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.

The game is rather eclectic; there is a display panel for the mix of atmospheric gases, with percentages to three decimal places, and a menu of natural disasters from hurricane to plague.

An interesting feature of the game is that most phyla of multicellular organisms are on an equal footing, and thus it is possible to evolve intelligent molluscs and the like. In addition to the familiar types, the long-extinct "trichordates" are included, because "we felt sorry for them, and are giving them a chance for survival in SimEarth". (A small number of mysterious fossils seem to exhibit trilateral symmetry, but this is controversial; see Vendian faunas, in particular Tribrachidium.)

Despite the humor, both in game and manual, game play itself is somewhat mystifying; species thrive or die out for no apparent reason, and it is easy to get the world into an unfixable state. No doubt this explains some of the lack of popularity, since players prefer to feel in control of events rather than condemned to fail by realistic science.