Plants, descended from aquatic green algal ancestors, first appeared on land more than 450 million years ago during or prior to the Ordovician period. This event preceded the colonization of land by four-footed animals (tetrapods), which occurred considerably later in the Devonian period (408 to 360 million years ago). Understanding the origin of plants is important because early plants were essential to the development of favorable terrestrial environments and provided a source of food for animals. In addition, the earliest plants were ancestral to all of the food, fiber, and medicinal plants upon which modern humans depend. The hominid lineage, leading to modern humans, is only about 4 million years old; most modern plant community types are considerably older.

Ancient, microscopic fossils and deoxyribonucleic acid (DNA) evidence indicate that the earliest land plants resembled modern bryophytes, the liverworts, hornworts, and mosses. Bryophytes are smaller and simpler than other...