The researchers hadn't come looking for poop. They arrived at the Neanderthal site known as El Salt in search of cooking utensils and food scraps -- poking around the ancient fire pits in search of evidence of food preparation and the cooking process.
They found remnants of food alright, only it was at the other end of the digestion process.
Though it was a surprise to find feces in a hearth, it's unlikely the perpetrator was pooping over a live flame. The hearth had likely been converted to a latrine.
"I thought they were cooking in there, so I was looking for lipids from cooking," explained Ainara Sistiaga, a researcher from Spain's University of La Laguna -- a school on the Canary Islands. "I don't think they were using (the site as a toilet) when the fire was active."
Most significantly, chemical analysis of the petrified dung revealed both plant and animal matter, further disrupting any remaining notion that the early humans were purely meat eaters. Plant matter previously found in Neanderthal caves and between recovered teeth had already cast doubt on such theories. But doubts have persisted.
Some researchers suggested the plants might have gotten there incidentally -- via the stomach of a consumed animal.
Human excrement, Sistiaga pointed out, "is the perfect evidence, because you're sure it was consumed."
Sistiaga is the lead author of the study of the ancient Spanish poop; it was published this week in the journal PLOS ONE.
Though it will remain difficult to determine exactly what food sources made up what portions of the early human diet, it's becoming more and more clear that Neanderthals were omnivores not carnivores.
"We believe Neanderthals probably ate what was available in different situations, seasons, and climates," Sistiaga said.