The story begins when a comet crashes to the Earth, raining down a strange green dust in its wake. The space dust is harmless to humans and goes unnoticed, but they failed to notice that flora, fauna and fungi including mushrooms, cacti, and kudzu gained sentience. Also, this strange green dust mutated some of the animals too (such as rabbits, moles, spiders, opossums, etc.). In order to survive, the newly conscious mushroom people soon formed into tribes, and war inevitably followed. The story of The Spore Wars on Wii also chronicles the life of the main character Pax, a lone bolete mushroom who is trying to find his place in mushroom society.
Source: Wikipedia, "Mushroom Men," available under the CC-BY-SA License.