This article attempts to place key plant innovations in a geological context. It concerns itself only with novel adaptations and events that had a major ecological significance, not those that are of solely anthropological interest. The timeline displays a graphical representation of the adaptations; the text attempts to explain the nature and robustness of the evidence.

Plant evolution is an aspect of the study of biological evolution, predominantly involving evolution of plants suited to live on land, greening of various land masses by the filling of their niches with land plants, and diversification of groups of land plants.

In the strictest sense, the name plant refers to those land plants that form the clade Embryophyta, comprising the bryophytes and vascular plants. However, the clade Viridiplantae or green plants includes some other groups of photosynthetic eukaryotes, including green algae. It is widely believed that land plants evolved from a group of charophytes, most likely simple single-celled terrestrial algae similar to extant Klebsormidiophyceae.

Chloroplasts in plants evolved from an endosymbiotic relationship between a cyanobacterium, a photosynthesising prokaryote and a non-photosynthetic eukaryotic organism, producing a lineage of photosynthesizing eukaryotic organisms in marine and freshwater environments. These earliest photosynthesizing single-celled autotrophs evolved into multicellular organisms such as the Charophyta, a group of freshwater green algae.

Fossil evidence of plants begins around 3000 Ma with indirect evidence of oxygen-producing photosynthesis in the geological record, in the form of chemical and isotopic signatures in rocks and fossil evidence of colonies of cyanobacteria, photosynthesizing prokaryotic organisms. Cyanobacteria use water as a reducing agent, producing atmospheric oxygen as a byproduct, and they thereby profoundly changed the early reducing atmosphere of the earth to one in which modern aerobic organisms eventually evolved. This oxygen liberated by cyanobacteria then oxidized dissolved iron in the oceans, the iron precipitated out of the sea water, and fell to the ocean floor to form sedimentary layers of oxidized iron called Banded Iron Formations (BIFs). These BIFs are part of the geological record of evidence for the evolutionary history of plants by identifying when photosynthesis originated. This also provides deep time constraints upon when enough oxygen could have been available in the atmosphere to produce the ultraviolet blocking stratospheric ozone layer. The oxygen concentration in the ancient atmosphere subsequently rose, acting as a poison for anaerobic organisms, and resulting in a highly oxidizing atmosphere, and opening up niches on land for occupation by aerobic organisms.

Fossil evidence for cyanobacteria also comes from the presence of stromatolites in the fossil record deep into the Precambrian. Stromatolites are layered structures formed by the trapping, binding, and cementation of sedimentary grains by microbial biofilms, such as those produced by cyanobacteria. The direct evidence for cyanobacteria is less certain than the evidence for their presence as primary producers of atmospheric oxygen. Modern stromatolites containing cyanobacteria can be found on the west coast of Australia and other areas in saline lagoons and in freshwater.

Early plants were small, unicellular or filamentous, with simple branching. The identification of plant fossils in Cambrian strata is an uncertain area in the evolutionary history of plants because of the small and soft-bodied nature of these plants. It is also difficult in a fossil of this age to distinguish among various similar appearing groups with simple branching patterns, and not all of these groups are plants. One exception to the uncertainty of fossils from this age is the group of calcareous green algae, Dasycladales found in the fossil record since the middle Cambrian. These algae do not belong to the lineage that is ancestral to the land plants. Other major groups of green algae had been established by this time, but there were no land plants with vascular tissues until the mid-Silurian.

The evidence of plant evolution changes dramatically in the Ordovician with the first extensive appearance of embryophyte spores in the fossil record. The earliest terrestrial plants lived during the Middle Ordovician around 470 million years ago, based on their fossils found in the form of monads and spores, with resistant polymers in their outer walls, from Turkey, Saudi Arabia and Argentina. Individual trilete spores resembling those of modern cryptogamic plants and vascular plants first appeared in the fossil record from the Late Ordovician. These plants probably resembled liverworts, and did not have any conducting tissues. They were able to reproduce with spores, important dispersal units that have hard protective outer coatings which not only allowed their preservation in the fossil record, but also protected them from the UV light, desiccating environment and possible microorganism attack.