You've heard the PSA: Recycle that plastic water bottle, or else archaeologists will be digging it up thousands of years from now. What you probably haven't heard is that archaeologists are already digging up plastic water bottles that are thousands of years old.

This is not evidence of time travel. These bottles aren't clear, and they don't have labels. They're pitch black—made by indigenous tribes who coated large, woven bulbs with a tar-like substance called bitumen. Scientists have known about these bottles for years. But what they hadn't considered was whether these plastic bottles contributed to the declining health in some old societies, like the Native American tribes that once lived off the coast of California. Skeletons dating back thousands of years evidence a mysterious physical decline. A new study, published today in the journal Environmental Health, measured the toxicity of making plastic from oily bitumen, and of storing liquid in the bottles.

Modern water bottles aren't that different, really. But frozen, reused, even microwaved, there's not much risk of the liquid in them leaching enough harmful molecules—BPAs, DEHA, PET—to cause health problems. These ancient plastics are a different story, however. Bitumen is basically asphalt. Yes, basically the same stuff (when mixed with rocks, sand, and aggregates) that is used to pave roads. It's dense, viscous or semi-solid when cool, but turns into a malleable slop when heated up. It also releases chemicals known as polycyclic aromatic hydrocarbons, or PAHs, known to cause cancer (cigarettes, burning wood, and other smoky sources produce PAHs) and other health problems.

California's Channel Islands sit a few miles off the coast of Los Angeles. Like the mainland, they are dry. "They are also one of few places in North America where you find a more or less continuous population in the Americas, at least until the Industrial Age," says Sabrina Sholts, an anthropologist at the Smithsonian’s National Museum of Natural History in Washington, DC. "The earliest evidence we have of of people on these islands comes from about 13,000 years ago." One of the outstanding mysteries about these island-dwelling tribes—collectively called the Chumash—is why their overall health began to decline, beginning around 5,000 years ago.

Skeletal remains dating back to that time start to exhibit poor bone quality, reduced stature, smaller skulls, and bad teeth. Now, lots of things can cause these issues. Some researchers posit that malnutrition, poor sanitation, infectious disease, and lack of resources brought about by increased population on the islands might be the culprits. But Sholts developed a different hypothesis.

On certain beaches in Southern California, you have to watch your step to avoid stumbling on a nasty little ball of tar. Some come from the oil rigs offshore, but these balls have actually been washing ashore for thousands of years, the result of submarine seepage. This is bitumen, and for thousands of years Native Americans in this region had used it to build boats, make weapons, and craft water bottles. Sholts went to graduate school at UC Santa Barbara, and she recalls that one of her colleagues working with bitumen advised her early on to wear gloves and a mask to handle the stuff. She had recently learned about how Native Americans in this region had stored water in bitumen bottles. "I became uncomfortably curious, and not sure how strongly I should consider this as a factor in some of the changes I had seen in the skeletal record," she says.

After getting her PhD and a job at the Smithsonian, she took the opportunity to explore that curiosity. She conscripted a colleague, Kevin Smith, to recreate the bottle-making process. Smith is an archaeologist at UC Davis who has permits to do work on the Channel Islands—much of which are protected.