Land plants have had an enormous effect on earth, and without them the world we live in would be completely unrecognizable. The changes to earths physical geography and atmosphere didn't come from nowhere - they cam about with the emergence of a few key land plant adaptations. Reconstructing the timeline of land plant evolutionary history allows us to travel back through time and track these colossal changes.
Construction of plant timeline evolution
Modern species can be traced back to their ancestors through phylogenetic trees. These trees combine fossil, chemical, morphological, and geological evidence to reconstruct an evolutionary timeline. The fossil record is our most reliable source when constructing an evolutionary timeline of land plants. The information provided by this record allows us to place the emergence and evolution of different traits along the geological time scale. This is particularly important as from this we can view new steps in evolution alongside the proposed environment they occurred in. By far the greatest finding shown within the fossil record is the increasing speed of evolution, and the complexity of communities as plants establish themselves within the newly conquered terrestrial environment.
Plant timeline evolution: evolutionary history of plants
From these reconstructions, we can see four key evolutionary stages in the plant evolution timeline. These being the evolution of:
These key steps likely reflect the strong selection pressures and challenges plants have faced throughout their evolutionary history.
Problems faced by land plants throughout their evolutionary history
While the move to land did not come without its difficulties, there were no doubt several advantages for the successful colonizing organisms. Since plants conquered the terrestrial world long before their animal counterparts, early land plants were afforded the initial luxury of a lack of predators. Other favorable aspects of the terrestrial environment include a greater abundance of CO2 and much more access to the sun's rays. Both of which allowed early plants to photosynthesize more, and therefore gain more energy to spread, grow, and reproduce.
Though these same favorable aspects of the terrestrial environment presented their own stressors and difficulties which early plant species had to overcome to successfully flourish into the diverse range of plants we know today. 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 not found on land. Nutrient diffusion may be slower in water, but the water within aquatic environments provides buoyancy, support and crucially water to its inhabitants. All of which the first land plants required new morphological adaptations to supplement. The risk of desiccation on land can be considered near-constant.
The major stressors of desiccation, UV radiation damage, lack of structural support and ‘swimmable’ environments for gamete exchange in reproduction highlight the importance of early land plant evolution. Once the first land plants had successfully established themselves within initial terrestrial environments they evolved many adaptations which contributed to the success of both species and populations, and their dispersal into the wider, harsher, terrestrial environment. However, these changes did not happen all at once and can be tracked through time following the fossil record.
Plant evolution timeline through geological periods
Plants underwent 4 key steps within their evolutionary history: the move to land, followed by the evolution of vascular tissue, seeds and pollen, and flowering plants. Each of these adaptations emerged in different geological periods and in a sequential order.
Earliest terrestrial plants (bryophytes & green algae)
Once the ancestral streptophyte algae migrated to land, it underwent several key evolutionary stages before producing the diversity of modern-day land plants. Early terrestrial plants in the initial stages of plant evolution are most closely related to bryophytes. Bryophytes likely arose 450 million years ago during the Ordovician Period. Liverworts are the most closely related extant plants to the ancestral green algae.1
The fossil record for early terrestrial plants is poor since they were made of soft organic material. By studying bryophytes and liverworts we can learn more about early terrestrial plants, and assume these key characteristics:
Early terrestrial plants were likely small. In the air, they lacked support, and the ability to transport water, and so survived close to the ground in damp environments.
Early terrestrial plants were non-vascular, meaning they have no specialized transport system.
Early terrestrial plants relied on water for sexual reproduction. The male gametes', or sperm's, only method of transportation was by swimming through water using their flagellum.
The evolution of vascular plants
Next up on the timeline of plant evolution is the development of vascular systems. Plant vascular systems are similar to our veins and arteries. They transport water and nutrients to different parts of the plant. Vascular Plants likely evolved in the late Ordovician Period, as a result of high competition for sunlight. Vascular systems allowed plants to grow taller and access more resources, as they were no longer constricted by diffusion and osmosis to transport key nutrients and water. With the evolution of vascular tissue, came the development of true roots. Roots allowed plants to access more water, and anchor the plants for stability as they grew taller. Early vascular plants predate seed evolution. Like non-vascular bryophytes, seedless vascular plants need water to reproduce sexually.
Ferns and club mosses are extant descendants of early vascular plants. They are both seedless plants that reproduce via spores.
The evolution of seeds in plant biology
Before the evolution of seeds, water was required for sexual reproduction in plants. But with the emergence of seeds, the game changed completely.
Whilst seedless land plants produce spores, which are often dispersed by wind. This is only half of the reproduction story. All plants have evolved to reproduce via alternation of generations. This means plants alternate between two different phases of organisms: gametophytes and sporophytes. Gametophytes reproduce using sex cells and mitosis, sporophytes produce spores which later germinate into gametophytes. During gametophyte reproduction in seedless land plants, the sperm fertilizes the egg, sperm cells are only adapted to reach the egg by swimming through water. Therefore water is essential for seedless fertilization.
With the evolution of seeds and pollen, this requirement for water to reproduce disappears. In seeded land plants, male gametes are contained within pollen, which provides a protective casing against desiccation. These pollen grains travel between individuals through wind or by sticking to insects. Female gametes stay attached to the parent plant in ovules until they are fertilized by pollen grains. When fertilization occurs an embryo is produced and the ovule develops into a seed. This seed forms a protective case within which the embryo is housed alongside some crucial nutrients.
The seed contains all the nutrients needed to ensure the embryo’s survival. Allowing the embryo to lay dormant, without desiccating, as the seed is dispersed. The evolution of seeds is a beneficial adaptation in the history of land plant evolution as:
Seeds removed the need for watery environments to reproduce.
Seeds contain nutrients that allow developing plant embryos to lay dormant until conditions are favorable for their growth and survival.
Seeds have a protective coating which has increased the dispersal range of new plants, reducing their competition for resources with their parents.
Gymnosperms and angiosperms emergence
The first seeded land plants to evolve were ‘naked seed’ plants or gymnosperms. Early gymnosperms which first developed in the Devonian or Carboniferous periods, produced specialized male and female spores in different cones. Many millions of years later, during the Cretaceous Period, angiosperms evolved, which produce male and female structures within flowers. Even though angiosperms evolved much later than gymnosperms, they are considered sister branches. The last common ancestor of all gymnosperms is also thought to be the last common ancestor of all gymnosperms and angiosperms.
Gymnosperm: Vascular plants which reproduce via an exposed seed. Exposed seeds are not protected by an ovary or fruit as they are in angiosperms.
Angiosperm: Flowering vascular plants which reproduce via an enveloped seed. Similarly to gymnosperms, angiosperm seeds arise from ovules. However, unlike gymnosperms, angiosperm ovules are contained within ovaries which following fertilization mature into fruits.
The evolution of seeds and pollen kickstarted land plants’ migration into every terrestrial niche, allowing plants to survive on dry land. Seed plant evolution played a large role in the success of the plant kingdom dominating the globe. Allowing gymnosperm land plants to dominate the terrestrial landscape from their evolutionary emergence through the Triassic and Jurassic periods. Only to be replaced as the 'king' of seed plants 100 million years ago by angiosperms and their flowers during the late Mesozoic Era.
Timeline of flowering plant evolution
The ancestors of flowering plants diverged from gymnosperms 300 million years ago during the Carboniferous Period. However from this point onward the timeline of flowering plant evolution becomes a bit of a mystery.
There is no continuous fossil evidence tracking the evolution of flowers. The emergence of angiosperms is tough to pin down. Fossil evidence seems to indicate angiosperms diverged from gymnosperms in the Triassic Period. Genetic analysis, however, suggests the split was far sooner in the Devonian Period. This is backed up by the occurrence of fossilized pollen similar to that of angiosperms.
Initially the emergence of flowering seed plants seemed to occur all at once. This boom was so drastic that Charles Darwin referred to this particular step in evolution as an “abominable mystery”. This was largely due to gaps in the fossil record, and whilst collections have grown since the timeline of flowering plant evolution has still not been resolutely cracked.
During the Cretaceous Period angiosperms, or flowering plants, spread far and wide replacing and outcompeting the previous fern inhabitants. During this period angiosperms are thought to have rapidly reduced their genome size, allowing them to reproduce far quicker thanks to the resulting faster cell division. This shift is likely the reason why angiosperms now make up 90% of all plants today.
Throughout their evolutionary history, most angiosperms have had a symbiotic relationship with a pollinator. Angiosperms and their relevant pollinators are a great example of coevolution. 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.
Plant Timeline Evolution - Key takeaways
Plant evolution can be tracked throughout the fossil record starting with the origins of life ~3.7 million years ago.
Land plants were up against a great number of stressors when they first appeared on land. These stressors caused strong selection pressures and shaped the evolution of land plants.
There are four key sequential steps throughout plants evolutionary history.
Whilst flowering plants make up 90% of today's flora, they possess significant gaps within the fossil record. Little is known about the exact evolutionary journey of flower plants after they branched from gymnosperms in the late Devonian period.
Angiosperms and their relevant pollinators are a great example of coevolution.
References
- Lumen, Bryophytes, Boundless Biology
- C. Jill Harrison et al, The origin and early evolution of vascular plant shoots and leaves, Philosophical Transactions of the Royal Society B, 2018.
- T. Jonathan Davies et al, Darwin's abominable mystery: Insights from a supertree of the angiosperms, Proceedings of the National Academy of Sciences of the United States of America, 2004.
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