Until now it was widely assumed that the ancient humans who lived alongside our ancestors ate almost nothing but meat.
It was thought to have contributed to their downfall, since early modern humans were able to exploit more food sources.
But a new analysis of fossilised Neanderthal teeth has revealed evidence of a much richer diet, including a wide range of vegetables and pulses, as well as an ability to cook.
Microscopic particles trapped in the teeth contained residues of wild grass, beans, roots and tubers, as well as palm dates.
Many had undergone physical changes that matched experimentally cooked starch grains.
Researchers are still trying to identify remains of other plants.
The evidence, from cave sites in Iraq and Belgium, suggests Neanderthals controlled fire in much the same way as early modern humans living more than 30,000 years ago.
The researchers, writing in the journal Proceedings of the National Academy of Sciences, pointed to "an overall sophistication in Neanderthal dietary regimes".