Evolutionary history of plants
The evolution of plants has resulted in a wide range of complexity, from the earliest algal mats of unicellular archaeplastids evolved through endosymbiosis, through multicellular marine and freshwater green algae, to spore-bearing terrestrial bryophytes, lycopods and ferns, and eventually to the complex seed-bearing gymnosperms and angiosperms of today. While many of the earliest groups continue to thrive, as exemplified by red and green algae in marine environments, more recently derived groups have displaced previously ecologically dominant ones; for example, the ascendance of flowering plants over gymnosperms in terrestrial environments.
There is evidence that cyanobacteria and multicellular thalloid eukaryotes lived in freshwater communities on land as early as 1 billion years ago, and that communities of complex, multicellular photosynthesizing organisms existed on land in the late Precambrian, around.
Evidence of the emergence of embryophyte land plants first occurs in the middle Ordovician. By the middle of the Devonian, fossil evidence has shown that many of the features recognised in land plants today were present, including roots and leaves. More recently geochemical evidence suggests that around this time that the terrestrial realm had largely been colonized which altered the global terrestrial weathering environment. By the late Devonian some free-sporing plants such as Archaeopteris had secondary vascular tissue that produced wood and had formed forests of tall trees. Also by the late Devonian, Elkinsia, an early seed fern, had evolved seeds.
Evolutionary innovation continued throughout the rest of the Phanerozoic eon and still continues today. Most plant groups were relatively unscathed by the Permo-Triassic extinction event, although the structures of communities changed. This may have set the scene for the appearance of the flowering plants in the Triassic, and their later diversification in the Cretaceous and Paleogene. The latest major group of plants to evolve were the grasses, which became important in the mid-Paleogene, from around. The grasses, as well as many other groups, evolved new mechanisms of metabolism to survive the low and warm, dry conditions of the tropics over the last.
Colonization of land
Divergence
s evolved from a group of freshwater green algae, perhaps as early as 850 mya, but algae-like plants might have evolved as early as 1 billion years ago. The closest living relatives of land plants are the charophytes, specifically Charales; if modern Charales are similar to the distant ancestors they share with land plants, this means that the land plants evolved from a branched, filamentous alga dwelling in shallow fresh water, perhaps at the edge of seasonally desiccating pools. However, some recent evidence suggests that land plants might have originated from unicellular terrestrial charophytes similar to extant Klebsormidiophyceae. The alga would have had a haplontic life cycle. It would only very briefly have had paired chromosomes when the egg and sperm first fused to form a zygote that would have immediately divided by meiosis to produce cells with half the number of unpaired chromosomes. Co-operative interactions with fungi may have helped early plants adapt to the stresses of the terrestrial realm.Challenges to land colonization
Plants were not the first photosynthesisers on land. Weathering rates suggest that organisms capable of photosynthesis were already living on the land, and microbial fossils have been found in freshwater lake deposits from, but the carbon isotope record suggests that they were too scarce to impact the atmospheric composition until around. These organisms, although phylogenetically diverse, were probably small and simple, forming little more than an algal scum.Since lichens initiate the first step in primary ecological succession in contemporary contexts, one hypothesis has been that lichens came on land first and facilitated colonization by plants; however, both molecular phylogenies and the fossil record seem to contradict this.
There are multiple potential reasons for why it took so long for land plants to emerge. It could be that atmospheric 'poisoning' prevented eukaryotes from colonising the land prior to the emergence of land plants, or it could simply have taken a great time for the necessary complexity to evolve. A major challenge to land adaptation would have been the absence of appropriate soil. Throughout the fossil record, soil is preserved, giving information on what early soils were like. Before land plants, the soil on land was poor in resources essential for life like nitrogen and phosphorus and had little capacity for holding water.
Adaptations to land colonization
Evidence of the earliest land plants occurs at about, in lower middle Ordovician rocks from Saudi Arabia and Gondwana in the form of spores known as cryptospores. These spores have walls made of sporopollenin, an extremely decay-resistant material that means they are well-preserved by the fossil record.These spores were produced either singly, in pairs or groups of four, and their microstructure resembles that of modern liverwort spores, suggesting they share an equivalent grade of organisation. Their walls contain sporopollenin – further evidence of an embryophytic affinity.
Trilete spores similar to those of vascular plants appear soon afterwards, in Upper Ordovician rocks about 455 million years ago. Depending exactly when the tetrad splits, each of the four spores may bear a "trilete mark", a Y-shape, reflecting the points at which each cell squashed up against its neighbours. However, this requires that the spore walls be sturdy and resistant at an early stage. This resistance is closely associated with having a desiccation-resistant outer wall—a trait only of use when spores must survive out of water. Indeed, even those embryophytes that have returned to the water lack a resistant wall, thus don't bear trilete marks. A close examination of algal spores shows that none have trilete spores, either because their walls are not resistant enough, or, in those rare cases where they are, because the spores disperse before they are compressed enough to develop the mark or do not fit into a tetrahedral tetrad.
The earliest megafossils of land plants were thalloid organisms, which dwelt in fluvial wetlands and are found to have covered most of an early Silurian flood plain. They could only survive when the land was waterlogged. There were also microbial mats.
Once plants had reached the land, there were two approaches to dealing with desiccation. Modern bryophytes either avoid it or give in to it, restricting their ranges to moist settings or drying out and putting their metabolism "on hold" until more water arrives, as in the liverwort genus Targionia. Tracheophytes resist desiccation by controlling the rate of water loss. They all bear a waterproof outer cuticle layer wherever they are exposed to air, to reduce water loss, but since a total covering would cut them off from in the atmosphere tracheophytes use variable openings, the stomata, to regulate the rate of gas exchange. Tracheophytes also developed vascular tissue to aid in the movement of water within the organisms, and moved away from a gametophyte dominated life cycle. Vascular tissue ultimately also facilitated upright growth without the support of water and paved the way for the evolution of larger plants on land.
Consequences
A global glaciation event called Snowball Earth, from around 720-635 mya in the Cryogenian period, is believed to have been at least partially caused by early photosynthetic organisms, which reduced the concentration of carbon dioxide and decreased the greenhouse effect in the atmosphere, leading to an icehouse climate. Based on molecular clock studies of the previous decade or so, a 2022 study observed that the estimated time for the origin of the multicellular streptophytes fell in the cool Cryogenian while that of the subsequent separation of streptophytes fell in the warm Ediacaran, which they interpreted as an indication of selective pressure by the glacial period to the photosynthesizing organisms, a group of which succeeded in surviving in relatively warmer environments that remained habitable, subsequently flourishing in the later Ediacaran and Phanerozoic on land as embryophytes. The study also theorized that the unicellular morphology and other unique features of the Zygnematophyceae may reflect further adaptations to a cold loving life style. The establishment of a land-based flora increased the rate of accumulation of oxygen in the atmosphere, as the land plants produced oxygen as a waste product. When this concentration rose above 13%, around 0.45 billion years ago, wildfires became possible, evident from charcoal in the fossil record. Apart from a controversial gap in the Late Devonian, charcoal has been present ever since.Charcoalification is an important taphonomic mode. Wildfire or burial in hot volcanic ash drives off the volatile compounds, leaving only a residue of pure carbon. This is not a viable food source for fungi, herbivores or detritovores, so it is prone to preservation. It is also robust and can withstand pressure, displaying exquisite, sometimes sub-cellular, detail in remains.
In addition to the advent of charcoal in the rock record, the terrestrialization of plants has made significant contributions to changes in geology and landscapes. The Ordovician and Silurian show a 1.4 times greater proportion of mudrock in the geologic record than the previous 90% of earth's history and this increase in mudrock is considered to be a result of land plants retaining muds in a terrestrial setting.
Evolution of life cycles
All multicellular plants have a life cycle comprising two generations or phases. The gametophyte phase has a single set of chromosomes and produces gametes. The sporophyte phase has paired chromosomes and produces spores. The gametophyte and sporophyte phases may be homomorphic, appearing identical in some algae, such as Ulva lactuca, but are very different in all modern land plants, a condition known as heteromorphy.The pattern in plant evolution has been a shift from homomorphy to heteromorphy. The algal ancestors of land plants were almost certainly haplobiontic, being haploid for all their life cycles, with a unicellular zygote providing the 2N stage. All land plants are diplobiontic – that is, both the haploid and diploid stages are multicellular. Two trends are apparent: bryophytes have developed the gametophyte as the dominant phase of the life cycle, with the sporophyte becoming almost entirely dependent on it; vascular plants have developed the sporophyte as the dominant phase, with the gametophytes being particularly reduced in the seed plants.
It has been proposed as the basis for the emergence of the diploid phase of the life cycle as the dominant phase that diploidy allows masking of the expression of deleterious mutations through genetic complementation. Thus if one of the parental genomes in the diploid cells contains mutations leading to defects in one or more gene products, these deficiencies could be compensated for by the other parental genome. As the diploid phase was becoming predominant, the masking effect likely allowed genome size, and hence information content, to increase without the constraint of having to improve accuracy of replication. The opportunity to increase information content at low cost is advantageous because it permits new adaptations to be encoded. This view has been challenged, with evidence showing that selection is no more effective in the haploid than in the diploid phases of the lifecycle of mosses and angiosperms.
There are two competing theories to explain the appearance of a diplobiontic lifecycle.
The interpolation theory holds that the interpolation of a multicellular sporophyte phase between two successive gametophyte generations was an innovation caused by preceding meiosis in a freshly germinated zygote with one or more rounds of mitotic division, thereby producing some diploid multicellular tissue before finally meiosis produced spores. This theory implies that the first sporophytes bore a very different and simpler morphology to the gametophyte they depended on. This seems to fit well with what is known of the bryophytes, in which a vegetative thalloid gametophyte nurtures a simple sporophyte, which consists of little more than an unbranched sporangium on a stalk. Increasing complexity of the ancestrally simple sporophyte, including the eventual acquisition of photosynthetic cells, would free it from its dependence on a gametophyte, as seen in some hornworts, and eventually result in the sporophyte developing organs and vascular tissue, and becoming the dominant phase, as in the tracheophytes. This theory may be supported by observations that smaller Cooksonia individuals must have been supported by a gametophyte generation. The observed appearance of larger axial sizes, with room for photosynthetic tissue and thus self-sustainability, provides a possible route for the development of a self-sufficient sporophyte phase.
The alternative hypothesis, called the transformation theory, posits that the sporophyte might have appeared suddenly by delaying the occurrence of meiosis until a fully developed multicellular sporophyte had formed. Since the same genetic material would be employed by both the haploid and diploid phases, they would look the same. This explains the behaviour of some algae, such as Ulva lactuca, which produce alternating phases of identical sporophytes and gametophytes. Subsequent adaption to the desiccating land environment, which makes sexual reproduction difficult, might have resulted in the simplification of the sexually active gametophyte, and elaboration of the sporophyte phase to better disperse the waterproof spores. The tissue of sporophytes and gametophytes of vascular plants such as Rhynia preserved in the Rhynie chert is of similar complexity, which is taken to support this hypothesis. By contrast, modern vascular plants, with the exception of Psilotum, have heteromorphic sporophytes and gametophytes in which the gametophytes rarely have any vascular tissue.