Race for the Galaxy's development began when Thomas Lehmann started work on a card game version of Puerto Rico in parallel with Andreas Seyfarth's own. Their independent efforts resulted in some degree of collaboration, and though some of Mr. Lehmann's design ideas did not make it into what eventually became San Juan, he continued to develop his concepts, exchanging the theme and incorporating elements from an unpublished customizable card game he had previously designed. For a more detailed narrative from the designer, himself, please see these three articles at Boardgamenews.com:
Race for the Galaxy benefits from an unusually thorough development process, involving thousands of test games and the deliberate design of an unusually coherent iconography (by onigame and the graphics designer, Mirko Suzuki) as well as a unique approach to player interaction that prevents direct conflict and encourages "parasitic conflict".
Two yet to be released expansions adding new cards, concepts, and starting planets, and supporting up to 6 players, are already completed.
Some of the significant differences between San Juan and Race for the Galaxy include:
ROLE SELECTION
San Juan: Players pick roles one at a time in turn order. A role that is picked is unavailable to be picked by other players. Each role has exactly one advantage for the one who picked it.
RftG: Players pick roles secretly and simultaneously. Multiple players can possibly pick the same role. Some roles have two advantages of which the player who picked the role must choose one.
THEME & GAMEPLAY
San Juan: Tightly linked to Puerto Rico creating a gameplay experience that feels like a lighter, faster version of the boardgame.
RftG: Space theme that is completely divorced from its Puerto Rico roots allowing for some divergence from the original's gameplay.
6-COST CARDS
San Juan: Their only function is to generate end-of-game victory point bonuses in exactly the same spirit as the "big buildings" of Puerto Rico
RftG: Function as end-of-game victory point bonuses but also provide benefits and advantages during gameplay
EXPANDABILITY & PLAYERS
San Juan: As of now there are no expansions announced and number of players is limited to 2-4
RftG: Two expansions already designed and planned to be released in the future. The base game is 2-4 players but the eventual expansions will increase that to 1-6 players.
The 1st expansion, Race for the Galaxy: The Gathering Storm, is tentatively scheduled by RGG for Q2 2008.
Race for the Galaxy microbadges: 
