In the Dune series Arrakis is said to be the only source of the spice, and the spice is the only reason people would go to Arrakis- so one wonders where someone got the idea to go to a planet regarded as a giant catbox in space and consume some sort of substance there and by doing so find out how it extends life, expands consciousness et al?
We never learn how the spice was discovered in Frank Herbert's original Dune series, beyond the fact that it happened thousands of years prior to the ascent of House Atreides.
In The Machine Crusade by Brian Herbert and Kevin J. Anderson, we learn that spice was discovered by ancestors of the Fremen (the Zensunni), who made it part of their culture and religion (as we see it is in Dune). Later, it was discovered by the Tlulaxa Tuk Keedair, who worked in the Zensunni slave trade. Together with Aurelius Venport, he became a spicing entrepeneur and formed VenKee Enterprises, which eventually became the basis of the Spacing Guild.
We also learn how and why Arrakis was settled in the first place in Brian and Kevin's prequel novels. The Fremen are the descendants of several diverse groups of Buddaislamics, including the Zensunni, who refused to fight the thinking machines.