[Wikipedia: http://en.wikipedia.org/wiki/Sim_earth]
SimEarth: The Living Planet is a life simulation computer game designed by Will Wright and published in 1990 by Maxis, in which the player controls the development of a planet. Versions were made for the Apple Macintosh, Commodore Amiga, IBM PC, SNES and TurboGrafx-16, which was also subsequently re-released on the Wii Virtual Console.
In SimEarth, the player can vary a planet's atmosphere, temperature, landmasses, etc, then place various forms of life on the planet and watch them evolve. Since it is a software toy, the game does not have any required goals. The big (and difficult) challenge is to evolve sentient life and an advanced civilization. The development stages of the planet can be reverted and repeated, until the planet "dies" 10 billion years after its creation, the estimated time when the Sun will become a red giant and kill off all of the planet's life.
The game models the Gaia hypothesis of James Lovelock (who assisted with the design and wrote an introduction to the manual), and one of the options available to the player is the simplified "Daisyworld" model.