WASHINGTON — Neanderthals, prehistoric cousins of humans, ate grains and vegetables as well as meat, cooking them over fire in the same way homo sapiens did, a new study reveals.

The new research published in the Proceedings of the National Academy of Sciences (PNAS) challenges a prevailing theory that Neanderthals' over reliance on meat contributed to their extinction around 30,000 years ago.

Researchers found grains from numerous plants, including a type of wild grass, as well as traces of roots and tubers, trapped in plaque buildup on fossilized Neanderthal teeth unearthed in northern Europe and Iraq.

Many of the particles "had undergone physical changes that matched experimentally-cooked starch grains, suggesting that Neanderthals controlled fire much like early modern humans," PNAS said in a statement.

Stone artifacts have not provided evidence that Neanderthals used tools to grind plants, suggesting they did not practice agriculture, but the new research indicates they cooked and prepared plants for eating, it said.

The squat, low-browed Neanderthals lived in parts of Europe, Central Asia and the Middle East for around 170,000 years but all evidence of them disappears some 28,000 years ago, their last known refuge being Gibraltar.

Why they died out is a matter of debate, because they co-existed alongside modern man.

The latest study was carried out by the Department of Anthropology at the Smithsonian natural history museum in Washington.