When plants moved ashore and changed the planet Understand article

Plants today are extremely diverse, abundant, and flamboyant. However, the first land plants, which initiated a great change in the flora and fauna on planet Earth, were very different.

In the beginning of the Palaeozoic era, 500 million years ago, the surface of the Earth was covered with water and barren rock and was uninhabitable to modern life forms.  The atmosphere contained 20 times more carbon dioxide but only half as much oxygen as today. For large animals living on Earth today, such conditions would be bad news – mitochondria, the powerhouses of the body, need oxygen to provide the energy for life. During the Palaeozoic era, there was not enough oxygen available to satisfy these energy requirements and thus the animals of the early Palaeozoic were small. They also mostly lived in aquatic environments because water filtered out harmful UV radiation. The ozone layer that today protects us from UV radiation requires oxygen to form. It starts with a process called photolysis, where the two oxygen atoms that make up an oxygen molecule (O₂) are split apart by energy from sunlight. Then a single oxygen atom combines with another O₂ molecule and ozone (O₃) is formed:
O + O₂ → O₃
In the early Palaeozoic, not enough oxygen was available to allow the buildup of ozone.

Photosynthesis on land pushed up atmospheric oxygen levels

287 million years later, towards the end of the Palaeozoic era, mid-sized to large animals were thriving in both aquatic and terrestrial habitats. What changed?

The evolution of large animals on land was made possible by the depletion of atmospheric carbon dioxide and the accumulation of atmospheric oxygen through photosynthesis, a process by which chloroplast-bearing organisms convert carbon dioxide and water into glucose and oxygen by using energy from sunlight. At the beginning of the Palaeozoic era, photosynthesis was carried out by cyanobacteria and green algae that lived in water. Green algae were single-celled, colonial, or filamentous, and lacked features for survival out of water. Because of their small size and their immersion in water, where carbon dioxide and sunlight are diluted, their photosynthetic activity was low.

Around 480 million years ago, a major transition happened that promoted photosynthetic activity on Earth – freshwater algae (charophytes) evolved features that enabled them to survive out of water. As a result, they had access to more sunlight and carbon dioxide and could photosynthesize more efficiently, producing more oxygen. The first plants that were able to grow on land had root-like structures called rhizoids for anchorage on sedimented rock, and a waxy and impermeable top layer (a cuticle) that protected them from drying out. Most of the first land plants later went extinct, but from the fossil record it is clear that they were small and lacked true stems, roots, leaves, and flowers.

What did the first land plants look like?

After the last common ancestor of all land plants appeared, two lineages evolved from it – vascular plants and non-vascular plants. The first group makes up the majority of Earth’s biomass today and is characterized by the presence of a vasculature, a water-conducting system that carries water taken up by the roots into the aerial parts of the plant. This allowed these land plants to grow very tall. A few hundred million years later, some vascular plants evolved flowers and these became the flowering plants that we are so familiar with today.

Non-vascular plants, on the other hand, grow close to the ground and in moist environments because they rely on the less efficient processes of diffusion and capillary action for the distribution of water. These two land plant lineages went on to follow their own evolutionary paths at different rates.^[3] The clade of non-vascular land plants that still exist today is called the bryophytes.

Because bryophytes have retained more ancestral characters (i.e., have changed less) than vascular plants in the course of their evolution, researchers study them to understand which essential adaptations had to evolve to enable the transition from water to land. The two most studied species are the moss Physcomitrium patens and the liverwort Marchantia polymorpha.^[4] Unless we find fossils that convince us otherwise, these bryophytes might be the closest we can come to visualising the first land plant. Their wild habitats are damp and often temperate – you might encounter them on the banks of streams or around the edges of pools of water. This dependence on moist environments for growth and reproduction is inextricably linked to their aquatic ancestry.

The first adaptations to life on land

Due to the lack of a fossil record and our current inability to match existing fossils to early groups of land plants, we do not know with certainty what the common land-plant ancestor looked like. However, from studying bryophytes we know that the following adaptations were among the first to evolve for plants to thrive and reproduce quickly on land.^{[2, 5]} Some of them are now shared by all land-plant groups, others have been lost over time in some groups.

• In charophytes, after fertilization of the egg cell by a sperm cell, the zygote falls off the maternal alga and is dispersed in water, which carries the zygote to a place where it will grow into a new plant. In all land plants, however, the zygote develops into amulticellular embryo that is retained within the mother plant. In Marchantia, the embryo develops underneath the ‘fingers’ of the umbrella-like female reproductive structure that is elevated above the plant body by a stalk.

• In the first land plants, the diploid embryo divided through meiosis to produce a vast number of haploid and genetically diverse spores. Spores are unicellular reproductive units that get dispersed in large numbers and grow into haploid plants through mitosis. On land, where fertilization was not facilitated by water, this mechanism ensured the production of many genetically diverse offspring. Fertilization to produce a new diploid embryo only happens after the haploid plants mature and develop sexual reproductive organs.

Nowadays, all bryophytes and some vascular plant groups, such as ferns, still reproduce via spores. Seeds, which are multicellular reproductive dispersal units, only evolved later in the vascular lineage.

• Land plants evolved flavonoids, secondary metabolite molecules that accumulate in the top layers of plant tissue and absorb UV radiation. In the absence of water as a filter for UV light, they act as a ‘sunscreen’ to protect from UV-induced DNA damage.
• All land plants have meristems with stem cells that allow them to grow directionally to explore the terrestrial environment. The stem cells have multiple division planes enabling three-dimensional growth and the development of multilayered, specialised organs. Meristems therefore enable land plants to direct growth in an environment that is much more spatially variable than water.
• Among such specialised organs are air pores or stomata that facilitate the exchange of carbon dioxide and oxygen without water as a transporting agent. 
• The first land plants developed rooting structures, which are specialised cells or organs for anchorage and water and nutrient uptake. These structures are simple rhizoids in non-vascular plants and complex, multilayered roots in vascular plants.
• Land plants undergo symbiosis with fungi to facilitate the uptake of nutrients like phosphate, which is crucial for plant growth. Fungi could break down minerals from the barren rocks of the Palaeozoic and supplied these to the plants in return for carbohydrates and fats.

With these essential features, the ability of plants to survive on land had been established. What followed was a rapid evolution of various lineages of vascular and nonvascular plants. Many vascular plants evolved and perfected incredible features, like tall stems, flowers, succulence, carnivory, and more, to conquer terrestrial habitats and to live and reproduce in dense communities without a requirement for much water. With the evolution of pollen, which is spread by wind and pollinators and not by water, some plant groups truly broke away from their aquatic ancestry.

The increase in atmospheric oxygen resulting from the proliferation of land plants promoted the diversification and spread of land animals. Not only did more oxygen satisfy their energy requirements but it also enabled the formation of a protective ozone layer. Flowering plants later rose to dominance and continue to feed us with the oxygen that we need to live today. Without them and their flowerless, primitive ancestors, our planet would likely be very different.

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