Cannabis plant evolved super high (on the Tibetan Plateau)

Cannabis may have had high origins. Where the plant comes from has been a bit of a mystery, but analysis of ancient pollen now suggests it evolved some 3 kilometres above sea level on the Tibetan Plateau. Intriguingly, this site is only a few hundred kilometres from a cave that researchers recently announced was once home to our ancient Denisovan cousins.

Humans began exploiting cannabis deep in prehistory. Its seeds are a good source of protein and fatty acids, while fibres from its stems can be spun into yarn and made into textiles. Its flowers, meanwhile, are a source of cannabinoids which have been used as a drug for at least 2700 years.

To find out where the plant evolved, John McPartland at the University of Vermont and his colleagues searched through scientific studies to pick out archaeological and geological sites across Asia where cannabis pollen has been found.

Identifying cannabis pollen isn’t easy because it looks identical to the pollen of a closely related plant called the common hop, which happens to be used for flavouring beer. But McPartland and his colleagues believe it is possible to work out which species the pollen belongs to by considering the other pollen present at an archaeological site.

This is because cannabis lives on open grassy steppes, so its pollen usually occurs with the pollen of steppe plants. The common hop, however, grows mostly in woodlands, so its pollen typically occurs with tree pollen.

When McPartland and his colleagues applied this rationale, they discovered that the earliest occurrence of cannabis pollen in the geological and archaeological record is in northern China and southern Russia. From the distribution of the pollen, the team concluded that cannabis probably emerged on the Tibetan Plateau in the vicinity of Qinghai Lake, which is about 3200 metres above sea level.

It may have evolved there about 28 million years ago, which is when biologists estimate cannabis and the common hop last shared a common ancestor.

The Tibetan Plateau may have driven the evolution of the plant. Cannabis thrives in arid, steppe-like environments and it was the formation of the Tibetan Plateau that helped promote the spread of those conditions in Asia.

The Tibetan Plateau formed because of the collision between the Indian and Asian landmasses, so in a sense we can thank plate tectonics for the evolution of cannabis, says McPartland.

“Most people agree that cannabis came from somewhere in central Asia,” says Robert Clarke, of BioAgronomics Group Consultants in Los Angeles. But he sees limitations in the approach McPartland and his colleagues used. For instance, trees may grow on the banks of rivers in steppe environments, so cannabis and tree pollen can co-occur.

Qinghai Lake lies just a few hundred kilometres northwest of Baishiya Karst Cave, which we now know was visited by Denisovans at least 160,000 years ago. But this doesn’t necessarily mean the ancient humans had the option of exploiting cannabis.

Jean-Jacques Hublin at the Max Planck Institute for Evolutionary Anthropology, Germany, who has analysed the Baishiya Karst Cave material, notes that Earth was deep in an ice age 160,000 years ago. We don’t yet know whether cannabis continued to grow on the Tibetan Plateau during such punishing conditions.

But Denisovans may have encountered the plant elsewhere. There is cannabis pollen in the sand and dirt of Denisova Cave in Siberia, where Denisovan fossils were first identified.

Journal reference: Vegetation History and Archaeobotany, DOI: 10.1007/s00334-019-00731-8