According to the online etymology sources, the terms "eventual" and "eventually" were in use in the early 1600s and held its current meaning by the mid 1800s. The etymologies point to French éventuel, but both the French word and German eventuell have very different meanings. (See this Q&A).
How did these meanings diverge? Did the French term evolve from an earlier Latin term? Did the English usage change from the French? Did they both change independent of each other?