The fairy-tale collector who worked out how Germanic languages evolved

Jacob Grimm — yes, of Grimm's Fairy Tales — built on Rasmus Rask's earlier findings and in 1822 systematised a wholesale pattern of consonant shifts separating the Germanic languages from their Indo-European relatives, like father/Vater and foot/Fuß. Grimm's Law showed sound change is regular and predictable rather than random drift, laying the groundwork for reconstructing lost ancestor languages.

— Jacob Grimm, Deutsche Grammatik — 2nd edition, 1822 — public domain