An artist’s illustration of Stone Age hunter-gatherers fishing.

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The real ‘paleo diet’ may have been full of toxic metals

You’ll be healthier if you ate as your ancestors did. At least that’s the promise of some modern fads such as the “caveman” or paleo diet—characterized by avoiding processed food and grains and only eating things like meat, fish, and seeds. But a new study suggests the food some early humans in Norway ate may have not only been unhealthy, but downright toxic. In some cases, these people may have consumed more than 20 times the levels of dangerous metals recommended for humans today.

“This study raises interesting ideas,” says Katheryn Twiss, an archaeologist at Stony Brook University who was not involved in the work. But, she notes, the findings are limited to a small number of animal remains from just a few sites, and therefore may not fully represent the diets of Norwegians from thousands of years ago.

Pollutants have been entering our food chain for millennia. In 2015, for example, researchers reported that cod caught off the North American coast around 6500 years ago by Stone Age hunter-gatherers contained high levels of mercury. This metal occurs naturally in Earth’s crust and is thought to have leached into the oceans in greater concentrations after sea level rise covered more land. Once in the water, fish absorb mercury through their gills and their food.

To find out whether this problem was more widespread, archaeologist Hans Peter Blankholm of the Arctic University of Norway and colleagues focused on Stone Age humans living on the shores of the Norwegian Arctic, in an area known as Varanger.

The researchers selected eight archaeological sites from the region, spanning 6300 to 3800 years ago. They did not study any human remains; instead, they analyzed the bones of dozens of Atlantic cod and harp seals found in ancient garbage pits. The majority of the cut marks on the seal bones suggest the animals were butchered for their meat, rather than simply skinned. Both species were among the main ingredients in the diet of the people who lived here, according to previous archaeological studies. The hunter-gatherers are also known to have eaten haddock, whale, dolphin, reindeer, and beaver.

The bones of the cod at these sites contained more than 20 times the maximum level of cadmium and up to four times the highest level of lead that the European Food Safety Authority considers safe in meat, the team reports in Quaternary International. Cadmium can cause kidney, liver, and lung disease, while lead can impair the brain and nervous system.

Seal bones at the sites contained up to 15 times the recommended levels of cadmium and up to four times the recommended levels of lead. The level of mercury—which can cause damage to the brain, kidneys, and immune system in humans—was also high in both animals.

As with the fish in the earlier study, the researchers believe rising sea level was responsible for the pollution in the food chain.

Blankholm calls these levels of heavy metals in seafood “unhealthy, if not unsafe.” But he says it’s unclear how much the diet of these prehistoric people would have harmed them. Balancing the seal and cod with fruit or meat from reindeer and rabbits could have blunted the effects of the heavy metals. The Varanger people also may not have lived long enough to feel the full effects of the accumulating pollutants.

The scientists may soon be able to shine a light on some of these outstanding questions: They have now acquired the remains of eight individual Stone Age humans from the Varanger region and can explore the potential effects of heavy metals on their health and lifespans. The researchers also hope to analyze additional animal remains.

All of this may help to address what Twiss sees as a key weakness of the work so far: Researchers are using only 40 bones to draw conclusions about pollution levels across various sites and some 2500 years.

Even if the cod and seal were contaminated by heavy metals, Twiss says, such meat would surely have also been a good source of protein and other key nutrients. So maybe this paleo diet wasn’t all bad after all.