Seed plants of today's world consist of two major groups, the “gymnosperms” and the angiosperms. Both evolved from a common ancestral group, the progymnosperms, during the Late Devonian period 365 million years ago. There are five phyla of seed plants with living representatives—four gymnosperms plus a single phylum of angiosperms, the Anthophyta. The fossils in several extinct phyla show evolutionary steps in the development of seeds and ultimately, the flowering plants.
The seed of the seed plants evolved from adaptations and changes to the megagametophyte, a reproductive structure common to all heterosporous plants (including the seed plants). The changes resulted first in the elimination of external water as the medium in which sperms reach the eggs, and secondly, produced a small, easily transportable package—the seed—to distribute the new sporophyte. Seeds give plants a tremendous advantage to spread and colonize new land.