Roger Summons is an astrobiologist at the Massachusetts Institute of Technology searching for organic matter that could be the hallmark of life on Mars. He has studied the earliest geological and biochemical cycles on earth. But last year he received an unusual proposal from a student who wondered if she could use the sophisticated biochemistry equipment in his laboratory for a somewhat less sweeping scientific investigation: a detailed study of Neanderthal poop.
"We do all sorts of chemistry focused on microbes and this is our first foray into humans, so I think it's exciting," Summons said.
Ainara Sistiaga, a graduate student at the University of Laguna in Spain, had access to sediment from a cave in southern Spain that had been used by Neanderthals around 50,000 years ago. Scientists had unearthed fireplaces, animal remains, Neanderthal remains, and stone tools at the site — as well as tiny specks in the sediment that appeared to be residues of excrement. There were telltale signs in the small pieces of sediment: that they had once contained the eggs of parasites, for example.
In a study published Wednesday in the journal PLOS ONE, she and Summons report that an analysis of five samples of sediment show signs of byproducts of human digestion. They found molecules that are created when food is broken down in the gut: digestion of meat produces coprostanol, whereas eating plants creates a byproduct called 5B-stigmastanol.
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Overall, the samples suggest that the Neanderthal diet was dominated by meat, as several studies have already suggested. But two of the samples also contained a strong signal of plant consumption. Even though there was comparatively less of the plant byproducts in the fecal residue, meat starts with a much higher concentration of the cholesterol that is broken down by digestion. That means that even the small amount of plant byproduct could signal that it was a considerable portion of Neanderthals' diet.
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The scientists cannot rule out that Neanderthals were carnivores with this data; it is always possible that they ingested plants indirectly, by consuming the full stomachs of their prey.
But the technique may provide a powerful new window into understanding early human ancestors. One of the major challenges for scientists trying to reconstruct how ancient people lived is that they are extrapolating meaning from fragmentary remains. Reports of plant matter interspersed with fossilized teeth could mean, for example, that the people were eating plants — or it could mean they were chewing or biting plants for other purposes.
Sistiaga said that other studies in the field have focused on ancient Greek latrines, mummified coprolites, and fossilized mammoth dung dating back to 20,000 years ago. The researchers believe this is the most ancient human fecal matter ever to be analyzed.
They are now applying for a grant with the hopes that the technique can be extended to sediments harvested from other, even older sites.
More science coverage:
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Carolyn Y. Johnson can be reached at cjohnson@globe.com. Follow her on Twitter @carolynyjohnson.
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