Life evolved in the sea, and for more than a billion years was confined to the oceans of the earth. On land, there was only bare rock. This bleak terrestrial picture changed over half a billion years ago, when the first fungi are thought to have invaded the land. The difficulty of the challenge this first invasion posed cannot be overstated. Animals could not be the first to invade. Animals are heterotrophs-what would they eat? Fungi, also heterotrophs, faced the same problem. Algae are photosynthetic, and so food did not present such a challenge for their invasion of land. Sunlight would be able to provide all the energy they might need. But how would they obtain nutrients? Phosphorus, nitrogen, iron, and many other chemical elements critical to life cannot be obtained by algae from bare rock. The solution to this dilemma was a sort of “You scratch my back, I’ll scratch yours” cooperation. Fungi called ascomycetes formed associations with photosynthetic algae, forming a two-member partnership called a lichen. The lichen you see in this photo is growing on bare rock. The algae within it harvest energy from sunlight, while the fungal cells derive minerals from the rock. In this chapter you will become better acquainted with fungi and then explore their partnerships with algae and plants.