The Elements of Evolution – Verdure Venture

Verdure Venture

Life changed the face of the Earth. Before there was life on land, there were no soils. The Earth’s surface was barren rock, and rates of erosion were vast, more than 10 times what they are today. Mountains were rocky crags, and lowland plains were dustbowls. ~ Michael Benton

The earliest land plants appeared ~500 MYA, during the Ordovician. The way had been paved by hardier life.

The first soils were prepared by microbes that had arrived much earlier: a diversity of bacteria and eukaryotic algae which secreted organic acids that dissolved rock, releasing iron and phosphorus. Fungi were among the earliest multicellular terrestrial eukaryotes.

Lichen, which is a symbiosis of a fungus and green alga or cyanobacterium, likely played a significant role in rendering soil upon which plants might survive. Following lichen as a role model, early plants were anchored in place by fungal symbionts. The symbiosis was instrumental in recovering water from the soil.

Plants adapted to life on land by internalising the external atmosphere and exploring the soil in an intimate way. ~ English botanists Martin Ingrouille & Bill Eddie

Plants descended from freshwater algae acquainted with dry times, as evidenced by their protective cell walls.

Algae were on land before they turned into plants. ~ Danish microbiologist Jesper Harholt

The algae that took to a terrestrial lifestyle planned their move in advance. Their ability to succor support from fungi was critical. While still in the water, algae preadapted their communication facility for mutualism with fungi.

Land plants got their start in a hothouse: 5 ºC warmer than now. Atmospheric CO2 was ~15 times that of today.

The Sun was 6% fainter, basking an Earth that had less of a greenhouse gas dynamic than now. As there was little atmospheric filter, sunlight was especially intense in the ultraviolet range.

The greatest challenge nascent land plants faced was desiccation. Defeating dehydration would repeatedly drive plant evolution.

To survive frequent cycles of dehydration and rehydration, the first land plants must necessarily have deployed efficient molecular/biochemical strategies to withstand drought. ~ English botanist Sean Stevenson et al

Among the first land plants were rock-hugging mosses. They extracted vital minerals from the rocks to which they clung: calcium, magnesium, phosphorus, and iron, causing chemical weathering on the Earth’s surface.

Roots are 1 of the 3 fundamental organ systems of vascular plants, and have roles in anchorage, symbiosis, and nutrient and water uptake. ~ English botanist Alexander Hetherington & Irish botanist Liam Dolan

Plant roots convergently evolved at least twice. Their evolution was a stepwise assemblage of structures for intended functionality.

In their mineral mining, early land plants had help from microbes that chewed rock for a living. This grew into mutual relations. Plants today cultivate specific root microbes when growing in nutrient-poor soil.

New carbonate rocks formed in the oceans. Chemical by-products from land plants flowed as runoff into the nearby seas, fueling productivity which further removed atmospheric carbon, burying it in the ocean depths when its consumers died.

Land plants inherited their biochemistry and cell biology from ancestral green algae, but their fundamental organs and tissues evolved on land. ~ English paleobotanist Paul Kenrick

The earliest land plants were only a few centimeters tall. They sprouted into photosynthetic pencils to soak up the readily available CO2.

To cope with desiccation and excessive UV, pioneer terrestrial plants developed a layer of epidermal cells coated by a waxy cuticle layer that helped limit water loss and selectively refract light wavelengths.

Cooksonia was a primitive land plant. It had a simple stalk that bifurcated a few times. This plant group evolved specialized tissue to transport water: the onset of vascular plants. Stems widened from less than a millimeter to a few millimeters to accommodate nutrient flow.

By the Late Ordovician, global climates had become more variable. Some regions became cooler and moister. Rain patterns changed. Glaciers formed at the South Pole.

Less volcanic outgassing and increased burial of organic carbon contributed to more favorable conditions for terrestrial plants. There was a dramatic reduction of atmospheric carbon, triggered by plant consumption of CO2.

As plants spread over the world, they adapted to the various conditions of temperature and moisture while playing a role in shaping climate on a planetary scale. Plant proliferation helped cool the planet by the close of the Ordovician. Global temperatures fell 7–8 °C. This rapid change devastated sea life, leading to a mass extinction event. This period was but a pause for plants in their ongoing evolution.

The glaciers and ice caps of the Ordovician receded during the Silurian (444–417 MYA). Sea level rose. Climates were equable, with relatively warm oceans and gradual latitudinal temperature gradient. By 430 MYA plants had completely colonized the continents.

Pteridophytes arose 390 MYA, during the Devonian. Pteridophytes are vascular plants that reproduce and disperse via spores, producing neither flowers nor seeds. Pteridophytes succeeded the land plants of the Silurian as the dominant plant group.

 Ferns

Ferns are pteridophytes that emerged over 360 MYA. They were initially quite successful. Like sharks, ferns evolutionary advance was modest for 180 million years.

Unlike sharks, ferns could not compete with more modern designs. The towering of trees and rise of flowering plants foretold the demise of ferns.

Ferns were desperate for an innovation to save them from extinction. The answer lay in learning to live in the shadows of more advanced plants. Moving forward required looking back. The trick that let ferns live was genetically picked up from hornworts, an earlier-evolved non-vascular plant (bryophyte). Ferns co-opted the hornwort gene for making neochrome: a photoreceptive protein that lets ferns thrive on shady forest floors.

The green algae Mougeotia scalaris invented neochrome, which fuses red-sensing phytochrome and blue-sensing phototropin modules into a single molecule. Neochrome is efficiently receptive to longer wavelength light than chlorophyll, enlivening photosynthesis in relatively low light.

Hornworts – an early land plant – independently evolved neochrome a few times. Hundreds of millions of years later, ferns latched onto neochrome from hornworts via horizontal gene transfer.

This retro innovation gave ferns a new lease on life. From 180 MYA, the 1 lineage of ferns that managed to survive proliferated into 12,000 species.

The early Triassic, after the devastating Permian extinction 252 MYA, was the golden age of ferns. Ferns also flourished after the Cretaceous came to a crashing close 66 MYA. Their lithe appearance disguising a spunky spirit, ferns are early colonizers of barren landscapes.

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Trees and the first seed-bearing plants evolved before the end of the Devonian, 360 mya. Forests of primitive plants grew, covering the land and changing the way rivers sashayed across the landscape.

The earliest trees shed spores for reproduction. These plants had extensive roots and megaphyll leaves.

The evolution in plants of a branching vein system, and thin, laminate (megaphyll) leaves, did not occur for at least 25 million years after their arrival on land. Megaphyll leaves evolved as an extension of the branching patterns of the earliest vascular plants.

The coming of megaphyll leaves in all but the driest habitats was an adaptive response to the success of plants in altering the carbon cycle: pouring oxygen into the atmosphere, which developed the planet’s atmospheric water cycle, increasing cloud cover and rainfall. Megaphyll leaves were designed to take advantage of these new conditions, especially moister air.

The vein patterns of leaves reflect a plant’s condition. Vein density indicates the resources dedicated to the leaf network, while the distance between veins shows how well veins are supplying energy to the leaf. Veining patterns emerge to allow alternate pathways, to minimize damage to the whole leaf if a portion is ruined.

The earliest land plants reproduced by spores, first of 1 size (homosporous), progressing to heterosporous: a plant with 2 different spore sizes. The large spores evolved into seeds: essentially, a megaspore with a protective coating. Small spores were the precursor to pollen.

The Devonian ended in a major extinction event, primarily affecting the marine community, particularly warmwater reef builders. Land plants and freshwater species were relatively unaffected.

In the early Carboniferous, 340 mya, gymnosperms arose: the origination of seeds as a reproductive device, conferring greater robustness in progeny, and granting greater dormancy potential. Extant gymnosperms include conifers, cycads, and gingkoes. Other early seed-bearing plant groups have come and gone.