Traci Watson

Special for USA TODAY

Maybe archaeologists should watch where they step. Researchers have stumbled across what may be the world's oldest human poop, deposited by Neanderthals about 50,000 years ago atop an ancient campfire in Spain.

If the discovery is truly a prehistoric latrine – a claim that has provoked skepticism among other researchers – it contradicts the pop-culture image of Neanderthals as hunters who subsisted on hunks of flesh. Two of the new poop samples contain the chemical footprints of both meat and plant consumption, providing the earliest known evidence that humans were omnivores who ate significant quantities of plant-based food.

To the uneducated eye, the newfound Pleistocene poop doesn't look like poop except for its light-brown color, and the scientists who found it weren't looking for coprolites – preserved dung – at all. They were poking around rudimentary hearths at the Neanderthal site known as El Salt, hoping to find evidence of food preparation.

"I thought they were cooking in there, so I was looking for lipids from cooking," says Ainara Sistiaga of Spain's University of La Laguna, one of the scientists reporting the new discovery in Wednesday's PLOS ONE. Instead, analysis revealed traces of an activity that doesn't usually take place atop a blazing fire, but "I don't think they were using (the site as a toilet) when the fire was active."

The poop samples come from rock layers dated to roughly 50,000 years ago. That's far older than other ancient wastes, such as those found at Turkey's Catalhöyük, one of the world's earliest large villages, dating back 6,000 to 7,000 years, and what might be 14,000-year-old human coprolites at a cave in Oregon.

Three of the five Neanderthal samples contained high levels of compounds created by the breakdown of cholesterol, which is found in animal foods. But two additional samples contained those same compounds as well as those produced by the breakdown of molecules found in plant-based foods. The ratio of the different compounds points to a human origin, the researchers say.

The researchers haven't calculated the percentage of vegetable matter in these ancient dinners, but Sistiaga speculates that they could have been up to 25% plant-based. Previous studies have shown that Neanderthals ate deer and horse as well as roots, tubers and fruit.

Other researchers call the new study intriguing but far from airtight. The compounds measured by Sistiaga and her colleagues have probably degraded over time, making them unreliable as indicators of human feces, says Michael Richards of the University of British Columbia. The study does not rule out bears, which are also omnivorous, as the source of the coprolites, says Hervé Bocherens of the University of Tübingen in Germany.

The new method described in the paper is "very exciting," says Amanda Henry of Germany's Max Planck Institute for Evolutionary Anthropology. But "they have not managed to convince me yet that the coprolite they're looking at is actually human."

Sistiaga responds that as far as she knows, bears don't produce the same chemical from cholesterol that humans do. She also says degradation would not affect her results and that prehistoric poop may not be all that rare.

"No one has been looking for it," she says. "It's not because it's not there."