Life on land refers to the diverse assembly of organisms inhabiting terrestrial ecosystems, ranging from microscopic fungi to towering redwood forests and complex animal communities. This planetary layer of living matter, known as the terrestrial biosphere, covers nearly one-third of Earth’s surface. The proliferation of life across this expanse established the foundational conditions necessary for modern existence. Terrestrial life forms have fundamentally reshaped the planet’s geology and chemistry, creating the environment they occupy and setting the stage for human civilization.
The Evolutionary Transition
The colonization of land was one of the most profound evolutionary events in Earth’s history, unfolding over hundreds of millions of years. The first life forms to venture onto land were likely microbes, such as cyanobacteria, perhaps forming mats on surfaces as far back as \(2.7\) billion years ago. The major shift that transformed the planet began around 500 million years ago with the arrival of plants and fungi.
Plants, in symbiotic partnership with fungi, were the first complex multicellular organisms to establish widespread terrestrial communities. Fungi aided in nutrient uptake for early plants by breaking down minerals in rocks. Following the establishment of this plant-based food source, invertebrates began their transition, with arthropods, including early insects and spiders, colonizing the land by approximately 420 million years ago.
The final major group to make the leap were the vertebrates, beginning around 390 million years ago. This transition involved the evolution of the first tetrapods, shifting from aquatic animals with fins to four-limbed creatures capable of terrestrial locomotion. The success of these pioneering groups fundamentally altered the structure of the terrestrial world, allowing for the subsequent explosion of biodiversity.
Key Adaptations for Terrestrial Survival
Life moving from water to air encountered three primary physical hurdles: desiccation, lack of buoyancy, and a different medium for gas exchange. Overcoming desiccation, or drying out, required biological innovations to retain internal moisture. Plants evolved a waxy, waterproof coating called the cuticle, which limits water loss from leaves and stems. Animals developed thick, keratinized skin or a hard exoskeleton to protect their moist internal tissues from the dry atmosphere.
The loss of water’s buoyancy necessitated the development of internal structural support to withstand gravity. Plants met this challenge through the evolution of lignin, a complex polymer that reinforces cell walls, allowing for upright growth and the formation of wood. Vertebrates developed robust internal skeletons, particularly in the limbs and pelvic girdle, to provide leverage and support for movement on a solid surface.
A different respiratory system was also required, as gills are ineffective out of water. Vertebrates evolved lungs, which provide a large, moist internal surface area for the diffusion of oxygen from the air directly into the bloodstream. This adaptation was accompanied by the eventual pairing of lungs, increasing the surface area for gas exchange and facilitating sustained activity on land. Insects evolved a system of tubes called the trachea, which delivers oxygen directly to internal tissues, representing an effective solution for air breathing.
Global Ecological Impact
The establishment of terrestrial life fundamentally transformed Earth’s entire operating system, extending its influence far beyond local ecosystems. Plants and fungi are the primary agents of pedogenesis, the process of soil formation. Fungi and microorganisms initiate the biological weathering of rocks by excreting organic acids that help break down minerals. When plants and microorganisms die, their organic residue accumulates and mixes with weathered minerals, creating the nutrient-rich topsoil known as humus.
The proliferation of land plants also became the dominant force in regulating the planet’s atmospheric composition. Through photosynthesis, terrestrial flora absorb vast amounts of atmospheric carbon dioxide, converting it into biomass and sequestering the carbon in soils and wood. This process acts as a massive carbon sink, playing a major role in stabilizing the global climate by modulating greenhouse gas concentrations.
Terrestrial ecosystems influence local and global climate patterns through processes like evapotranspiration and the regulation of surface temperature. Forests, for example, release large amounts of water vapor, which contributes to regional rainfall and moderates temperature extremes. The health of these land-based systems directly determines the efficiency of biogeochemical cycles, including the movement of nitrogen and phosphorus, which are necessary for all life.
Human Dependence on Terrestrial Ecosystems
Human societies rely on terrestrial ecosystems for nearly all provisioning and support services, anchoring global food security and economic stability. Plant life, primarily sourced from terrestrial agriculture, constitutes about 80% of the human diet. Domesticated animals that provide meat, dairy, and labor are also integrated into these land-based systems, emphasizing the central role of soil fertility and ecosystem health in feeding billions.
Beyond food, terrestrial environments provide a wealth of raw materials essential for industry and daily life. Forests are the source of timber for construction and paper products, while natural fibers like cotton and flax originate from cultivated land plants. The immense biodiversity of terrestrial life represents an irreplaceable genetic library for human medicine and science. A significant portion of the most prescribed pharmaceuticals in the United States, around 57%, are derived from biological resources, highlighting the potential for discovering new treatments.