A million years of marijuana: How cannabis became our favourite weed

It’s the kind of question that might be raised in a smoky room, as the asker flicks back the wheel on a lighter, about to hold the flame to a joint: Where did weed even come from? Not the marijuana dispensary down the street or that guy your cousin goes to – but long, long before that?  

Pinning down precisely when cannabis evolved is tricky business because the plant’s iconic leaves …