Recreating bottles made by Native California Indians 5,000 years ago, scientists have found they may have been inadvertently harming their health in the process.

This study, published in Environmental Health and conducted by the Smithsonian Institution’s National Museum of History, looked at a type of water bottle favored by ancient Indians made from bitumen. This is a material derived from petroleum, essentially tar washed up on beaches, which was collected, traded, and used by Native Americans.

They would store water in bottles made from bitumen to cope with periods of drought. It may also have been used to fix arrowheads and even produce smoke signals.

To make the bottles and other objects, the bitumen would be heated up, allowing it to be molded into particular shapes and used as a sealant. However, doing so would have released polycyclic aromatic hydrocarbons (PAHs) inside the bitumen.

PAHs are today linked with a number of health problems, including cancer, hormone imbalance, organ damage, and developmental impairments. We’re exposed to them by the burning of fossil fuels and tobacco smoke.

A bottle recreated by the researchers. Sabrina Sholts, Smithsonian Institution

The people using these bottles lived on California’s Channel Islands, a few miles off the coast off Los Angeles. Known as the Chumash, their health mysteriously began to decline about 5,000 years ago, despite having lived on the islands for at least 8,000 years.

According to WIRED, skeletal remains from the time show that people experienced poorer bone quality, smaller skulls, and bad teeth. Bitumen could therefore be the cause, something that’s not been explored before.

"To our knowledge, this is the first time that experimental archaeology has been used to quantify PAH exposure in an ancient population,” Dr Sabrina Sholts, who led the study, said in a statement. “We believe that our research provides a deeper temporal context for understanding health impacts and evolutionary changes related to harmful chemical exposure caused by human technology."

The researchers remade the bottles using two methods, one with soft bitumen or “malak” – found washed ashore – and another with hard bitumen or “woqo”, found in land deposits. The air was sampled as they made the bottles, using gas chromatography and mass spectrometery to measure levels of PAHs.

The results showed that the levels were equal to or greater than that found in cigarette smoke. While the water stored in the bottles after the manufacturing process was generally safe to drink, olive oil quickly became contaminated.

More research will of course need to be done, but it looks like Native Americans were unknowingly causing themselves considerable harm.