The oldest known Neanderthal poo, uncovered in Spain, shows that cavemen ate not only meat but vegetables too, according to a study published on Wednesday.
The discovery was made at the archeological site of El Salt, where researchers have found signs that Neanderthals lived some 45,000-60,000 years ago.
The study in the journal PLOS ONE is the first to analyze feces in an attempt to show precisely what kinds of foods our long-extinct kin were eating.
Researchers dug into the sediment and ground the samples to a powder for analysis at a sophisticated Massachusetts Institute of Technology (MIT) lab.
They discovered biomarkers in the poo that showed coprostanol, a lipid formed when the gut metabolizes cholesterol, particularly from eating animals.
They also found 5B-stigmastanol, a substance that is made when plants are broken down in the digestive process.
That means Neanderthals ate mostly meat, as experts have believed for some time, but that there was also evidence of a considerable amount of plants in their diet, including tubers, berries and nuts.
"We believe Neanderthals probably ate what was available in different situations, seasons and climates," said Ainara Sistiaga, a graduate student at the University of La Laguna who performed the research while studying at MIT.
Previous studies have suggested that Neanderthals probably ate nuts and plants, based on residue found in their teeth.
However, these studies were not definite because Neanderthals often used their teeth as tools, and could have been chewing or grasping plant matter but not eating it.
It was also possible that the traces of plant microfossils in their teeth came from the stomach contents of the prey they ate.
For that reason, the 50,000-year-old poo samples provide a more direct path to finding out what they actually consumed -- a varied diet, the researchers said.
Before the Neanderthals went extinct some 35,000 years ago, their capacity for eating multiple food types might have helped them survive, said co-author Roger Summons.
"It's important to understand all aspects of why humanity has come to dominate the planet the way it does," said Summons, a professor of geobiology in MIT's Department of Earth, Atmospheric and Planetary Sciences.
"A lot of that has to do with improved nutrition over time."
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