Neanderthal diet was heavy on meat, but also included plant tissues such as tubers and nuts, claimed researchers.
Scientists from MIT and the University of La Laguna in Spain identified human fecal remains from El Salt, a known site of Neanderthal occupation in southern Spain that dates back 50,000 years.
The researchers analysed each sample for metabolized versions of animal-derived cholesterol, as well as phytosterol, a cholesterol-like compound found in plants. While all samples contained signs of meat consumption, two samples showed traces of plants - the first direct evidence that Neanderthals may have enjoyed an omnivorous diet.
Ainara Sistiaga, a graduate student at the University of La Laguna who led the analysis as a visiting student at MIT, said they have passed through different phases in our interpretation of Neanderthals.
Other researchers recently identified plant microfossils trapped in Neanderthal teeth - a finding that suggests the species may have led a more complex lifestyle, harvesting and cooking a variety of plants in addition to hunting prey. But Sistiaga says it is also possible that Neanderthals didn't eat plants directly, but consumed them through the stomach contents of their prey, leaving traces of plants in their teeth.
Equally likely, she says, is another scenario: "Sometimes in prehistoric societies, they used their teeth as tools, biting plants, among other things. We can't assume they were actually eating the plants based on finding microfossils in their teeth."
The study has been published in the journal PLoS ONE.