Fairy tales are transmitted through language, and the shoots and branches of the Indo-European language tree are well-defined, so the scientists could trace a tale’s history back up the tree—and thus back in time. If both Slavic languages and Celtic languages had a version of Jack and the Beanstalk (and the analysis revealed they might), for example, chances are the story can be traced back to the “last common ancestor.” That would be the Proto-Western-Indo-Europeans from whom both lineages split at least 6800 years ago (see image). The approach mirrors how an evolutionary biologist might conclude that two species came from a common ancestor if their genes both contain the same mutation not found in other modern animals. But it’s not quite so simple. —Science | AAAS