Origins of Plants on Land

The origins of the plant kingdom can be traced all the way back to the Archean Eon of geological time some 3.8 billion years ago. At that time small volcanic islands rose up from the seabed through the warm, shallow oceans. The continents had yet to form and the Moon was much closer to Earth than it is today.

Archaean Eon showing large moon, volcano and methane skiesnovaspaceart.com

The Earth’s atmosphere contained little oxygen but instead vast amounts of carbon dioxide. Such high levels of carbon dioxide, if present in the atmosphere today, would be toxic to the majority of living organisms.

Archean Landscape

It was 3.8 billion years ago, shortly after the planet had cooled sufficiently to sustain the presence of liquid water, that microscopic single celled bacteria called archean sun one third as bright as todays sunprokaryotes first appeared.

These primitive bacteria comprised nothing more than an outer membrane containing a store of chemicals with free DNA floating around inside. The early Archean bacteria were confined to obtaining their energy from the chemical scraps of nutrients they happened to absorb into their bodies from sediment on sea floors.

Then sometime between 3 and 2.7 billion years ago something truly amazing took place. These primitive prokaryotic bacteria evolved the ability to harness an unlimited source of energy. This source of energy was the near infrared light provided by the Archean Sun. At that time our Sun was just ‘hotting up’ and was far cooler that our Sun is today.

This new type of prokaryotic bacteria was able to harness the energy of the Sun by producing pigments within their cell walls capable of absorbing near infrared light. These light absorbing pigments were called bacteriochlorophylls, the all important predecessors of chlorophyll.

Cross section through a prokaryote bacterium showing cell wall and cell membrane

This new type of prokaryotic bacteria used the Sun’s light energy to convert carbon dioxide and hydrogen based organic chemicals ingested from sea water into sulfur and sulphates.  It was the sulfur and sulphates that provided these bacteria with the nutrients to grow and gave them the energy to move around using their flagella ‘tails’.

prokaryote

Example of a more advanced prokaryotic cell

The ability of these early bacteria to harness energy from the Sun to manufacture organic food was the first occurrence of photosynthesis – a significant advance in evolutionary terms and the origin of all plant life that we see on our planet today.

Provided with light energy from the Sun these bacteria evolved significantly and rapidly spread throughout the warm, shallow Archean seas. Over the course of the Archean Eon, from 3.8 to 2.5 billion years ago, these bacteria evolved into an array of colors (red, purple and green) and shapes.

Some Bacterial Morphology shapes including coccus bacillus coccobacillus fusiform bacillus vibrio spirillum spirochete

Around 2.7 billion years ago a new type of single celled prokaryote bacteria emerged -the cyanobacteria.

cyanobacteria-types

Whereas the earlier bactetia had pigments in their cell walls which could only absorb and exploit near infrared light, the new cyanobacteria was able to exploit visible light as a means of breaking down chemical compounds to manufacture food.

 Cyanobacteria was able to exploit visible light as a method of breaking down chemical compounds to manufacture food

To help them absorb the spectrum of visible light more effectively these cyanobacteria developed a range of photosynthetic pigments. Phycobilins absorbed red, orange, yellow and green light; carotenoids absorbed very strongly in the blue-violet range; other forms of photosynthetic pigment that we call chlorophyll absorbed light in the red (long wavelength) and the blue (short wavelength) regions of the visible light spectrum.

Absorbion spectrum of Photosynthesis showing carotenoids absorbing very strongly in the blue-violet range and chlorophylls absorbing light in the red (long wavelength) and the blue (short wavelength) regions of the visible light spectrum.

Whilst the chlorophylls were able to absorb blue and red light they were unable to absorb green light, leading to their green appearance.

cyanobacteria blue green algae chlorophyll absorbed light in the red (long wavelength) and the blue (short wavelength) regions of the visible light spectrum.

Whereas the waste product of the first prokaryotes that had evolved 3.8 billion years ago had been sulfurous gases, the waste product of these cyanobacteria prokaryotes that evolved 1.1 billion years later was oxygen!

how cyanobacteria produces oxygen as a byproduct of photosynthesis

Evidence for the presence of cyanobacteria has been found in the world’s oldest known fossils- the 2.7 billion year old fossils of stromatolites such as this one from Western Australia.

stomatolite fossil 2.7 billion years old

Stromatolites-what are they?

Stromatolites are dome like structures with a mat of cyanobacteria resting on top. The mat of cyanobacteria can include up to ten different species of bacteria growing in population densities in excess of 3000 million individuals per square meter.

Stromatolites in shark-bay-australia

  • So how are these stromatolites formed?

The cyanobacteria cells secrete a sticky film of mucus. Sediment adheres to the sticky mucus as it washes across the cyanobacteria mat. With accumulating sediment blocking off sunlight the cyanobacteria could cease to photosynthesise. To avoid this happening the cyanobacteria move above the sediment so they can continue the process of photosynthesis. Over time the layers of sediment gradually build on top of each other.

cyanobacteria or blue green algae showing layers of sediment and mucus layer

The layers of sediment react with calcium carbonate present in the water to form limestone. These limestone formations grow very slowly – it can take a stromatolite 100 years to grow just 5 cm; a one meter tall stromatolite might be 2,000 years old.

Some 176 different forms of stromatolite have been known to exist, a few of which can be seen below.

different types of stromatolite including large domes, small columns and domes
During the Archean Eon the photosynthetic cyanobacteria, present in stomatolites and elsewhere, continuously absorbed carbon dioxide from sea water whilst at the same time discharged oxygen as a waste product into the sea. Inevitably some of this oxygen ended up in the atmosphere as ‘free oxygen’.

Elemental Oxygen and Free Oxygen

Until the appearance of cyanobacteria 2.7 billion years ago oxygen atoms had been predominantly locked up in chemical relationships with other elements including sulfur dioxide (SO2) carbon dioxide (CO2) and water (H2O). Elemental oxygen, as it is called, is incredibly reactive and will bind to any available molecule if given the opportunity.

sulfur dioxide or SO2

The process of photosynthesis split single oxygen atoms away from water molecules allowing 2 single atoms of oxygen to bind together to form free oxygen (O2). Henceforth oxygen was to exist in a stable state allowing for its accumulation in the atmosphere.

The Great Oxidation Event, which started 1.5 billion years ago, marked the time when vast quantities of cyanobacteria pumped out huge volumes of free oxygen into the atmosphere. It would take a further billion years for atmospheric oxygen to reach 10%- only half of the oxygen present in Earth’s atmosphere today.

Graph of Great Oxidation Event where atmospheric oxygen increased as a result of oxygen pumped into the atmosphere by cyanobacteria

Stomatolites- where are they today?

During the Archean Eon stromatolites proliferated because no organisms yet existed to feed off the mats of cyanobacteria. Stromatolites began to decline in abundance and diversity about 700 million years ago during the Proterozoic Eon. This was largely as a result of the evolution of metazoans (including sponges, cnidarians, and worms) which grazed on the cyanobacteria thus preventing the development of stromatolites.

sponges cnidariand and worms grazed on cyanobacteria and reduced number of stromatolites

Today stomatolites are quite rare; they only exist where metazoan grazing is absent due to extreme conditions including high salinity, low nutrient levels, elevated or decreased temperatures, strong wave actions and a reduced occurrence of competing algae and plants.

Shark Bay in Western Australia is one of those rare locations in which stromatolites still exist in abundance.

Living stomatolites in Shark Bay Western Australia

Evolution of eukaroyotic cells

1.6 million years ago during the mid-Proterozoic Eon of geological time other types of photosynthesing life appeared including multicellular red, brown and green algae.

red green brown algaewere among the first eukaroyotic life forms

These algae were among the first eukaroyotic life forms to evolve comprising a type of complex cell characteristic of nearly all life forms today (apart from bacteria, blue-green algae and other primitive micro organisms).

eukaryotic plant cell showing chloroplast nucleus vacuole mitochrondrion and cell wall

Unlike the cyanobacteria before them the eukaryotic cell packed all its DNA into a single nucleus. More importantly their light-absorbing chlorophyll pigments became stacked and enclosed within a double cell membrane called a chloroplast. Eukaryotic cells are incredibly efficient at photosynthesising.

Chloroplasts are approximately 4 to 6 micrometers in diameter and are shaped like a satellite dish with the concave face toward the light. This shape, together with their alignment along the inner surface of the cell, maximizes their ability to capture light. There can be as many as two hundred chloroplasts in a single cell.

chloroplast of a tobacco leaf

Huge colonies of red algae (also colored purple and greenish black)  populated the ocean floor where they had the capacity to photosynthesise the shorter wavelengths of light that penetrated the murky depths.

red algae photosynthesises at shorter wavelengths of light

Brown algae adapted to colonise submerged rocks to which they attached themselves using structures called ‘holdfasts’.

Oarweed-holdfast-attached-to-rock

Green algae, known as Charophyceae, evolved the ability to survive in shallow water around land masses where they harnessed the energy of the blue and red wavelengths of light.

bladder-wrack-buoyed-up-underwater-by-air-bladders
Around 500 million years ago green algae from marine habitats was washed ashore and became stranded on land. Some of this stranded algae evolved into the first land plants. These first land plants were the ‘bryophytes’ and included liverworts…

marchantia-polymorpha liverwort

hornworts….

hornwort Phaeoceros

….and mosses.

moss next to rocks

The first land plants soon lost any resemblance to their algal ancestors as they developed adaptions to help them cope with the much drier conditions on land.

The following are just some of those adaptions:

  • Some (but by no means all) developed a thin protective layer, called a waxy cuticle, on the outermost layer of their ‘leaves’ to help them resist dessication.

liverwort thallus showing waxy cuticle

  • The male sex organ, the antheridium, evolved a  jacket of sterile cells to protect the sperm cells from dessication.

antheridium and sterile cells prevent dessication of developing sperm cells

  • Likewise the female sex organ, the archegonium, evolved a jacket of sterile cells surrounding the egg thus preventing the egg from dessicating.

archegonium and sterile cells surrounding egg

Bryophytes allude to their aquatic ancestry in other ways; sperm from the antheridium can only reach eggs in the archegonium….

Antheridium of bryophyte releases sperm mass and funnels of archegonia ready to receive water transported sperm

….if transported in water.

sperm of bryophytes need to be transported in water

Without the presence of any vascular tissue…..

vascular plant shown as diagram

…bryophytes are unable to transport any water or dissolved foods from one part of a plant to another. As a consequence the height of bryophtes is limited; the tallest living bryophytes, including Dawsonia longifolia moss, can only grow to a maximum height of 60 cms.

Dawsonia longifolia can grow to a height of 50cm

The first known vascular plants were species of Cooksonia,  such as this Cooksonia pertoni….

Cooksonia pertoni

….which first appeared during the Silurian epoch some 443.8 million years ago.

silurian landscape with cooksonia plants

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