The evolutionary history of plants is recorded in fossils preserved in lowland or marine sediments. Some fossils preserve the external form of plant parts; others show cellular features; and still others consist of microfossils such as pollen and spores. In rare instances, fossils may even display the ultrastructural or chemical features of the plants they represent. The fossil record reveals a pattern of accelerating rates of evolution coupled with increasing diversity and complexity of biological communities that began with the invasion of land and continued with the progressive colonization of the continents.
At present, fossil evidence of land plants dates to the Ordovician Period (about 485.4 million to 443.8 million years ago) of the Paleozoic Era. However, research using “molecular clock” methodology, which uses genetics to estimate how long species have been separated from a common ancestor, suggests that plants started to colonize terrestrial environments around 500 million years ago, about the middle of the Cambrian Period.
By far the most diverse and conspicuous living members of the plant kingdom are vascular plants (tracheophytes), in which the sporophyte phase of the life history is dominant. (See above Life histories.) Fossil remains of vascular plants provide evidence for evolutionary changes in the structure of the plant body (sporophyte and gametophyte), in the variety of plant forms, in the complexity of the life history, in the tolerance for ecological conditions, and in systematic diversity.
Nonvascular plants, or bryophytes (mosses, liverworts, and hornworts), are much smaller and less diverse than vascular plants. The first evidence for liverworts occurs in rocks laid down between 473 million and 471 million years ago, during the Ordovician Period, whereas the earliest moss fossils are from the Permian Period (298.9 million to 251.9 million years ago). In contrast to tracheophytes, most fossil bryophytes are relatively similar to living forms. Understanding of the evolution of nonvascular plants is, therefore, less complete than for tracheophytes.