Accurately determining when and where cannabis evolved was extremely difficult due to the lack of a strong print fossil record – impression of leaves or fruits in rocks. For a plant, like cannabis, that lacks a good fossil record, paleobotanists can use a “molecular clock”. This allows them to estimate when cannabis and its sister species Humulus (hops) diverged from a common ancestor. The molecular clock uses DNA to measure time, and calibrates the clock with fossil dates of related plants.
Using this method, they estimated that cannabis first diverged from a common ancestor 27.8 million years ago.