Algae have a long fossil record on Earth, dating to the Precambrian (more than 550 million years ago). Fossil cells from an 850-million-year-old Australian site may even be from algae. Being soft and fragile, algae do not preserve well. Also, the biochemical characteristics used to identify algae are usually absent in fossils. Many algae fossils are just shape outlines left on rocks. Some algae leave a record of their activities, such as those which precipitate calcium carbonate, leaving deposits behind. When algae came onto land, they started all terrestrial life. Paleobotanists point to a group of simple green algae called Charophyceae as the likely ancestors to all green plants. Sheets of plant cells have been found at Ordovican sites (about 450 million years old). Once plants became terrestrial, the way was paved for animals to live on land as well.

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.