The only clue I know of in the original Frank Herbert-written books is in Appendix II, the religion appendix in Dune:
Mankind's movement through deep space placed a unique stamp on religion during the one hundred and ten centuries that preceded the Butlerian Jihad. To begin with, early space travel, although widespread, was largely unregulated, slow, and uncertain, and, before the Guild monopoly, was accomplished by a hodgepodge of methods. The first space experiences, poorly communicated and subject to extreme distortion, were a wild inducement to mystical speculation.
So there doesn't appear to be any exact canon answer. But the idea appears to be that early space travellers experimented with FTL drives, wormholes, cryogenic colonization ships, and whatever else they could manage before the invention of the Foldspace drive.