Scientists from the Massachusetts Institute of Technology (MIT) and the University of La Laguna in Spain have identified 50,000-year-old human fecal remains from El Salt, a known site of Neanderthal occupation in southern Spain.
The researchers analysed each sample for metabolised 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 La Laguna who led the analysis as a visiting student at MIT looked for fecal remains in El Salt, an excavation site in Alicante, Spain, where remnants of multiple Neanderthal occupations have been unearthed.
She and her colleagues dug out small samples of soil from different layers, and then worked with co-author Roger Summons, a professor at MIT's Department of Earth, Atmospheric and Planetary Sciences, to analyse the samples.
In the lab, Sistiaga ground the soil into a powder, then used multiple solvents to extract any organic matter from the sediment.
Next, she looked for certain bio-markers in the organic residue that would signal whether the fecal remains were of human origin.
Specifically, Sistiaga looked for signs of coprostanol, a lipid formed when the gut metabolises cholesterol. As humans are able to break down more cholesterol than any other mammal, Sistiaga looked for a certain peak level of coprostanol that would indicate the sample came from a human.
She and Summons then used the same geochemical techniques to determine the proportions of coprostanol - an animal-derived compound - to 5B-stigmastanol, a substance derived from the breakdown of phytosterol derived from plants.
Each sample contained mostly coprostanol - evidence of a largely meat-based diet.
However, two samples also held biomarkers of plants, which Sistiaga says may indicate a rather significant plant intake.
As she explains it, gram for gram, there is more cholesterol in meat than there is phytosterol in plants - so it would take a significant plant intake to produce even a small amount of metabolised phytosterol.
In other words, while Neanderthals had a mostly meat-based diet, they may have also consumed a fairly regular portion of plants, such as tubers, berries, and nuts.
"We believe Neanderthals probably ate what was available in different situations, seasons, and climates," said Sistiaga.
The research was published in the journal PLoS ONE.