Thousands of years ago, indigenous groups living on the California Channel Islands made leak-proof water bottles by weaving rush plants together and coating them with bitumen, a type of raw petroleum that turns sticky when melted. Researchers at the University of California recently set out to recreate this ancient technique—and found the bottle-making process exposed people of the Channel Islands to the toxic chemicals that today are associated with fossil fuel burning and cigarette smoke.
Detailing their results in the journal Environmental Health, researchers explain the chemical exposure came from the bitumen, which washes up to the Channel Islands from underwater seeps. The ubiquitous substance appeared among many ancient groups. For some 70,000 years, people in California, Mexico and the Near East made regular use of bitumen: chewing it, painting their skin, sealing watercraft and making water bottles with it.
As Mira Abed reports for the Los Angeles Times, the team suspected bitumen water bottles might have been harmful to ancient people's health because petroleum releases toxic pollutants called polycyclic aromatic hydrocarbons, or PAHs. This class of chemicals is associated with burning coal, oil, gasoline, wood and tobacco. The Environmental Protection Agency labels 16 PAHs as priority pollutants; exposure to these chemicals can cause cancer, among other health issues.
Consequently, researchers wanted to see if PAH exposure might provide an explanation for the mysterious decline in the health of the Channel Islands indigenous groups known collectively as the Chumash. Archaeologists have observed skeletal lesions, poor dental health and decreasing head size among the remains of the Chumash, starting about 5,000 years ago—but aren't sure what caused the shift.
“We weren't looking to finger PAH exposure as the cause of any of this, but we knew that it has been attributed to this kind of effect in modern populations,” says Dr. Sabrina Sholts, an anthropologist at Smithsonian Institution and one of the authors of the study. “We just wanted to know if we should be ruling this out as another factor in that health decline.”
Sholts and her team set out to mimic ancient Chumash methods of manufacturing water bottles, using archaeological evidence and ethnographic records. Perhaps the most famous of these records describes the discovery of Juana Maria, a Native American woman who lived alone on an abandoned Channel island for nearly 20 years. When an expedition found her, according to Sholts, she was coating a basket with melted bitumen.
Guided by evidence from the past, researchers used a stone flake and a bird bone awl to weave rushes into bottles. Sholts’s co-author Kevin Smith then placed bitumen in an abalone shell, melted it with heated rocks, and spread it over the rushes with a sea mammal bone.
While the bitumen melted, researchers used mass spectrometry to measure pollutants released into the air. Once the containers were done, the team filled one with water and another with olive oil, taking samples every few weeks to see if chemicals were seeping out. (The Chumash didn’t have access to olive oil, but may have used bitumen-coated vessels to store oily substances like fish.)
After two months, researchers found eight PAHs in the water and all 16 high-priority PAHs in the oil. The concentrations of the chemicals were much higher in the oil than in the water—PAHs are lipophilic—but in neither case were the concentrations high enough to pose a health risk.
The levels of PAHs found in the air above the melting bitumen, however, did exceed the safety limits imposed by the EPA. Anyone standing over the smoke would have inhaled PAH concentrations “somewhat higher than those in cigarette smoke,” the authors of the study write.
Does that mean PAH exposure caused the decline of the Channel Islands indigenous groups? “It’s not a simple answer,” Sholts says. People who melted bitumen may have been at risk, but as Nick Stockton points out in Wired, it isn’t clear if they melted the substance frequently enough to cause significant damage. And the study only looked at the risks of adult exposure; young children are highly susceptible to PAHs.
“You have a much more sensitive window of development and growth in that early life period,” Sholts explains. “One thing we could do and may do [in the future] is to think about how we could look at early life exposure levels.”
For now, the concentrations of PAHs observed in the study cannot be counted as a major factor in the decline of the Channel Islands groups. But as Sholts points out, “It's very cool that we can say that.”
Experts have long been interested in the effect of toxins on ancient peoples; many have looked into lead poisoning among ancient Romans. Lead and other metals are stored in the skeleton, making it relatively easy for archaeologists to measure them. But organic pollutants like PAHs, many of which are metabolized and eliminated from the body shortly after exposure, are more difficult to trace among human remains.
“To be able to look at a modern health problem like PAH exposure—which is the same thing we're talking about when we talk about polluted air, and asphalt, and cigarette smoke, and other factors in our modern environment—[and] recognize a parallel in the past I think is interesting,” she explains. “It helps us to understand human health now, within this much longer story of human health and evolution.”