It's no surprise that humans pollute their waterways, but now it appears we've been doing it a lot longer than thought. In a study published this month in Science of the Total Environment, researchers say they've found evidence of a polluted river from 7,000 years ago. Sediment from the river—now a dry riverbed—in Jordan shows a high level of copper pollution, according to a press release. And there aren't really any explanations for it except early human experimentation with metallurgy. "These populations were experimenting with fire, experimenting with pottery, and experimenting with copper ores," anthropologist Russell Adams says.
Researchers were looking for clues on how humans first started transitioning from stone tools to metal ones, Motherboard reports. And while the pollution in the river came before the widespread use of metallurgy, it may have been a precursor to it. It's probable people living near the river were combining charcoal and copper ore then heating it to create beads and other small items. The smelting of copper can produce waste ranging from lead and zinc to arsenic and mercury, which likely led to widespread health problems during the later Bronze Age. (The "Great Pacific Garbage Patch" is worse than we thought.)