Plants moving from the oceans to land probably occurred as the result of two main conditions: oxygen and changes in some photosynthetic organisms. Initially, the Earth’s atmosphere had very low concentrations of oxygen; about one billion years ago, oxygen-generating organisms—bacteria and algae—eventually produced enough oxygen not only for plants to evolve on land, but also allowed the development of the ozone layer. This layer absorbed much of the Sun’s ultraviolet rays—responsible for disrupting DNA—and thus, protected the organisms.

The changes in some of the photosynthetic organisms also contributed to the origin of land plants. In particular—and this took millions of years—some algae developed the ability to withstand short periods in which no water was available (desiccation); this probably included the development of a thicker layer to protect the cells. Eventually, these organisms would have to develop ways of moving nutrients from the ground (no longer the water) to the cells and special cells to help the plant support itself on land.