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Historians are well aware of the fact that humans have been using cannabis seeds, fibers, and flower for millennia, but the actual origins of the cannabis plant itself have remained a mystery. That mystery may now be solved, thanks to a new study by researchers from the University of Vermont.

Researchers have traditionally struggled to accurately identify cannabis pollen in archaeological samples, as it closely resembles the pollen of the hop, an essential mainstay of beer production. But because cannabis naturally grows on grassy steppes, and hops generally grow in woodlands, the research team realized they might be able to accurately separate the two kinds of pollen. John McPartland and his colleagues realized that cannabis pollen would normally be found mixed with the pollen of other steppe plants, while hop pollen is usually mixed with tree pollen.

By using this rationale to separate the two kinds of pollen, researchers found that the earliest known appearance of cannabis pollen in the geological record occurred in northern China and southern Russia. By analyzing the distribution of this pollen, the researchers concluded that cannabis may have originated from the Tibetan Plateau, near Qinghai Lake, nearly two miles above sea level. Weed may have been growing on this plateau as much as 28 million years ago, which is the last time biologists estimate that cannabis and hops had a common ancestor.

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The new study, published last week in the Vegetation History and Archaeobotany journal, provides a possible solution to an old mystery. But Robert Clarke of BioAgronomics Group Consultants in Los Angeles believes that the study may have its limitations. Clarke told New Scientist that while “most people agree that cannabis came from somewhere in central Asia,” the study fails to account for areas where tree and cannabis pollen can co-occur, like the banks of rivers that cross steppes.

The jury may still be out on the exact origin of the cannabis plant, but it is known that Denisovans, an extinct hominin ancestor of current humans, probably co-existed with cannabis. Qinghai Lake is within a few hundred miles of Baishiya Karst Cave, which was visited by Denisovans at least 160,000 years ago. And even if the Denisovans didn't discover cannabis there, researchers did positively identify cannabis pollen in the sand of Denisova Cave in Siberia, where fossils of this mysterious hominin were first discovered.