“The Hazy Evolution of Cannabis” is an episode from the PBS Eons series that uncovers the ancient, deep-time roots of the cannabis plant.
Because fossilized leaves are exceptionally rare, the episode highlights how scientists turned to DNA sequencing and fossil pollen analysis to track its lineage. The data reveals that the genus diverged from its closest relative, Humulus (the hops used to brew beer), roughly 28 million years ago on the Tibetan Plateau. This evolutionary shift was largely driven by plate tectonics; when the collision of India and Asia formed the plateau, it created the vast, arid steppe environments where early cannabis variants flourished.
The documentary then shifts from geographical origins to the plant’s long relationship with humanity. Genome mapping indicates that ancestral foragers first domesticated cannabis around 12,000 years ago in East Asia. Initially utilized as a versatile multi-purpose crop for its oily seeds and tough fibers, a major genetic split occurred roughly 4,000 years ago. Humans began practicing artificial selection, intentionally separating the lineages into two distinct variants: tall, fibrous hemp for tools and textiles, and short, highly resinous, flower-rich varieties bred specifically for their psychoactive properties.
Finally, the episode tackles the biological mystery of why cannabis gets humans high. It details how the plant evolved microscopic, mushroom-like structures called glandular trichomes to survive intense ultraviolet radiation and ward off mountain herbivores. These trichomes produce a sticky resin packed with cannabinoids like THC. By evolutionary coincidence, these chemical compounds are capable of hacking the ancient endocannabinoid system—a biological receptor network shared by nearly all animals—which explains why the plant has such a profound, psychoactive impact on the human body and brain.