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.

18.1. Complex Multicellularity

The algae are structurally simple multicellular organisms that fill the evolutionary gap between unicellular protists and more complex multicellular organisms (fungi, plants, and animals). In complex multicellular organisms, individuals are composed of many highly specialized kinds of cells that coordinate their activities. There are three kingdoms that exhibit complex multicellularity:

1. Plants. Multicellular green algae were almost certainly the direct ancestors of the plants (see chapter 17) and were themselves considered plants in the 19th century. However, green algae are basically aquatic and much simpler in structure than plants and are considered protists in the six-kingdom system used widely today.

2. Animals. Animals arose from a unicellular protist ancestor. The simplest (and seemingly most primitive) animals today, the sponges, seem clearly to have evolved from a kind of flagellate.

3. Fungi. Fungi also arose from a unicellular protist ancestor, one different from the ancestor of animals. Certain protists, including slime molds and water molds, have been considered fungi (“molds”), although they are usually classified as protists and are not thought to resemble ancestors of fungi. The true protist ancestor of fungi is as yet unknown. This is one of the great unsolved problems of taxonomy.

Perhaps the most important characteristic of complex multicellular organisms is cell specialization. If you think about it, having a variety of different sorts of cells within the same individual implies something very important about the genes of the individual: Different cells are using different genes! The process whereby a single cell (in humans, a fertilized egg) becomes a multicellular individual with many different kinds of cells is called development. The cell specialization that is the hallmark of complex multicellular life is the direct result of cells developing in different ways by activating different genes.

A second key characteristic of complex multicellular organisms is intercellular coordination, the adjustment of a cell’s activity in response to what other cells are doing. The cells of all complex multicellular organisms communicate with one another with chemical signals called hormones. In some organisms like sponges, there is relatively little coordination between the cells; in other organisms like humans, almost every cell is under complex coordination.

Key Learning Outcome 18.1. Fungi, plants, and animals are complexly multicellular, with specialized cell types and coordination between cells.