Relatives of the club mosses were among the first vascular plants (with specialized water conducting cells). Dating to at least the early Devonian (about 400 million years ago), club mosses dominated many ancient landscapes, particularly swamps and other wetlands. Tree-form (arborescent) club mosses grew fast and reached heights of 30 meters (100 feet). Their crowns leafed out last, most of their lives being spent as poles covered with leaves. The formation of the crown and cone-like reproductive organs signaled the end of the plant's life. Root systems of arborescent club mosses were long and shallow, extending out tens of feet from the trunk. They frequently fossilized, thanks to their hollow centers filling with sediment, acting as a 3-dimensional mold. Warming and drying pulses from the later Carboniferous through the Jurassic caused a series of extinctions, eliminating arborescent club mosses, but leaving smaller, herbaceous ones like what we see today. The coal we use is largely made up of giant club moss remains.

Plants on Earth began life in the water. The earliest plants were aquatic algae, living in the warm seas more than 500 million years ago. By the Silurian and possibly the latest Ordovician (as much as 440 million years ago), plant life on land had begun. While terrestrial plants had more access to sunlight, challenges included supporting their weight and distributing water throughout their tissues. During the Devonian explosion (55 million years, which is brief in geologic time), plants evolved from small, simple forms to a huge variety of larger, complex forms adapted to life on land. Club mosses, horsetails, ferns, and probably sphenopsids originated. The incredible seed, which safely packages the developing plant embryo, also evolved. Plants extended their reach underground with root systems, providing the anchor and water supply to support large trees. Although plants were to undergo many changes over time, by the end of the Devonian (the golden age of plant evolution), the groundwork had been laid for the terrestrial ecosystems of today.