Flowering plants (or angiosperms) are the dominant group of plants today, but newcomers compared to others. The earliest, clear evidence of angiosperms is from the Cretaceous (about 100 million years ago). Classifying the earliest angiosperm fossils is difficult because they tend to be leaves and pollen, rather than flowers that would permit conclusive identification. Competing theories explain angiosperm origins: that they lived in disturbed areas along stream corridors from which they invaded lowland habitats; that they began as understory plants in dark forests; that they originated in coastal areas and moved inland; or that they started as aquatic plants. Questions about angiosperm origins led Charles Darwin to describe their origins as an abominable mystery. After they appeared on the scene, angiosperms gradually and then rapidly replaced conifers and seed ferns in ecosystems. Advantages may have been faster reproductive cycles, their intimate relationship with insects for pollination, large photosynthetic leaves, and improved systems to transport water and nutrients. Which combination of characteristics allowed angiosperms to become so successful is a question of continuing debate for paleobotanists.

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.