Cannabis appears to have been domesticated at multiple times and sites across Eurasia, as suggested by Nikolai Vavilov back in the 1920s.
Among the earliest evidence of for cultivation is domesticated pollen from sites on the Western Steppe, most of which are associated with the Yamnaya culture (3300–2600 BCE). Recent genetic studies indicate the Yamnaya are the most likely candidates for the Proto-Indo-Europeans. This supports the “Kurgan hypothesis” of Marijia Gimbutas.
Here’s an authoritative lecture on the Indo-Europeans by ace archaeologist, Colin Renfrew.
The photograph shows a sculptural reconstruction of the face of a Yamnaya man.