First Land Plants: How Plants Colonised the Land

The first land plants appeared roughly 470 million years ago, during the Ordovician Period, when descendants of freshwater green algae began to grow on damp ground. Over the following 80 million years plants evolved stems, water-conducting tissues, roots and finally trees, transforming bare landscapes into vegetated ones. Their spread created soils, altered the chemistry of the atmosphere and oceans, and made it possible for animals to live on land.

Background: a bare land

For most of Earth’s history the continents were barren. Microbial mats and crusts of bacteria and algae may have coated damp surfaces for hundreds of millions of years, but there were no plants rooted in soil.

Life on land posed serious challenges for organisms adapted to water. They had to avoid drying out, support themselves against gravity without the buoyancy of water, obtain water and nutrients from the ground, cope with stronger ultraviolet light, and reproduce without water to carry their cells.

Genetic evidence shows that land plants, or embryophytes, evolved from a group of freshwater green algae called charophytes. Their closest living relatives appear to be the Zygnematophyceae, simple algae often found in ponds and on damp surfaces.

The earliest evidence: spores

The oldest convincing fossils of land plants are not plants at all but their spores. Microscopic spores with tough walls, called cryptospores, appear in Middle Ordovician rocks about 470 million years old, including finds from Argentina reported in 2010. Their structure resembles that of the spores of modern liverworts.

In 2003 Charles Wellman and colleagues described tiny fragments of spore-bearing tissue from Ordovician rocks in Oman, showing that these spores came from plants similar to liverworts. The plants themselves were probably small, flat or low-growing and lived in damp places near water.

Molecular clock studies, which estimate dates from rates of genetic change, suggest land plants may have originated earlier, perhaps in the middle Cambrian around 500 million years ago. The fossil record has not yet confirmed this.

Stems and vascular tissue: Cooksonia

The next major step was the evolution of upright stems and, eventually, vascular tissue, the internal plumbing that carries water and nutrients. Vascular plants could grow taller and survive in drier places.

The best-known early example is Cooksonia, which first appears in Silurian rocks around 430 million years ago. It was a tiny plant, usually only a few centimetres tall, with smooth stems that forked into pairs, each ending in a round spore capsule. It had no leaves or true roots.

Cooksonia was named in 1937 by the British palaeobotanist William Henry Lang in honour of the Australian botanist Isabel Cookson. Well-preserved specimens are known from Wales and the Welsh Borderland, from Bohemia in the Czech Republic and from several other continents. Not all fossils assigned to Cooksonia show clear evidence of vascular tissue, and some may represent intermediate forms.

The Rhynie chert: a snapshot of early land life

The Rhynie chert in Aberdeenshire, Scotland, is one of the most important fossil sites for early land life. It formed around 407 million years ago in the Early Devonian, when hot springs flooded a wetland with silica-rich water that petrified plants and animals where they grew.

The site was discovered by the doctor and geologist William Mackie in 1912, and its plants were described by Robert Kidston and William Henry Lang between 1917 and 1921. The fossils preserve individual cells, so clearly that scientists can see fungi living inside plant tissues, early mites and other arthropods, and even the stomata (breathing pores) on plant stems. The fungal partnerships show that plants relied on fungi to obtain nutrients from the earliest stages, a relationship still found in most plants today.

From small plants to the first forests

During the Devonian, plants evolved leaves, deep roots, wood and seeds. By around 385 million years ago the first forests had appeared, with tree-like plants such as Wattieza, known from Gilboa in New York State, and later Archaeopteris, a woody tree that in some cases may have reached 20 metres or more.

These forests prepared the way for the vast swamp forests of the Carboniferous coal forests, whose buried remains became much of the world’s coal.

Effects on soils, air and climate

Plants did not simply occupy the land; they changed it. Their roots and the fungi associated with them broke rocks apart and released nutrients, building the first true soils. Plants also stabilised river banks, changing rivers from broad braided channels to meandering ones.

Increased chemical weathering of rocks removed carbon dioxide from the atmosphere. In 2012 Timothy Lenton and colleagues proposed that even early, non-vascular plants may have weathered rocks enough to contribute to the cooling that led to the Hirnantian glaciation and the Late Ordovician mass extinction. This hypothesis remains debated. The later spread of deep-rooted trees in the Devonian is more widely accepted as a cause of major falls in carbon dioxide.

Plant growth also contributed oxygen and organic matter, and the nutrients washed into rivers and seas may have influenced ocean chemistry.

Adaptations for life on land

To survive out of water, early plants needed several new features, many of which evolved step by step.

• Cuticle. A waxy outer layer reduced water loss from the plant’s surface.
• Stomata. Small pores, which could open and close, allowed carbon dioxide in for photosynthesis while limiting water loss. They are found in the Rhynie chert plants and in most later land plants.
• Tough spores. Spores protected by a resistant substance called sporopollenin could survive drying and ultraviolet light and be carried by wind. This toughness is why spores are the first plant fossils to appear.
• Protected embryos. All land plants nourish their young embryos on the parent plant, which is why they are called embryophytes.
• Vascular tissue. Tubes of specialised cells, later strengthened with lignin, carried water upwards and supported taller stems.

These innovations allowed plants to grow larger, compete for light and spread from riverbanks and wetlands into drier inland areas. Height also helped plants scatter their spores on the wind, which may help explain why upright stems evolved.

The early plants lived alongside other organisms. Fungi, lichens and bacteria were already present on land, and fossils such as Prototaxites, a trunk-like organism several metres tall that is now usually interpreted as a giant fungus, show that early land ecosystems contained forms with no modern equivalent.

Timeline

• c. 500 million years ago: Molecular estimates suggest land plants may have originated around this time.
• c. 470 million years ago: Earliest widely accepted fossil spores of land plants.
• c. 430 million years ago: Cooksonia appears in Silurian rocks.
• c. 407 million years ago: The Rhynie chert preserves an Early Devonian wetland in Scotland.
• c. 385 million years ago: First forests, including those at Gilboa, New York.
• 1912: William Mackie discovers the Rhynie chert.
• 1937: William Henry Lang names Cooksonia.
• 2003: Fragments of the earliest land plants are described from Oman.

Why it matters

The colonisation of the land by plants was one of the most important changes in the history of Earth. It created soils and terrestrial food chains, reshaped rivers and landscapes, and influenced climate for hundreds of millions of years. Every land animal, including humans, ultimately depends on the ecosystems these first small plants began.