Pollen from ancient plants helped scientists to track down where Cannabis first appeared in Asia.

Cannabis may have originated high on the Tibetan Plateau, according to an analysis of fossil pollen.

While this medicinal and psychotropic plant was long thought to have first evolved in central Asia, scientists were hazy on the precise location. That's because there isn't much evidence of ancient cannabis in fossil impressions — imprints that plants leave behind in rock.

But there was abundant fossil pollen representing the Cannabis genus, scientists recently reported. However, past evaluations of fossil pollen in Asia lumped together Cannabis pollen with related plants in the Humulus genus (some of which produce hops used in beer).

For the new study, researchers separated Cannabis and Humulus pollen from 155 studies and mapped them to regions across Asia, to clarify where and when Cannabis emerged. [25 Odd Facts About Marijuana]

The scientists identified fossil pollen as belonging to Cannabis plants if it appeared alongside other types of pollen from a steppe ecosystem — open, treeless habitats where Cannabis is known to thrive. They discovered that the earliest Cannabis fossil pollen placed the genus in northwestern China, and dated to about 19.6 million years ago.

But Cannabis diverged from Humulus around 28 million years ago, suggesting that it might have originated somewhere else, the study authors wrote in the new study.

While the researchers didn't find any Cannabis pollen dating to 28 million years ago, they did find 28-million-year-old pollen from Artemisia, another genus of steppe plant that grew abundantly alongside Cannabis millions of years later. This earliest evidence of Artemisia showed up on the Tibetan Plateau near Qinghai Lake, a location about 10,700 feet (3,260 meters) above sea level.

Using a statistical model, the study authors estimated that since the assembly of plants in that location — including Artemisia — were found with Cannabis in other locations millions of years later, it was likely that Cannabis was also present in this high-altitude ecosystem, even if there was no direct evidence of Cannabis pollen, they wrote in the study.

From the Tibetan Plateau, Cannabis reached Europe approximately 6 million years ago, and spread as far as eastern China by 1.2 million years ago, the scientists reported.

The findings were published online May 14 in the journal Vegetation History and Archaeobotany.

Originally published on Live Science.