Timeline of Plant Evolution - Earliest Classifiable Plants

Earliest Classifiable Plants

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. Of these organisms, the cyanobacteria are photosynthetic bacteria (prokaryotes), while the 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 3000 Ma, in rock records and fossil evidence 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.

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 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.

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.

Key innovations in
early plant evolution -450 — – -440 — – -430 — – -420 — – -410 — – -400 — – -390 — – -380 — – -370 — – -360 — O S D Pragian Emsian Eifelian Givetian Frasnian Famennian Lochkovian Wenlock Llandovery Pridoli Ludlow Sandbian Katian Hirnantian ← Spore tetrads ← Rhynie chert ← Sporangia ← Stems ← Roots ← Seeds ← Wood ← Leaves Axis scale: millions of years ago.