Humans used ceramic pots to cook wild plants more than 10,000 years ago, well before the advent of agriculture, researchers reported Monday.

The findings are the earliest direct proof that our species processed plants for food.

The vessels, discovered in Libya, contained traces of wild grasses such as cattail or cockspur, as well as the leaves and fruit from fig trees, and the family of plants that includes cinnamon, nutmeg and star anise.

There were even aquatic plants, some of which are still consumed today.

Learning to cook was crucial to human evolution. It broadened our diet and made available new sources of energy.

Many of the plants discovered would have been poisonous or indigestible eaten raw.

No one knows exactly when our ancestors began to roast meat over an open fire, but the first heat-resistant ceramic vessels date from about 15,000 years ago.

Evidence that they were used soon after that to boil animal products -- whether meat or milk -- is abundant.

Surprisingly, however, telltale chemical traces in cookware that wild plants were prepared in the same way, have been lacking.

"Until now, the importance of plants in prehistoric diets has been under-recognised," said lead author Julie Dunne, a researcher at the University of Bristol in England.

"What we have found is the first direct evidence of the cooking of plants, which seems to have happened at the same time pottery was invented in North Africa," she told AFP.

The study was published in the journal Nature Plants.

It analysed 110 pot fragments discovered at two sites in the Libyan Sahara -- which would have been lush and oasis-like at the time -- called Takarkori and Uan Afuda.

The Takarkori rock shelter is one of the few sites which records the transition from hunter-gatherers, who thrived from about 8200 BC to 6400 BC, to plant domestication and agriculture.

The technique used to detect the plants, called organic residue analysis, looks for signature molecules that worked their way into unglazed pots.

The Neolithic humans who cooked fruit-and-veggie stews were not necessarily vegetarians.

"Some pots seemed to be used to process plants only, some to process meats," Dunne said. "And some were used to process a mixture of both."

Most experts agree that pottery was independently invented twice in human history, first in East Asia about 16,000 years ago, and then again in North Africa some 4,000 years later.

"Ceramics are a major technological invention which allows the processing of foodstuffs in different ways," said Dunne.