How Plants Conquered the Land: Plant evolution history

Plants are the basis of life on Earth, providing oxygen, food, medicine, and habitats for countless animals. But plants did not always dominate the land. In fact, they originated from aquatic ancestors that colonized the terrestrial environment millions of years ago.

The First Land Plants: algae

The first organisms to colonize the land were algae. The shallow areas of the coastline transition between wet and dry conditions due to tides. Algae developed new mutations that allowed them to survive in such an environment. By analyzing the DNA of the cell nucleus, mitochondria and chloroplasts, it has been determined green algae called charophytes are the closest relatives of plants. They appeared on land about 600 million years ago, during the Precambrian era. They helped to create soil by producing acids that eroded the rocks. These algae were considered semi-aquatic plants, as they still depended on water for reproduction and dispersal.

The First True Land Plants: Mosses

The first true land plants were mosses that evolved from green algae during the Cambrian period, about 510 million years ago. They had simple structures, such as short filamentous roots (rhizoids) that anchored them to the soil and had a weak absorption capacity. They also had stem and leaves that contain chloroplasts for photosynthesis. Their stems and leaves had a cuticle that prevents water loss. Moss did not have vascular tissues, which are specialized cells that transport water and nutrients, and support the whole plant. Therefore, they were short, non-vascular plants that preferred moisture and shade. Their ability to withstand drought and to absorb water were better than algae. Mosses were not semi-aquatic plants, but they still had to live near water. They secreted acids that continue to erode rocks to produce soil.

The First Vascular Plants: Ferns and First Forests

Due to intense crustal movements around 420 million years ago, the seabed rose to become land. Algae that once thrived in seawater were forced to be exposed to the air. Many algae perished, but a certain type of green algae managed to survive on land over time, gradually evolving into the first wave of terrestrial vascular plants. Primitive vascular tissues for support and water absorption allowed them to grow to around 1 meter in height. They lacked differentiated roots and leaves, resembling bare branches. They harnessed photosynthesis through these green branches. Following the successful colonization of land by these primitive ferns, a variety of ancient fern plants gradually diversified. With xylem and vascular structures, these fern plants grew remarkably large, reaching heights of over ten meters. Not only did ferns have tall trunks, but they also developed true roots and leaves. The cuticle layer and stomata on the leaf surface prevented water loss. Although seedless vascular plants better adapted to terrestrial conditions than algae, such as greater height, wider distribution, and better water conservation, ferns still required the assistance of dew or rainwater for their sperm to reach the female gametophyte. Hence, they continued to live near water sources.

The first forests were composed of seedless vascular plants, such as ferns and clubmosses. These plants dominated the land during the Devonian period, about 360 million years ago. They formed large and diverse ecosystems that supported many animals, such as insects, amphibians, and early reptiles. These first primeval forests continuously photosynthesized to release oxygen. The oxygen content of the Earth's atmosphere reached over 30% at that time. Fallen leaves and plant bodies were buried deep underground to form the coal we use today.

Frequently Asked Questions

What are the benefits and challenges of plants colonizing land?

Plants left aquatic environments and ventured onto land due to the rising of the seabed. This new world offers significant advantages for plants. There is more abundant sunlight and a richer carbon dioxide on land than in water. Herbivores and pathogens are relatively scarce here, and there is no competition from other algae. Soils along water sources contain abundant minerals. However, there are substantial challenges. The land is drier, making plants susceptible to dehydration. There's no buoyancy from water to counteract gravity, and solar ultraviolet radiation is more intense.

How do plants overcome challenges on land?

Plant leaves have stomata on their surfaces to regulate the entry and exit of water. The leaf surfaces also covered a cuticle layer to prevent water evaporation. To combat UV radiation, some plants produce protective pigments like carotenoids. The woody tissue in the stems, primarily composed of lignin, provides high strength to withstand gravity.