Stories evolve. As they are told and retold to new audiences, they accumulate changes in plot, characters, and settings. They behave a lot like living organisms, which build up mutations in the genes that they pass to successive generations.
This is more than a metaphor. It means that scientists can reconstruct the relationships between versions of a story using the same tools that evolutionary biologists use to study species. They can compare different versions of the same tale and draw family trees—phylogenies—that unite them. They can even reconstruct the last common ancestor of a group of stories.
In 2013, Jamie Tehrani from Durham University did this for Little Red Riding Hood, charting the relationships between 58 different versions of the tale. In some, a huntsman rescues the girl; in others, she does it herself. But all these iterations could be traced back to a single origin, 2,000 years ago, somewhere between Europe and the Middle East. And East Asian versions (with several girls, and a tiger or leopard in lieu of wolf) probably derived from these European ancestors.
That project stoked Tehrani's interest, and so he teamed up with Sara Graça da Silva, who studies intersections between evolution and literature, to piece together the origins of a wider corpus of folktales. The duo relied on the Aarne Thompson Uther Index—an immense catalogue that classifies folktales into over 2,000 tiered categories. (For example, Tales of Magic (300-749) contains Supernatural Adversaries (300-399), which contains Little Red Riding Hood (333), Rapunzel (310), and more amusing titles like Godfather Death (332) and Magnet Mountain Attracts Everything (322).