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Plant Evolution

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Plant Evolution

Plant evolution has played a critical role in shaping the world we know today. Since first appearing millions of years ago, plants have molded the earth to suit their needs. From changing the balance of gasses in our atmosphere, to physically altering earth's landscapes. There is no doubt that our planet would be fundamentally different without the emergence and evolution of the plant kingdom.

The definition of plant evolution

Plants have evolved through the same mechanisms affecting all life on earth. Much like animals, bacteria, and fungi, the different conditions plants faced influenced their evolution.

Diversity within populations and between individuals occurs naturally through genetic variation. A phenomenon where, thanks to differences in the DNA sequences from one individual to another (i.e. different allele frequencies), different morphological traits are present within a species. Genetic variation can be caused by mutations, sexual reproduction or genetic drift, but no matter the cause the outcome is always the same: slight differences between individuals.

Some differences caused by genetic variation can be beneficial, or harmful, to an individual's chance of survival. What is beneficial varies across different periods, environments, and in the presence or absence of predators and resources. The environmental factors affecting survival are known as selection pressures.

Left to time, populations may change to the point where they are no longer recognizable as descendants of their ancestors. These morphological changes are the result of a gradual change, over many generations, in the genetic makeup of a population. This process is known as evolution. Natural selection is the mechanism by which evolution occurs.

Evolution: a gradual and cumulative change in the heritable genetic traits of a population of organisms over the course of many generations.

Natural selection: a process where individuals with traits that help them survive in their environment are more likely to survive and reproduce because of those traits. These beneficial traits become more and more common within the population with each passing generation.

Tracking these changes in individuals and conditions through time to paint a picture and better understand how the diversity of plant life across the terrestrial world came to be. This knowledge can help us to predict responses to climate change, droughts, and other challenges our society will face, and may even influence how we respond to these threats.

The origin and evolution of plants

Whilst the origins of life itself are hotly contested, it is mostly agreed that all life stems from a single common ancestor. This Last Universal Common Ancestor (LUCA) formed roughly 3.5 billion years ago.1 LUCA gave rise to all living organisms we see today, plants, animals, fungi and bacteria alike.

Early life forms were simple unicellular organisms, reliant on diffusion to gather all the energy and nutrients needed from their surrounding environment. With time, life evolved complex processes to make its own energy. This early cellular evolution underpinned the processes of glycolysis, respiration, and photosynthesis.2

Photosynthesis is thought to have originated in bacteria and allowed organisms to harness sunlight for energy.3 Early plant ancestors, in the form of simple eukaryotic cells, are believed to have absorbed photosynthesizing cyanobacteria. These previously free-roaming cyanobacteria gave rise to chloroplasts, the photosynthetic organelles found in plants.

This symbiotic relationship may have occurred due to food scarcity. In an environment lacking prey, capitalizing on freely available sunshine for energy would be very beneficial. By absorbing rather than consuming photosynthesizing cyanobacteria plant ancestors would have also gained this beneficial trait.3

Plant evolution: the move to land

It's widely believed that life started underwater. Roughly 430 million years ago the first organisms migrated to terrestrial land and gave rise to today's land plants. Similarly to the ‘universal common ancestor’, ancestral streptophyte algae is thought to have been the only plant ancestor to survive the move onto land4.

Modern-day plants have complex stress signaling pathways with many similarities to the ancestral streptophyte algae4. This indicates it was not an easy transition from water to land and strong selection pressures would have been at work.

The importance of plant evolution

In the eyes of evolution, you either adapt or face extinction.

Initial terrestrial environments were rife with available sunlight, and space to grow whilst lacking predators and competitors. However, the move to land was still a particularly stressful time for early land plants.

Land plants that couldn’t adapt to their new environment were outcompeted for resources, and simply couldn't survive the harsh conditions. The threat of Extinction was constant for early land plants. Some of the deadly threats and consequences of terrestrial life included:

  • Desiccation - Early land plants couldn’t transport water, so relied heavily on damp conditions.

  • UV radiation - Water may filter sunlight, and reduce the amount of energy absorbed by chlorophyll pigments, but it also acts as a barrier against harmful UV radiation. A barrier absent on land.

  • Lack of support - Water offers aquatic plants support and buoyancy, but in terrestrial environments plants must devote energy and nutrients to rigid features like cell walls.

The harsh selection pressures of early terrestrial environments shaped land plant's evolutionary journey. Yet, since their emergence in the late Ordovician Period, land plants have reworked our planet to suit their own needs. Paving the way for some species to blossom, whilst ensuring the extinction of others.

The development of roots changed the earth's physical environment. As plants spread across land, previously bare riverbeds became flourishing plant habitats. Plant roots held the earth together and reduced erosion on river banks. This resulted in an increase of meandering rivers, rather than the wide braided channels common before the emergence of land plants.

Land plants drove early mass extinctions. As plant roots burrowed down into the earth, the rocks beneath were worn down. Releasing minerals that found their way into earth's river systems and oceans. This sudden increase in nutrients caused the eutrophication and anoxia of past oceans, killing half of marine life in the Devonian Mass Extinction.

Plants and algae changed the earth's atmosphere. Plants and algae are autotrophs. They absorb carbon dioxide and energy from the sun, whilst releasing oxygen. Plants and algae dramatically increased the ratio of oxygen in the atmosphere during the carboniferous period, allowing a boom in animal evolution. With oxygen no longer a limiting factor, huge arthropods emerged.

Plants influence the global climate. Photosynthesis directly increases atmospheric oxygen concentrations, but plant roots also played a role by breaking up the earth and releasing minerals which react with carbon dioxide. These reactions draw down atmospheric carbon dioxide and lock it away in the earth and oceans. This dramatically increased the concentration of oxygen in the atmosphere, leading to global cooling periods and ice ages. The mass extinctions which occurred during ice ages, opened up niches for surviving species to adapt and colonize.

Plant evolution timeline

Land plants' ability to flourish is largely attributed to adaptations gained through four key evolutionary steps (Fig. 1) , which no doubt evolved under harsh selection pressures.

Plant Evolution, Timeline, StudySmarterKey steps in plant evolution throughout different geological periods, IST Austria

Angiosperms, which underwent each key stage of plant evolution, are now the most abundant of all land plant types.

Examples of plant evolution

Billions of years of plant evolution have allowed land plants to conquer every corner of the globe. So much so that land plants now make up 82% of global biomass.

Adaptation

Example of the Benefits Bestowed on Plants.

Waxy cuticle

Prevent water loss, reducing the risk of desiccation.

Stomata & guard cells

Increased gas exchange needed for respiration and photosynthesis. Guard cells control how open or closed the stomata are, reducing water lost by transpiration and desiccation risk.

Rhizoids

Provide structure and some uptake of water in bryophytes.

Vascular system

Transport nutrients, water and energy in the form of ATP from where they are absorbed, or produced, to tissues where they are needed. Vascular plants are able to grow much taller, outcompeting other plants for sunshine, because of their vascular systems.

Vascular plants also have increased rigidity and support.

True roots

Anchor and support plants, and aid in the absorption of water in vascular plants.

Protective flavonoids and pigments

Protect plants from UV radiation by filtering harmful UV light whilst still allowing for some energy absorption for photosynthesis.

Nectar & variations in color, scent and size of flowers

Sweet nectar encourages insects and other pollinators to travel deep into the flowers, where sticky pollen attaches to their skin or fur. As pollinators visit multiple flowers in a day, some of this pollen will rub off on future plants, fertilizing and spreading the initial plant's genes. Plants make their flowers more inviting for pollinators through the use of bright colors, appealing scents, and different sized and shaped petals. Angiosperms, or flowering plants, have coevolved with pollinators.

Seeds and pollen

Allow the genetic material, and eventually fertilized embryos of plants to travel far away from their parents and reduce competition for resources. Seeds and pollen both also have protective coats which protect their contents from mechanical damage and desiccation. Seed plants are able to survive and spread through much harsher environments thanks to this adaptation.

Fruit surrounding seeds

Angiosperms' seeds are surrounded by fruits or ovaries. Sweet fleshy fruits invite animals to eat them, dispersing the seeds contained within through their feces. This increased dispersal reduces competition from parent plants in angiosperms.

Other fruits are dry and hard. These fruits confer additional protection to the seed. Some of which may have hooks that attach to the fur of pollinators aiding dispersal. Not all seed plants benefit from the evolution of protective fruit, as gymnosperms lack ovaries.

Plant Evolution - Key takeaways

  • All plants originated from the same common ancestor as all other life on earth.
  • Photosynthesis evolved in bacteria. Early plant eukaryotes engulfed cyanobacteria gaining the ability to photosynthesis themselves. Engulfed cyanobacteria gave rise to chloroplasts.
  • Plant evolution has shaped our natural world, by changing the composition of the atmosphere and the geology of their environments.
  • The move to land brought about many stressors and strong selection pressures. Natural selection ensured plants that didn't adapt faced harsh competition, predation and eventually went extinct.
  • There are many examples of plant evolution seen in the adaptive features land plants require to survive within their niches.

References

  1. Christie Wilcox, Evolution: Out Of The Sea, Scientific American, 2012.
  2. Cooper GM, The Origin and Evolution of Cells, The Cell: A Molecular Approach. 2nd edition, 2000.
  3. Robert E. Blankenship, Plant Physiology: Early Evolution of Photosynthesis, 2010.
  4. Jan De Vries et al, Plant evolution: landmarks on the path to terrestrial life, New Phytologist, 2018.
  5. Lumen, Bryophytes, Boundless Biology
  6. C. Jill Harrison et al, The origin and early evolution of vascular plant shoots and leaves, Philosophical Transactions of the Royal Society B, 2018.
  7. Ada Linkies et al, The Evolution of Seeds, New Phytologist, 2010
  8. Hannah Ritchie and Max Roser, Biodiversity, Our World In Data, 2021.

Frequently Asked Questions about Plant Evolution

Gymnosperm plants were the first seeded plants to evolve from vascular plants during the late devonian or early carboniferous period. Gymnosperm plant’s novel seed and pollen adaptations allowed them to disperse throughout and survive the dry conditions. 

The major events in the evolution of plants were the move to land, evolution of the vascular system, seeds, and flowers. These adaptations increased the survival of plants within the dry terrestrial conditions as they conquered the land. 

The proposed sequence of plant evolution starts with the move of the ancestral streptophyte algae onto land. Next plants evolved vascular systems, gymnosperm seeds and eventually the angiosperm seeds and flowers which make up 80% of known plant life. 

The four major periods of plant evolution were the move to land, development of vascular tissue, and gymnosperm and angiosperm emergence. Occurring within the ordovician, late ordovician, devonian & carboniferous period and cretaceous period respectively. 

There are numerous examples of plant evolution under natural selection. In the late devonian period vascular plants evolved ‘naked seeds’ as an adaptation to dry conditions. 

Final Plant Evolution Quiz

Question

If life evolved in water, how did land plants migrate to the terrestrial world?

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Answer

Lands plants migrated to land once. All land plants share a single common ancestor known as ancestral streptophyte algae


The similarity in the stress signaling pathways between modern plants and ancestral streptophyte algae has been shown to be very high suggesting these pathways were beneficial for adapting to lands many stressors. 

Show question

Question

When did early ancestral plants make the move to land?

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Answer

Ancestral streptophyte algae moved to land roughly 430 million years ago. 

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Question

Once on land why was it important for plants to adapt to their environment?

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Answer

Modern plants and ancestral streptophyte algae share complex signalling pathways. This suggests it was harsh transition from aquatic to terrestrial environments. In the novel terrestrial environment it was crucial plants evolved desiccation strategies to survive the dry conditions. 


Further evolution was critical to reduce competition for resources, resist against UV damage, and disperse into new environments. 

Show question

Question

Which feature of ancestral streptophyte algae may have been beneficial for surviving early terrestrial environments?

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Answer

Cell walls containing lignin like compounds

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Question

Which of the following could NOT be considered a beneficial adaptation to avoid dessication?

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Answer

Protective Flavonoids and Pigments

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Question

In what period did early angiosperms evolve?

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Answer

Cretaceous period

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Question

True or False: Angiosperms (flowering plants) and pollinators are thought to have coevolved?

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Answer

True

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Question

When did early vascular plants emerge? 

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Answer

Late Devonian Period

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Question

What group of plants are the most abundant in modern times? 

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Answer

Angiosperms / Flowering Plants

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Question

What is the name for the evolutionary mechanisms which favours traits that confer increased survivability to the individual? 

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Answer

Natural Selection

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Question

Which theory best describes the origins of all life on earth?

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Answer

Universal Common Ancestor 

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Question

How long ago did the first life form evolve?

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Answer

6 Billion years ago

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Question

Which group of seed producing plants evolved first? 

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Answer

Gymnosperms

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Question

In what chronological did the 4 key steps in plant evolution occur? 

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Answer

Vascular systems    Move to land    Angiosperms    Gymnosperms

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Question

Which of the following is not a known evolutionary stage predating all extant land plants?


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Answer

Eukaryiotic organisms obtain mitochondria and chloroplasts as laid out in the endosymbiotic theory.

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Question

Give an example of a seedless vascular plant.


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Answer

Ferns &/ Club Mosses

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Question

What are the benefits seeds provide to reproducing plants?


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Answer

  1. Reproduction is no longer reliant on water for gamete exchange.
  2. The seed coating protects the inner embryo from mechanical damage.
  3. Seeds can stay dormant until favourable conditions arise. 
  4. The protection and nutrients seeds offer allow for further dispersal and less competition with the offsprings parents. 

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Question

What is the main differences between gymnosperms and angiosperms?


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Answer

Gymnosperms produce naked seeds and lack ovaries and flowers. 


Angiosperms are flowering plants whose ovaries mature into fruits encasing their seeds after fertilisation and ripening. 

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Question

What characteristics did early terrestrial plants have?


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Answer

Early terrestrial plants were likely small. In air they lacked support and, being non vascular, the ability to transport water, and so survived close to the ground in damp environments.

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Question

Which of the following characteristics does not apply to all plants?


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Answer

Contain vascular tissue for nutrient and water transport

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Question

Name a harsh environmental condition which would have applied a survival-based selection pressure to early land plants?

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Answer

Desiccation / UV Radiation / Lack of Support

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Question

Which event in earths history is not likely to be a consequence of the emergence and evolution of land plants?

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Answer

Global Cooling Periods

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Question

How are plants thought to have caused the Devonian mass extinction? 

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Answer

As plant roots evolved, the earth below was broken up releasing free minerals. These minerals entered the river systems and travelled towards the oceans where they caused eutrophication and anoxia. 

Show question

Question

Give and example of how plants and algae changed earths atmospheric composition?

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Answer

Plants and algae photosynthesis releases oxygen. The ratio of oxygen to other gases, like carbon dioxide and nitrogen, increased. 


Plant roots also released free nutrients and molecules which reacted with atmospheric carbon dioxide. This drew down carbon dioxide and trapped it in the oceans and ground, further unbalancing the ratio of atmospheric gasses. 

Show question

Question

How did land plant evolution alter earths climate? 

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Answer

Several of land plants adaptations increased the percentage of oxygen in earths atmosphere:

1. The development of roots resulted in carbon dioxide drawdown. 

2. As plants evolved they grew into more niches and throughout the time their biomass increased. More plants meant more photosynthesis was occurring releasing oxygen into the atmosphere. 


This increase in oxygen caused global climatic cooling events and ice ages. 

Show question

Question

Which was the last of the 4 key stages of plant evolution that liverworts underwent? 

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Answer

The Move to Land

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Question

What name do we use to refer the lifecycle of plants as they switch between diploid and haploid phases?

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Answer

Alternation of Generations

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Question

Which of the following do angiosperms and gymnosperms have in common? 

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Answer

They both produce seeds which arise from ovules

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Question

Why did Charles Darwin refer to the emergence and evolution of angiosperms as an "abominable mystery"

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Answer

Gaps in the fossil record made it appear as if angiosperms sprung up out of nowhere and with no indeterminate species showing the progression from flowerless to flowering plants.

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Question

What percentage of plant life do modern day angiosperms make up? 

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Answer

90%

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Question

In what order did the 4 key stages of plant evolution occur? 

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Answer

  1. the move to land. T
  2. he development of vascular tissue. 
  3. The development of seeds.
  4. The development of flowers & fruits.

Show question

Question

Give an example of coevolution relevant to land plants?

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Answer

Throughout their evolutionary history, most angiosperms have had a symbiotic relationship with a pollinator. 


Angiosperms evolved to have brightly colored flowers of different sizes, shapes and scents, containing sweet nectar, to attract and cater to pollinators. 


The angiosperm plant's pollen sticks to the pollinators and is carried with them as they visit, and hopefully pollinate, the next plant. 


Pollinators have evolved traits like longer tongues and beaks to better reach the nectar they feed on. 


Whilst angiosperm plants continue to make this nectar sweeter more appealing for pollinator friends. 

Show question

Question

What percentage of today's land plants are angiosperms?

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Answer

90%

Show question

Question



Fossil evidence seems to indicate angiosperms diverged from gymnosperms in the _______ (A) Period. Genetic analysis, however, suggests the split was in the _______(B) Period. 

Show answer

Answer

(A) Triassic (B) Devonian

Show question

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