The Evolution of Plant Life from Algae
Life on Earth as we know it would not be possible without the evolution of plants, and without the transition of plants to live on land. Land plants are a lineage embedded within the green algae. Green algae as a whole are among the oldest lineages documented in the fossil record, and are well over a billion years old, while land plants are about 450–500 million years old.
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Much of green algal diversification took place before the origin of land plants, and the land plants are unambiguously members of a strictly freshwater lineage, the charophyte green algae. Contrary to single-gene and morphological analyses, genome-scale analyses indicate the sister taxon of land plants to be a group of mostly unbranched filamentous or single-celled organisms.
Plants dominate the terrestrial environment. Remarkably, a single lineage of land plants, also called the ‘Kingdom Plantae’, accounts for the vast majority of land cover, biomass, and named biological diversity. Marine environments are a different story, with a diversity of oxygenic phototrophs largely unrelated to the terrestrial flora; known as ‘algae’, although land plants are phylogentically placed deeply within the green algae.
Two lineages of algae — the red and the green algae — have ancient fossil records. Red algae account for some of the earliest fossils known, from ∼1200 Ma sediments. The fossil record for green algae is more difficult to interpret, although there are ancient acritarchs that could well be attributed to green algae (but to several other clades as well) in sediments ∼1500 Ma, and molecular analyses suggest that red and green algae are of comparable age. It has been suggested that preservation bias favoring marine over freshwater sediments may account for the relatively greater age of red algae in the fossil record.
The Begin of plant life on Earth – green algae
Despite some popular images to the contrary, the link between the marine and terrestrial floras is found in freshwater. Freshwater environments harbor a broad diversity of algae. Among these are the green algae, which are exclusively freshwater. They constitute one of two great lineages of green algae, the other being the chlorophytes. Chlorophytes account for the bulk of green algal diversity and are found in both freshwater and marine environments, but are relatively distantly related to land plants. As noted above, green algae are extremely ancient eukaryotes, and the divergence between the charophyte and chlorophyte lineages may be well over a billion years old. The fact that the ancestral habitat for charophyte algae was clearly in freshwater provides strong evidence that the common ancestor of land plants lived in freshwater as well. This idea is bolstered by the observation that marine shoreline splash zones in high-energy environments (i.e., where there are strong waves) are generally barren, while less energetic shorelines have a very specialized flora (e.g., mangroves and salt marshes).
Over 470 million years ago, during the Silurian or late Ordovician Period, a lineage of charophyte green algae underwent an evolutionary transition that allowed it to remain hydrated and reproduce while in full contact with the atmosphere, and to access subsurface water. In so doing, these organisms gained access to atmospheric CO2 and sunlight unfiltered by water. They were probably not the first photosynthetic organisms to occupy the terrestrial environment, but they diversified into the land plants that now occupy all but the harshest terrestrial environments. They constitute the basis for agriculture, as well as lumber, paper, plant fibers (cotton, linen, flax, etc.), and other key industrial products. Their diverse biochemistries give rise to secondary metabolites that are crucial pharmaceuticals, recreational drugs, and pesticides. Dead land plants are the primary source not only of coal, but also of the organic components of soil, which is the single largest reservoir of stored carbon on Earth. Thus, the origin of a terrestrial flora was one of the most profound geobiological transitions in the history of the planet, and established the basis for the environment in which we live. Because land plants are a monophyletic group, the peculiarities of this lineage are responsible for many of the properties of the terrestrial flora (the several independent colonizations of land by cyanobacteria and algae not withstanding), and understanding the early history and biology of land plants and their close relatives, the charophytes, can provide valuable insights into why land plants are they way they are.
To understand the evolution of terrestrial organisms from aquatic organisms, it is important to remember that life remains a fundamentally aquatic process. The ancestral habitat for all of the charophyte lineages is almost certainly freshwater, although a few of these groups have members that have become secondarily adapted to brackish or alkaline waters, and many of them have semi-terrestrial or ‘subaerial’ members. Because rainfall is essentially distilled water, terrestrial environments are intermittently available to freshwater organisms. However, occupancy of intermittently wet habitats requires dormancy, desiccation-tolerance, or desiccation-resistance mechanisms that allow them to survive between wetting events. Temporal factors are also important — if an organism can only survive desiccation in a specialized dormant state, the hydrated environment must persist long enough for the organism to emerge, grow, and return to its dormant state. Conceptualized in this way, all plants inhabit a hydrological gradient, their location on which is determined by their ability to maintain hydrated conditions under varying degrees of desiccation pressure. Indeed, given that many freshwater environments such as pools and streams are subject to occasional or periodic drying, it is unsurprising that many charophytes (and other lineages of freshwater algae) have terrestrial members.
The emergence of progressively less expensive, high throughput DNA sequencing techniques, which has accelerated greatly within the last decade, has opened up the possibility of genome-scale analyses in non-model organisms. Such studies have revealed that the genomes of diverse charophyte algae are very plant-like, consistent with and expanding on the earlier ultrastructural, biochemical, and single-gene phylogenetic evidence for a common ancestry. Genomic studies have revealed properties that were difficult to study with other methods.
Thus, many of the adaptations displayed by land plants seem to have roots among the charophyte algae, probably because the freshwater environment is so closely intertwined with the terrestrial environment.
Sea to Land transition
Some of the most fundamental adaptations are biochemical, and involve photosynthesis at increased CO2 concentrations. Because the concentration of CO2 in water is typically less than 2% its concentration in the overlying atmosphere, there is a substantial benefit to performing photosynthesis in direct contact with the atmosphere, if this is possible without lethal dehydration. The higher concentration of CO2 permits relatively rapid photosynthesis, but with the penalty of increased intracellular concentration of oxygen, which means increased photorespiration and formation of reactive oxygen species (ROS).
Another mixed blessing of the terrestrial environment is the increased photon flux, which facilitates rapid photosynthesis, but at the cost of increased photo damage. This is exacerbated by substantially higher exposure to ultraviolet light (UV) on land. It has been proposed that one of the key geobiological prerequisites to the colonization of the land was the formation of an ozone layer because it led to a reduction in surface UV; an ozone layer could develop only after oxygenic photosynthesis had converted the atmosphere to oxidizing, although the importance of ozone for the colonization of the land has been questioned.
The Hydraulic system was poorly developed in the earliest diverging lineages of land plants, but spectacularly successful in, for example, the towering coast redwood (Sequoia sempervirens). For the most part bryophytes (liverworts, mosses, and hornworts) cope with life in a desiccating environment by inhabiting sheltered sites in humid environments or with moist substrates. Those that survive in drier environments do so through their capacity to survive near complete loss of cellular water and rapidly resume normal metabolism when rehydrated, thus allowing them to make use of transiently available water. The earliest land plants presumably lived close to the ground in moist habitats, but cells specialized for water transport soon arose, and vascular tissues were well developed by the time the Rhynie Chert formed in the Early Devonian, about 410 million years ago. The hydraulic system of extant vascular plants consists of regulated pores for gas exchange (cells), an impermeable coating (bark) to control water loss to the atmosphere, a network of pipes (xylem) to distribute water, and an extensive root system to extract water from, often deep, underground.
This new water management and distribution system enabled plants to develop the second key structural property, large body size. A great advantage of multicellularity is – that it permits strength. Unlike animals, plants are largely free from the need to move. That means that they were and are free from the need to compromise between exo- and endo-skeletons.
With this new development in wooden stems, root systems and leaves, plantlife started its aeons long conquer of the planet. Never ending evolutionairy changes, to adapt to predators like for example the plant eating dinosaurs, to which the plants reacted with aggressive defense tactics, starting with thorns and spikes and ending in the development of chemical and biological defense weapons.
After the extinction of the dinosaurs 66 million years ago, the Cenozoic Era began. It is sometimes called the Age of the Mammals, because these once-tiny creatures rose to dominance, filling the voids left behind by the dinosaurs and other reptiles, but it could just as well be called the Age of Grasses.
Fossils from Chile show us that the first real grassland communities began to spread around just over 30 million years ago. With them followed numerous ancient rodents, for a while the main herbivores of these new ecosystems. They were soon joined by bigger grazing mammals as the grasslands carpeted the continents. This turnover occurred during the Miocene, the time when mammal evolution was in overdrive thanks to the advent of grassy plains. This was also a time of great mountain-building, with the Andes still being relatively young at the time. The mighty Himalayas would start to affect Asia’s weather patterns for millions of years.
Fossils show us that around 21 million years ago the early forests of North America’s Great Plains region began to die away due to several cooling periods. During such events, there was less moisture in the air with much of it locked up at the poles. This made the world not just colder but also drier and far more open. Grasses were able to spread due to their ability to dominate arid or drought-prone areas. They can do this because they evolved a more efficient method of photosynthesis that loses less water than many other plants.
Six million years later, a sudden climatic shift called the Middle Miocene Disruption began when the Antarctic ice sheets began to grow, and shifting ocean currents caused global temperatures to drop. As the world grew colder, grass easily spread in these now dry and cooler plains and so did herbivores that fed on them. In response, grass evolved various methods of defense. Their leaves are covered with little hairs on their surface that make them waterproof and highly unpalatable to many creatures. They are also filled to the brim with phytoliths, bits of stone-like silica that they take up through their roots from the soil. Not only are they already distasteful to the tongue and tough to digest, they literally have rocks inside their leaves. One might imagine that nothing would eat grass and that it would be quite safe from consumption by animals. Of course, we all know that this is far from the truth.
Interestingly, grasses often thrive on being eaten and constantly grazed upon. The meristem, the part of the plant responsible for the growth of its organs and leaves, is closer to the bottom of the plant. This allows them to recover quickly from almost any attacks by herbivorous animals, as large grazers are unable to destroy the roots of a grass, which lay comfortably beneath the soil. What’s more, the hooves of these very same grazers trample saplings and preclude the growth of trees, keeping the grasslands open.
The story of plants of course, is not yet over and they will continue to blanket the Earth for hundreds of millions of years to come. Even though many see plants as a passive feature of our landscapes, they are living organisms that evolve and adapt just like animals – shaping the history of those around them. By pumping oxygen into the early atmosphere, creating a green carpet on which the first land animals could tread on, and using them to spread far and wide via their seeds, plants have shown they impact the entire world. They were the first living things to conquer the land, and may well be some of the last to stand.
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