I just took a brief glance at some Wikipedia entries and find it treated as historical fact that the Septuagint existed, also that Philo referred to it in the century before Christ and that Jerome used it in his translation of the Vulgate Bible. So that is more than just Origen given as source of information, Jerry.

A comparison of a verse in Genesis is given that does show an amazing divergence between the Septuagint and the Masoretic text. Obviously there's a lot of stuff going on here I'll never have the time to slog through.