said Omar Cornejo, from Washington State University in the US.

The study, published in the journal Communications Biology, also found that cacao's domestication ended up selecting for flavour, disease resistance and the stimulant theobromine. However, that came at the cost of retaining genes that lowered crop yields.

Researchers sequenced the Theobroma cacao genome in 2010. That laid out what Cornejo refers to as an archetype of the cacao genome, while this study, by sequencing 200 plants, teases out variations in the genome that can reveal the plant's evolutionary history.

They looked at "the prince of cocoas," Criollo -- rare, flavourful and the first to be domesticated.

They found that it was domesticated in Central America 3,600 years ago, but originated in the Amazon basin, near the modern-day border of southern Colombia and northern Ecuador, from an ancient germplasm known as Curaray.

Chances are it was introduced to Central America by traders, said Cornejo.

The tree's population at the time consisted of between 437 and 2,674 individual trees, and most likely about 738 trees.

The time of domestication 3,600 years ago, with margins of 2,481 and 10,903 years ago, is consistent with traces of theobromine found in Olmec pottery and large-scale analyses of ancient and modern human DNA that put colonization of the Americas at roughly 13,000 years ago.

The researchers also saw support for a hypothesis that domestication carries a cost as growers, in choosing plants with desirable traits, can ultimately make plants that accumulate counterproductive genes -- "deleterious mutations" -- making them less fit.

Insights from the study could help identify genes behind specific traits that breeders can emphasize, including yield.

"What we would like to have is a way to combine plants from populations with high productivity -- like Iquitos -- with plants of Criollo origin, while retaining all these desirable traits that make Criollo cacao be the best in the world," said Cornejo.