Ferns are the second most diverse plant group on Earth today, following only the flowering plants in number of species. The origin of ferns is not well understood, though. Weedy, fern-like plants appear in the latest Devonian, specialized to colonize disturbed sites such as volcanic landscapes. The oldest, definite fern fossils are from the early Carboniferous (about 350 million years ago). By the middle Carboniferous, all major fern growth forms were present: trees, vines, and ground cover, mostly specialized to exploit disturbances. Among these plants, however, were tree ferns up to 10 meters (32 feet) tall that lived in swampy wetlands, and whose remains fossilized into coal. Many of these early groups of ferns went extinct in the Paleozoic but were eventually replaced by new forms, many of which have survived to the present day. A huge diversification of ferns happened during the Cretaceous (150 million years ago,) and new kinds of ferns have continued to evolve into modern times.

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.