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 ofbiological evolution, predominantly involving evolution of plants suited to live on land, greening of various land masses by the filling of theirniches with land plants, and diversification of groups of land plants.
In the strictest sense, the nameplant refers to those land plants that form thecladeEmbryophyta, comprising the bryophytes and vascular plants. However, the cladeViridiplantae or green plants includes some other groups of photosynthetic eukaryotes, includinggreen algae. It is widely believed that land plants evolved from a group ofcharophytes, most likely simple single-celled terrestrial algae similar to extantKlebsormidiophyceae.[1]
Chloroplasts in plants evolved from anendosymbiotic relationship between acyanobacterium, a photosynthesisingprokaryote and a non-photosyntheticeukaryotic 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 theCharophyta, 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, photosynthesizingprokaryotic organisms. Cyanobacteria use water as areducing agent, producing atmospheric oxygen as a byproduct, and they thereby profoundly changed the earlyreducing atmosphere of the earth to one in which modern aerobic organisms eventually evolved. This oxygen liberated by cyanobacteria thenoxidized dissolvediron in the oceans, the iron precipitated out of the sea water, and fell to the ocean floor to form sedimentary layers of oxidized iron calledBanded 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 theultraviolet blockingstratospheric ozone layer. The oxygen concentration in the ancient atmosphere subsequently rose, acting as a poison foranaerobic 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 ofstromatolites in the fossil record deep into thePrecambrian. Stromatolites are layered structures formed by the trapping, binding, and cementation of sedimentary grains by microbialbiofilms, 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 cyanobacteriacan be found on the west coast of Australia and other areas in saline lagoons and in freshwater.
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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 noland plants with vascular tissues until the mid-Silurian.
The evidence of plant evolution changes dramatically in the Ordovician with the first extensive appearance ofembryophyte spores in the fossil record. The earliest terrestrialplants lived during theMiddle Ordovician around470 million years ago, based on their fossils found in the form of monads and spores, with resistant polymers in their outer walls, fromTurkey,Saudi Arabia andArgentina.[3][4] Individual trilete spores resembling those of moderncryptogamic plants andvascular plants first appeared in the fossil record from theLate Ordovician.[5][6] These plants probably resembledliverworts,[4] and did not have any conducting tissues.[7] They were able to reproduce withspores, 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.[4]
The first fossil records ofvascular plants, that is, land plants withvascular tissues, appeared in theSilurian period. The earliest known representatives of this group (mostly from the northern hemisphere) are placed in the genusCooksonia. They had very simple branching patterns, with the branches terminated by flattened sporangia. By the end of the Silurian much more complex vascular plants, thezosterophylls, had diversified[8] and primitivelycopods, such asBaragwanathia (originally discovered in Silurian deposits in Victoria, Australia),[9] had become widespread.
By the Devonian Period, the colonization of the land by plants was well underway. Thebacterial and algal mats were joined early in the period by primitiveplants that created the first recognizablesoils and harbored some arthropods likemites,scorpions andmyriapods. Early Devonian plants did not have roots or leaves like the plants most common today, and many had no vascular tissue at all. They probably relied onarbuscular mycorrhizal symbioses with fungi to provide them with water and mineral nutrients such asphosphorus.[10][11] They probably spread by a combination ofvegetative reproduction forming clonal colonies, and sexual reproduction via spores and did not grow much more than a few centimeters tall.
By the Late Devonian, forests of large, primitive plants existed:lycophytes,sphenophytes,ferns, andprogymnosperms hadevolved. Most of these plants have true roots and leaves, and many were quite tall. The tree-likeArchaeopteris, ancestral to the gymnosperms, and the giantcladoxylopsid trees had truewood. These are the oldest known trees of the world's first forests.Prototaxites was the fruiting body of an enormous fungus that stood more than 8 meters tall. By the end of the Devonian, the first seed-forming plants had appeared. This rapid appearance of so many plant groups and growth forms has been called the "Devonian Explosion". The primitive arthropods co-evolved with this diversified terrestrial vegetation structure. The evolving co-dependence of insects and seed-plants that characterizes a recognizably modern world had its genesis in the late Devonian. The development of soils and plant root systems probably led to changes in the speed and pattern oferosion and sediment deposition.
The 'greening' of the continents acted as acarbon dioxidesink, andatmospheric concentrations of thisgreenhouse gas may have dropped.[12] This may have cooled the climate and led to a massiveextinction event. seeLate Devonian extinction.
Also in the Devonian, bothvertebrates and arthropods were solidly established on the land.
Early Carboniferous land plants were very similar to those of the preceding Latest Devonian, but new groups also appeared at this time.
The main Early Carboniferous plants were theEquisetales (Horse-tails),Sphenophyllales (scrambling plants),Lycopodiales (Club mosses),Lepidodendrales (arborescent clubmosses or scale trees),Filicales (Ferns),Medullosales (previously included in the "seed ferns", an artificial assemblage of a number of earlygymnosperm groups) and theCordaitales. These continued to dominate throughout the period, but duringlate Carboniferous, several other groups,Cycadophyta (cycads), theCallistophytales (another group of "seed ferns"), and theVoltziales (related to and sometimes included under theconifers), appeared.
The Carboniferous lycophytes of the order Lepidodendrales, which were cousins (but not ancestors) of the tiny club-mosses of today, were huge trees with trunks 30 meters high and up to 1.5 meters in diameter. These includedLepidodendron (with its fruit cone calledLepidostrobus),Halonia,Lepidophloios andSigillaria. The roots of several of these forms are known asStigmaria.
The fronds of some Carboniferous ferns are almost identical with those of living species. Probably many species were epiphytic. Fossil ferns includePecopteris and the tree fernsMegaphyton andCaulopteris. Seed ferns or Pteridospermatophyta includeCyclopteris,Neuropteris,Alethopteris, andSphenopteris.
The Equisetales included the common giant formCalamites, with a trunk diameter of 30 to 60 cm and a height of up to 20 meters.Sphenophyllum was a slender climbing plant with whorls of leaves, which was probably related both to the calamites and the modern horsetails.
Cordaites, a tall plant (6 to over 30 meters) with strap-like leaves, was related to the cycads and conifers; thecatkin-like inflorescence, which bore yew-like berries, is calledCardiocarpus. These plants were thought to live in swamps and mangroves. True coniferous trees (Walchia, of the order Voltziales) appear later in the Carboniferous, and preferred higher drier ground.
The Permian began with the Carboniferous flora still flourishing. About the middle of the Permian there was a major transition in vegetation. The swamp-loving lycopod trees of the Carboniferous, such asLepidodendron andSigillaria, were replaced by the more advanced conifers, which were better adapted to the changing climatic conditions. Lycopods and swamp forests still dominated theSouth China continent because it was an isolated continent and it sat near or at the equator. The Permian saw the radiation of many important conifer groups, including the ancestors of many present-day families. Theginkgos and cycads also appeared during this period. Rich forests were present in many areas, with a diverse mix of plant groups. Thegigantopterids thrived during this time; some of these may have been part of the ancestralflowering plant lineage, though flowers evolved only considerably later.
On land, the holdover plants included thelycophytes, the dominantcycads,Ginkgophyta (represented in modern times byGinkgo biloba) andglossopterids. Thespermatophytes, or seed plants came to dominate the terrestrial flora: in the northern hemisphere,conifers flourished.Dicroidium (aseed fern) was the dominant southern hemisphere tree during the Early Triassic period.
The arid, continental conditions characteristic of the Triassic steadily eased during the Jurassic period, especially at higher latitudes; the warm, humid climate allowed lush jungles to cover much of the landscape.[13]Conifers dominated the flora, as during the Triassic; they were the most diverse group and constituted the majority of large trees. Extant conifer families that flourished during the Jurassic included theAraucariaceae,Cephalotaxaceae,Pinaceae,Podocarpaceae,Taxaceae andTaxodiaceae.[14] The extinct Mesozoic conifer familyCheirolepidiaceae dominated low latitude vegetation, as did the shrubbyBennettitales.[15]Cycads were also common, as wereginkgos andtree ferns in the forest. Smallerferns were probably the dominant undergrowth.Caytoniaceous seed ferns were another group of important plants during this time and are thought to have been shrub to small-tree sized.[16] Ginkgo-like plants were particularly common in the mid- to high northern latitudes. In the Southern Hemisphere,podocarps were especially successful, while Ginkgos andCzekanowskiales were rare.[15][17]
Flowering plants, also known asangiosperms, spread during this period, although they did not become predominant until near the end of the period (Campanian age).[18] Their evolution was aided by the appearance ofbees; in fact angiosperms and insects are a good example ofcoevolution. The first representatives of many modern trees, includingfigs,planes andmagnolias, appeared in the Cretaceous. At the same time, some earlier Mesozoicgymnosperms, likeConifers continued to thrive, although other taxa likeBennettitales died out before the end of the period.
The Cenozoic began at theCretaceous–Paleogene extinction event with amassive disruption of plant communities. It then became just as much the age of savannas, or the age of co-dependent flowering plants and insects. At 35 Ma,grasses evolved from among the angiosperms.About ten thousand years ago, humans in theFertile Crescent of the Middle East develop agriculture. Plant domestication begins with cultivation ofNeolithic founder crops. This process of food production, coupled later with the domestication of animals caused a massive increase in human population that has continued to the present. In Jericho (modern West Bank, Palestine), there is a settlement with about 19,000 people. At the same time, Sahara is green with rivers, lakes, cattle, crocodiles and monsoons.At 8 ka, Common (Bread) wheat (Triticum aestivum) originates in southwest Asia due to hybridisation of emmer wheat with a goat-grass,Aegilops tauschii.At 6.5 ka, two rice species are domesticated: Asian rice,Oryza sativa, and African riceOryza glaberrima.
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