Nitrogen-15 levels in putrefying meat could explain high levels of the isotope in hominid fossils
WASHINGTON — Kimberly Foecke has a great relationship with her local butcher.
Though she buys loads of meat, Foecke is not a chef or the owner of a small zoo. She’s a paleobiologist who studies what Neandertals ate. And that involves, in her words, “experimental putrefaction, which is a fancy way of saying, I rot meat, all day, every day.”
Scientists know Neandertals ate a lot of meat. Fossilized bones from the hominids tend to have high levels of a heavier form of nitrogen, nitrogen-15, compared with the lighter form, nitrogen-14. Nitrogen-15 is least abundant in plants, and becomes more concentrated further up the food chain because it’s harder to break down than nitrogen-14.
But exactly how much meat these hominids ate — and what else was in their diet — is somewhat controversial. Evidence such as tooth scrapings suggests that Neandertals also ate a variety of plants. But the nitrogen-15 measurements point to "an unreasonably huge amount of meat" in the diet, says Foecke, a researcher at George Washington University in Washington, D.C. Those levels tend to be even higher than what’s seen in top carnivores like hyenas, which nosh almost entirely on meat.
Grocery store steaks wouldn’t cut it for this experiment. Instead, Foecke calls her butcher in Maryland, who makes sure she receives meat that is fresh and from animals raised as close to Pleistocene-style as is possible in 2018 — after all, no hormones or antibiotics were fed to animals hunted 200,000 years ago. She needs animals raised on organic diets she can sample.
Foecke leaves the steaks to rot for 16 days in a mesh-covered box in her family’s backyard, or sometimes in a greenhouse, and samples nitrogen values daily. She plans future sampling for longer periods.
Her preliminary results suggest that nitrogen-15 ratios do fluctuate as meat rots. In the first week, levels increase. The meat is moist, and there’s lots of microbial activity that breaks down the lighter nitrogen-14 faster than the nitrogen-15, Foecke reported December 14 at the American Geophysical Union meeting. It smells “pretty terrible,” she says — though over time, the stench diminishes as the meat blackens and takes on a more jerkylike consistency.
Foecke’s research so far suggests that eating rotting meat could at least partly explain the high nitrogen-15 signatures in Neandertal fossils. And it makes sense that Neandertals weren’t feasting on fresh grub, particularly when they killed large animals. A carcass from a large animal might last days. Foecke is also measuring what happens biochemically as she cooks or smokes meat — food prep steps that Neandertals might have taken that could also affect nitrogen-15.
K. Foecke, A. Brooks and C. France. Alternatives to Neanderthal hypercarnivory - Experimental study of the δ15N values of meat during putrefaction. American Geophysical Union meeting, December 14, 2018. Washington, D.C.
E. Engelhaupt. Neanderthals reveal their diet with oldest excrement. Science News Online, June 25, 2014.