Evolution of Plants
Earliest classifiable plants lived millions of years ago
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 3000 Ma, in rock records and fossil evidence of cyanobacteria, photosynthesizing prokaryotic organisms. Cyanobacteria use water as a reducing agent, thereby producing atmospheric oxygen as a by-product, and profoundly changing the early reducing atmosphere of the earth to one in which modern aerobic organisms eventually evolved. This oxygen liberated by cyanobacteria then oxidised dissolved iron in the oceans; the iron precipitated out of the sea water, and fell to the ocean floor to form sedimentary layers of oxidised 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 oxidising 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 photosynthesising eukaryotic organisms in marine and freshwater environments. These earliest photosynthesising single-celled autotrophs later led to organisms such as Charophyta, a group of freshwater green algae.
Major eras in Earth's evolution
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 480–440Ma and fungi on land around 1 Ga. However it is not yet clear whether the fossil evidence supports this interpretation of the molecular clock.
The evidence for plant evolutionary history changes dramatically in the Ordovician with the first extensive appearance of spores in the fossil record (Cambrian spores have been found, also). The first terrestrial plants appeared in the form of tiny plants resembling liverworts when, around the Middle Ordovician, evidence for the beginning of the terrestrialisation of the land is found. These early plants did not have conducting tissues, severely limiting their size. They were, in effect, tied to wet terrestrial environments by their inability to conduct water, like extant liverworts, hornworts, and mosses, although they reproduced with spores, important dispersal units that have hard protective outer coatings, allowing for their preservation in the fossil record, in addition to protecting the future offspring against the desiccating environment of life on land. With spores, plants on land could have sent out large numbers of spores that could grow into an adult plant when sufficient environmental moisture was present.
The first fossil records of vascular plants, that is, land plants with vascular tissues, appeared in the Silurian period. The earliest known representatives of this group (mostly from the northern hemisphere) are placed in the genus Cooksonia. They had very simple branching patterns, with the branches terminated by flattened sporangia. By the end of the Silurian much more complex vascular plants, the zosterophylls, had diversified and primitive lycopods, such as Baragwanathia had become widespread.
By the Devonian Period, life was well underway in its colonisation of the land. The bacterial and algal mats were joined early in the period by primitive plants that created the first recognisable soils and harboured some arthropods like mites, scorpions and myriapods. 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 spread largely by vegetative growth, and did not grow much more than a few centimetres tall.
By the Late Devonian, forests of small, primitive plants existed: lycophytes, sphenophytes, ferns, and progymnosperms had evolved. Most of these plants have true roots and leaves, and many were quite tall. The tree-like ancestral fern Archaeopteris and the giant cladoxylopsid trees grew as a large tree with true wood. 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 metres 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 characterises a recognisably 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 of erosion and sediment deposition. The 'greening' of the continents acted as a carbon dioxide sink, and atmospheric levels of this greenhouse gas may have dropped. This may have cooled the climate and led to a massive extinction event.
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 the Equisetales (Horse-tails), Sphenophyllales (scrambling plants), Lycopodiales (Club mosses), Lepidodendrales (scale trees), Filicales (Ferns), Medullosales (previously included in the seed ferns, an artificial assemblage of a number of early gymnosperm groups) and the Cordaitales. These continued to dominate throughout the period, but during late Carboniferous, several other groups, Cycadophyta (cycads), the Callistophytales, and the Voltziales appeared. The Carboniferous lycophytes of the order Lepidodendrales, which are cousins of the tiny club-moss of today, were huge trees with trunks 30 metres high and up to 1.5 metres in diameter. These included Lepidodendron, Halonia, Lepidophloios and Sigillaria. The roots of several of these forms are known as Stigmaria.
The fronds of some Carboniferous ferns are almost identical with those of living species. Probably many species were epiphytic. Fossil ferns and seed ferns include Pecopteris, Cyclopteris, Neuropteris, Alethopteris, and Sphenopteris; Megaphyton and Caulopteris were tree ferns. The Equisetales included the common giant form Calamites, with a trunk diameter of 30 to 60 cm and a height of up to 20 metres. Sphenophyllum was a slender climbing plant with whorls of leaves, which was probably related both to the calamites and the lycopods. Cordaites, a tall plant with strap-like leaves, was related to the cycads and conifers; the catkin-like inflorescence, which bore yew-like berries, is called Cardiocarpus. These plants were thought to live in swamps and mangroves. True coniferous trees 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 as Lepidodendron and Sigillaria, were replaced by the more advanced conifers, which were better adapted to the changing climatic conditions. Lycopods and swamp forests still dominated the South China continent because it was an isolated continent and it sat near or at the equator. Oxygen levels were probably high there. The Permian saw the radiation of many important conifer groups, including the ancestors of many present-day families. The Ginkgos and cycads also appeared during this period. Rich forests were present in many areas, with a diverse mix of plant groups. The gigantopterids thrived during this time; some of these may have been part of the ancestral flowering plant lineage, though flowers evolved only considerably later.
On land, the holdover plants included the lycophytes, the dominant cycads, ginkgophyta and glossopterids. The spermatophytes, or seed plants came to dominate the terrestrial flora: in the northern hemisphere, conifers flourished. Dicroidium 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. 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 the Araucariaceae, Cephalotaxaceae, Pinaceae, Podocarpaceae, Taxaceae and Taxodiaceae. The extinct Mesozoic conifer family Cheirolepidiaceae dominated low latitude vegetation, as did the shrubby Bennettitales. Cycads were also common, as were Ginkgos and tree ferns in the forest. Smaller ferns 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. Ginkgo-like plants were particularly common in the mid- to high northern latitudes. In the Southern Hemisphere, podocarps were especially successful, while Ginkgos and Czekanowskiales were rare.
Flowering plants, also known as angiosperms, spread during this period, although they did not become predominant until near the end of the period. Their evolution was aided by the appearance of bees; in fact angiosperms and insects are a good example of co-evolution. The first representatives of many modern trees, including figs, planes and magnolias, appeared in the Cretaceous. At the same time, some earlier Mesozoic gymnosperms, like Conifers continued to thrive, although other taxa like Bennettitales died out before the end of the period.
The Cenozoic is just as much the age of savannahs, 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 the Fertile Crescent of the Middle East develop agriculture. Plant domestication begins with cultivation of Neolithic 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 Israel), 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 wheat with a goat-grass, Aegilops tauschii. At 6.5 ka, two rice species are domesticated: Asian rice, Oryza sativa, and African rice Oryza glaberrima.