Ants cultivated designer crops in controlled environments millions of years before humans figured out how to push seeds into the ground to grow food, scientists reported in a study Wednesday.
It has long been known that dozens of ants species tend and harvest fungi in sub-terranean farms, mostly to feed a colony's larvae.
A few species have taken that process to the next level, modifying fungi so thoroughly they can no longer survive in the wild, much in the way some genetically altered crops consumed by humans are not viable without pesticides or other inputs.
"Over the course of millions of years, the fungus has become domesticated," said lead author Michael Branstetter, an ant specialist at the US National Museum of Natural History.
The new research shows for the first time that some ants transitioned to this more sophisticated level of farming about 30 million years ago, probably in response to a cooling and drying climate.
"We discovered that domestication likely occurred in dry habitats in South America," Branstetter told AFP.
"These habitats would have prevented the ant's fungi from escaping the nest and interbreeding with other free-living fungi."
Moisture-loving fungi evolved in wet forests, and would have been poorly equipped to survive on their own in this changing environment.
The findings, published in the journal Proceedings of the Royal Society B, are the fruit of intense genetic sleuthing.
Using powerful new tools, scientists compared some 1,500 stretches of DNA in 119 modern ant species, two-thirds of them farming ants.
By identifying the non-farming ant most closely related to the fungi-cultivating species, they were able to construct an evolutionary tree going back in time.
"Higher agricultural-ant societies have been practising sustainable, industrial-scale agriculture for millions of years," said lead researcher Ted Schultz, the museum's curator of ants.
There may be lessons there for our own species, he added.
"They provide all the nourishment needed for their societies using a single crop that is resistant to disease, pests and droughts at a scale and level of efficiency that rivals human agriculture," he said in a statement.
Just as humans living in a dry or temperate climate might raise tropical plants in a greenhouse, agricultural ants carefully maintain the humidity within their climate-controlled fungal gardens.
"If things are getting a little too dry, the ants go out and get water and they add it," Shultz explained. "If they're too wet, they do the opposite."
Fungi, which include yeasts and moulds, are neither plants nor animals, but form a "kingdom" of their own.