Stone Age Seafood-Based Diet Was Full Of Toxic Metals

Cod fishes floating in aquarium, Alesund, Norway.

Modern paleo-diets, characterized by avoiding agricultural products and eating things like meat, fish, and naturally occurring seeds and fruits, gained quite some popularity in the last years. But a new study suggests that the original paleo-diet, some 9,500 years ago at the end of the last ice age, was downright toxic.

In 2015, researchers reported that cod caught off the North American coast around 6,500 years ago by Stone Age hunter-gatherers contained more than 20 times the levels of mercury recommended for humans today. To find out whether this problem was more widespread, a team of archaeologists studied the feeding habits of human societies living near the sea. The researchers selected eight archaeological sites from the Norwegian Arctic, spanning 6,300 to 3,800 years in age. They analyzed the chemical composition of bones of animals, like Atlantic cod and harp seals, disposed of in ancient garbage pits, and so preserved to this day. Both species were among the main ingredients in the diet of the local people, even if the early hunter-gatherers, based on cut marks found on the bones, also successfully hunted for haddock, whale, dolphin, reindeer and beaver.

The analyzed 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 today food safety authorities consider safe. Cadmium can cause kidney, liver, and lung diseases, while lead attacks the nervous system. The studied seal bones contained up to 15 times the recommended levels of cadmium, up to four times the recommended levels of lead, and high levels of mercury. Mercury can cause kidney problems and also damages the nervous system.

Mercury, cadmium and lead, like other potentially toxic metals, occur naturally in rocks and water. At the beginning of the last ice-age large quantities of water became trapped as ice on the poles or inland ice-shields. During the Last Glacial Maximum, about 20,000 years ago, sea levels were more than 900 feet (300 meters) lower, exposing large parts of the shelf areas of the continents Weathering and erosion caused rocks to break down, releasing heavy metals into the soil. At the end of the last ice-age, between about 14,000 and 6,000 years ago, the sea levels rebounded. The previously exposed areas were inundated and the metals became dissolved in the seawater. Over time the heavy metals will accumulate in the marine food chain, with humans acting as apex predator and getting the highest potentially toxic levels.

According to the study, the level of heavy metals in the seafood was unsafe if compared to modern food safety regulations; however, it is unclear how the pollutants affected prehistoric people. The researchers note that the hunter-gatherer likely had a more varied diet, even if fish and seal dominate the garbage pits, mitigating the possible toxic effects of the heavy metals. With life expectancy being around 30 to 40 years in early societies, people also may not have lived long enough to feel the full effects of the accumulating pollutants. In the next step, the researchers will try to analyze the chemical composition of human bones, to determine if the levels of toxic metals correlate with the levels found in the consumed seafood.