The genus Cannabis is part of the Cannabaceae, a small family of flowering plants with 10 genera and some 120 species (Jin et al., 2020; Yang et al., 2013). The Cannabaceae have been estimated to have originated ca. 70 to 90 million years ago, and are distributed in temperate and tropical regions throughout the world (Figure 3) (Jin et al., 2020; Magallón et al., 2015). Most species of the Cannabaceae are trees or shrubs, Cannabis as a herb is, therefore, the exception rather than the rule in the family. However, a trait Cannabis shares with many other species in the family is the inconspicuous unisexual flowers (Yang et al., 2013).
The closest relative of Cannabis is the genus Humulus(Yang et al., 2013), which consists of three species, among which Humulus lupulus(hop) is economically important for the beer brewing industry. Both hop and Cannabis produce separate male and female flowers, and the trichomes in the female inflorescences are the site of secondary compound production that make both of those plants economically valuable (Page and Nagel, 2006).
Within the angiosperm phylogeny, Cannabaceae are most closely related to the Moraceae (mulberry or fig family) and Urticaceae (nettle family). Together with the Ulmaceae (elms and relatives) they form a group known as the urticalean rosids (Figure 3) (Sytsma et al., 2002). It is interesting to note that unisexual flowers appear to be prevalent in the urticalean rosids, whereas bisexual flowers are by far the dominant system in angiosperms in general (Renner, 2014; Sytsma et al., 2002). The evolution of sex expression and sex determination in this group is an interesting area of future research.