Around 480 million years ago, plants began to invade the land masses.
In the strictly modern sense, the name plant refers to the biological classification kingdom Plantae. However, other photosynthetic organisms, including protists, green algae, and cyanobacteria have evolutionary significance to modern plants. While this article is directly about the evolutionary history of the Plant kingdom, these other organisms provide clues to the evolution of all photosynthetic organisms. All of these organisms - plants, green algae, and the protists - are primary photosynthetic eukaryotic organisms.
Scientists start the search for fossil evidence of plants with indirect evidence for their presence, the evidence of photosynthesis in the geological record. The evidence for photosynthesis in the rock record is varied, but primary evidence comes from around 3.0 billion years ago, in rock records and fossil evidence of cyanobacteria, photosynthesizing prokaryotic organisms. Cyanobacteria use water as a reducing agent, thereby producing atmospheric oxygen as a byproduct, and profoundly changing the early reducing atmosphere of the earth to one in which modern aerobic organisms eventually evolved. The 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.
Evidence for cyanobacteria also comes from the presence of stromatolites in the fossil record deep into the Precambrian. Stromatolites are layered structures thought to have been formed by the trapping, binding, and cementation of sedimentary grains by microorganisms, such as cyanobacteria. The direct evidence for cyanobacteria is less certain than the evidence for their presence as primary producers of atmospheric oxygen. Modern day stromatoloid structures containing cyanobacteria can be found on the west coast of Australia.
Chloroplasts in eukaryotic plants evolved from an endosymbiotic relationship between cyanobacteria and other prokaryotic organisms producing the lineage that eventually led to photosynthesizing eukaryotic organisms in marine and freshwater environments. These earliest photosynthesizing single-celled autotrophs later led to organisms such as Charophyta, a group of freshwater green algae.
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Early plants were small, unicellular or filamentous, composed mostly of soft body tissues, with simple branching. The identification of plant tissues 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. Generally it is accepted that there were no land plants with vascular tissues at this time although the molecular clock points to an earlier Cambrian or perhaps Precambrian origin of land plants at around 480–440 million years ago and fungi on land around 1 billion years ago. However, it is not yet clear whether the fossil evidence supports this interpretation of the molecular clock.
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