Plants have lived on land for roughly half a billion years, far longer than dinosaurs or mammals have existed.1PubMed. Evolution of mycorrhiza systems Their story is less a single event than a long series of inventions, each one solving a problem of life out of water. Some of the most familiar plants are surprisingly recent: grass-covered landscapes as we know them took over only in the last few million years.2PubMed. The origins of C4 grasslands: integrating evolutionary and ecosystem science Along the way, plants reshaped the air, the soils and even the oceans.
Where Plants Came From
The closest living relatives of land plants are most likely a group of freshwater green algae that includes common pond scum.3PubMed Central. Charophytes: Evolutionary Giants and Emerging Model Organisms Because they live in fresh water, they hint that plants came ashore from ponds and streams rather than the sea. The more complex-looking stoneworts were long favored as plant ancestors, but they turn out to be more distant relatives.4PubMed Central. Origin of land plants: Do conjugating green algae hold the key?
“Closest relative” does not mean ancestor. Today’s pond algae are cousins, not grandparents. Both lines kept evolving separately after they split, and the shared ancestor is long extinct and left no clear fossils.
The date of that split is old and uncertain. Genetic estimates place it at roughly 725 million years ago in one analysis.5PubMed. Dating the early evolution of plants: detection and molecular clock analyses of orthologs Another analysis puts it near 912 million years ago, with a range of about 870 to 962 million.6PubMed. Land plant evolutionary timeline: gene effects are secondary to fossil constraints in relaxed clock estimation of age and substitution rates
The Move Onto Land About 470 to 500 Million Years Ago
The living plant groups had begun branching apart by about 475 million years ago.7PubMed. Land plant evolutionary timeline: gene effects are secondary to fossil constraints in relaxed clock estimation of age and substitution rates By then, the ancestors of mosses and of today’s vascular plants were already separate lines.
These pioneers were small, low, moss-like plants hugging damp ground. They had no true roots, leaves, flowers or wood. The land before them was mostly bare rock and crusts of microbes.
Waterproofing was the first problem. Out of water, a plant dries out fast and is exposed to strong sunlight. Early land plants evolved waxy, water-repellent materials that cut water loss, block germs and shield against harmful UV light.8PubMed. The evolution of hydrophobic cell wall biopolymers: from algae to angiosperms One such coating, the cuticle, is the waxy film still found on leaves today.9PubMed. The evolution of hydrophobic cell wall biopolymers: from algae to angiosperms
A sealed surface creates a new problem, since the plant still has to breathe. Stomata, tiny leaf pores that open and close, were a key invention for life on land.10PubMed. The origin and evolution of stomata They let the plant take in carbon dioxide for food while limiting water loss.11PubMed. The origin and evolution of stomata The ability to open and close them was already present before mosses and vascular plants split.12PubMed. The origin and evolution of stomata
Fungi were partners from the start. Root fungus partnerships appeared at the same time as the first land plants and persist in most plants today.13PubMed. Evolution of mycorrhiza systems Some scientists argue these fungi made the move onto land possible.14Global Soil Biodiversity Initiative. Understanding the evolution of the mycorrhizal symbioses: a matter of phylogenetics or paleontology? Fungal threads reach into the soil far beyond the plant and deliver water and minerals such as phosphorus. The plant pays with sugars it makes from sunlight.
Veins, Leaves and Standing Upright
Vascular plants have tubes that carry water, strengthened by lignin, a tough material that stiffens wood.15PubMed Central. Evolution of xylem lignification and hydrogel transport regulation Water evaporating from the leaves pulls water up through the tubes the way sipping on a straw does. That pull creates suction strong enough to crush weak tubes. Lignin works like the ribbing in a vacuum hose, keeping the tubes open and letting plants stand upright as leaves pull water up.16PubMed. The origin and evolution of lignin biosynthesis Without it, plants stayed short.
Leaves evolved independently in several plant groups rather than once.17PubMed. Megaphylls, microphylls and the evolution of leaf development Club mosses grew their own small leaves separately from ferns and seed plants.18PubMed. Challenging the paradigms of leaf evolution: Class III HD-Zips in ferns and lycophytes The different leaf types may still borrow the same set of genes first used to build spore-making structures.19PubMed. Challenging the paradigms of leaf evolution: Class III HD-Zips in ferns and lycophytes
Club mosses have tiny leaves with a single vein. Ferns and most garden plants have broad leaves with branching veins. The two leaf types do similar work, but they grew from different starting points.
Wood and the First Forests Around 400 Million Years Ago
Trees exist because some plants learned to grow wider each year, not just taller.20PubMed. Evolution of development of vascular cambia and secondary growth A thin layer under the bark adds new wood each growing season, which is also what produces tree rings. Thicker trunks can support greater height. Many plants, such as grasses, never do this.
The earliest evidence of this thickening growth dates to about 407 million years ago.21Digital Commons @ Cal Poly Humboldt. Exploring the origins of secondary growth – two Lower Devonian (Emsian) euphyllophytes from Gaspé, Canada, and their implications for the evolution of secondary growth Within a few million years it had spread to all major groups of these plants by the middle of the Devonian period.22Digital Commons @ Cal Poly Humboldt. Exploring the origins of secondary growth – two Lower Devonian (Emsian) euphyllophytes from Gaspé, Canada, and their implications for the evolution of secondary growth The same ability produced both massive forest trees and woody climbing vines.23PubMed. Evolution of development of vascular cambia and secondary growth
The Devonian lasted from roughly 419 to 359 million years ago. Its trees were relatives of ferns and club mosses, not oaks or pines, and they reproduced with spores rather than seeds. These forests created the first deep soils and the first shade on land. Tree form evolved repeatedly in unrelated groups, and palm, pine and oak trees are not closely related.
Seeds and the Rise of Conifers
A seed combines several features, including protective coats and pollen, that seem to have arisen separately before coming together.24PubMed Central. Origins of the seed: The “golden-trio hypothesis” One genetic estimate dates the split between conifer-type seed plants and flowering plants to about 385 million years ago.25PubMed. Dating the early evolution of plants: detection and molecular clock analyses of orthologs
Swampy forests around 300 million years ago were ruled by spore plants and early seed plants.26Annual Review of Earth and Planetary Sciences. Response of Late Carboniferous and Early Permian Plant Communities to Climate Change Then a drying global climate shifted the world toward seed plants, first in cooler regions and then in the tropics.27Annual Review of Earth and Planetary Sciences. Response of Late Carboniferous and Early Permian Plant Communities to Climate Change
Spores are single cells with little food. A seed carries a baby plant plus a food supply, and a hard coat lets it wait for rain. Pollen removes the need for water to fertilize the plant.
You can still see seed plants without flowers: pines, firs, cycads and ginkgo. Their seeds sit exposed, often on cone scales. They were dominant before flowering plants arrived.
Flowers and Fruit in the Age of Dinosaurs
The flower’s key part is the carpel, which holds seeds and becomes fruit.28PubMed. The origin and evolution of carpels and fruits from an evo-devo perspective Exactly how the carpel arose is still one of plant science’s great unsolved puzzles.29PubMed. The origin and evolution of carpels and fruits from an evo-devo perspective The earliest flowers were probably small, with both male and female parts, and carpels sealed by a sticky secretion.30Journal of Experimental Botany. An evolutionary perspective on the regulation of carpel development
A cone exposes its seeds on scales, while a flower encloses them inside the carpel, and the enclosed seed becomes a fruit. Petals attract pollinators and protect the inner parts.
Insects joined early. By about 100 million years ago, roughly three quarters of flowering plants in one fossil site relied on animals, mostly insects, for pollination.31PubMed Central. Early steps of angiosperm pollinator coevolution Some of those were already matched to specialized pollinators.32PubMed Central. Early steps of angiosperm pollinator coevolution The rise of flowers appears to have boosted new insect pollinator families and protected them from extinction.33PubMed Central. The angiosperm radiation played a dual role in the diversification of insects and insect pollinators
Fruit spread the seeds. Fleshy fruits are mostly spread by animals, while dry fruits are often spread by wind.34Journal of Systematics and Evolution. Evolution of fruit types and seed dispersal: A phylogenetic and ecological snapshot Fruit types seem to have evolved to suit the dispersers common in each habitat.35Journal of Systematics and Evolution. Evolution of fruit types and seed dispersal: A phylogenetic and ecological snapshot Berries feed birds that drop seeds elsewhere, and forests have more birds and mammals to carry fleshy fruit. Wet tropical forests shaped by flowering plants may also have helped ferns diversify, including those living on tree branches.36International Journal of Plant Sciences. Ferns in an Angiosperm World: Cretaceous Radiation into the Epiphytic Niche and Diversification on the Forest Floor
Why Flowering Plant Dates Are Still Debated
Genetic age estimates agree with fossils for all major land plant groups except flowering plants.37PubMed. Land plant evolutionary timeline: gene effects are secondary to fossil constraints in relaxed clock estimation of age and substitution rates One genetic estimate puts the split within flowering plants at about 301 million years ago.38PubMed. Dating the early evolution of plants: detection and molecular clock analyses of orthologs In some fossil-rich regions, though, genetic clocks give much younger ages than the rock dates.39Penn State University. Turn back the molecular clock, say Argentina’s plant fossils
A genetic clock works because DNA changes build up slowly over generations. Counting the differences between two species estimates how long ago they split. Fossils of known age calibrate the clock, but if the rate of change varies, the dates drift.
Grasslands and Other Recent Innovations
C4 photosynthesis is a way of capturing carbon that works better when carbon dioxide is scarce. It arose about 25 to 32 million years ago, linked to low carbon dioxide levels.40PubMed Central. Nature’s green revolution: the remarkable evolutionary rise of C4 plants Even so, C4 grasses rose to dominate large areas only 3 to 8 million years ago.41PubMed. The origins of C4 grasslands: integrating evolutionary and ecosystem science
Corn, sugarcane and many lawn and prairie grasses use it, and they thrive in hot, sunny places. Wheat and rice mostly do not.
Other adaptations arrived along the way. Meat-eating plants evolved in wet, open, nutrient-poor places such as bogs.42Oxford University Press. Evolution of carnivory in angiosperms Plants have also repeatedly doubled their entire genome, giving spare genes for new functions.43PubMed Central. Pervasive genome duplications across the plant tree of life and their links to major evolutionary innovations and transitions Genome doubling alone does not cause new varieties, though; environmental change seems needed too.44PubMed Central. Nested whole-genome duplications coincide with diversification and high morphological disparity in Brassicaceae
How Plants Changed the Planet
Plants and their fungi may have up to roughly doubled the rate of chemical breakdown of rock.45PubMed Central. Evaluating the effects of terrestrial ecosystems, climate and carbon dioxide on weathering over geological time: a global-scale process-based approach That matters because the breakdown draws carbon dioxide from the air. Some scientists argue that plants overall slowed rock breakdown, because burying plant carbon in swamps did the work instead.46Geology. Land plant evolution decreased, rather than increased, weathering rates
Coal is that buried carbon. Ancient swamp forests fell and were buried, and over millions of years they became coal. Burning coal returns that old carbon to the air.
The effects reached the sea. In the Devonian, rivers may have carried extra nutrients from weathered land into shallow seas.47Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences. Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events This may have fed algal blooms that stripped oxygen from deep waters.48Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences. Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events