33.7
The Colonization of Land
Changes in the environment of the early Earth drove the evolution of organisms. As prokaryotic organisms in the oceans began to photosynthesize, they…
Over three and a half billion years ago, lifeforms could survive only in aquatic environments. Later, some prokaryotes, such as cyanobacteria, began fixing carbon dioxide to produce sugars through photosynthesis.
These photosynthetic organisms released oxygen, and it accumulated in the atmosphere.
Evidence suggests that single-celled photosynthetic microbes colonized moist terrestrial surfaces more than two billion years ago.
While land environments provided abundant sunlight and carbon dioxide, two crucial components for photosynthesis, they also presented challenges such as desiccation, UV exposure, and lack of structural support.
Multicellular plants and fungi gradually evolved the ability to live on land around 470 million years ago.
This transition required several adaptations. Some cells formed shoots that capture light, while others formed root-like structures that anchor the plant and absorb water and nutrients.
Other structural changes also helped plants survive on land. For example, a waxy cuticle limited evaporation.
Cell walls provided structural support, while vascular tissue helped move water through larger plants.
Around 450 million years ago, arthropods were the first animals to evolve the ability to live on land.
These organisms had external skeletons and belonged to the group that includes modern insects, arachnids, and crustaceans.
Early tetrapods evolved around 380 million years ago. Their lobe-finned fish ancestors had features like lungs and stronger limb bones that helped them move from water onto land. They later diversified into amphibians, reptiles, birds, and mammals.
Plants that had already colonized land formed the base of early terrestrial food webs, supporting both arthropods and tetrapods. Through this gradual process, plants, fungi, and animals colonized Earth’s landmasses and eventually evolved into the diverse species that exist today.
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Q1: Why did early life remain in oceans for billions of years?
Early organisms lacked adaptations to survive terrestrial challenges. Land environments presented desiccation, UV exposure, and lack of structural support that aquatic life could not withstand. Only after photosynthetic prokaryotes produced oxygen and atmospheric oxygen accumulated could organisms develop the physiological and structural adaptations necessary for land colonization.
Q2: What adaptations allowed plants to survive on land?
Plants evolved a waxy cuticle to limit evaporation, cell walls for structural support, and vascular tissue to transport water through larger organisms. They also developed root-like structures to anchor and absorb water and nutrients, plus shoots to capture light. These structural changes enabled plants to thrive in drier terrestrial environments around 470 million years ago.
Q3: How did the oxygen revolution influence terrestrial colonization?
Photosynthetic prokaryotes released oxygen that accumulated in the atmosphere approximately 2.3 billion years ago. This oxygen revolution allowed organisms capable of cellular respiration to gain a survival advantage. Increased atmospheric oxygen made terrestrial environments more habitable, supporting the eventual evolution of multicellular organisms that could colonize land.
Q4: What was the relationship between early plants and fungi on land?
Plants and fungi colonized land simultaneously through a mutually beneficial coevolution. Fungi aided plants in absorbing water and nutrients from soil, while plants provided fungi with sugars from photosynthesis. This symbiotic relationship, evident in both modern organisms and early plant fossils, was essential for successful terrestrial establishment.
Q5: Which animals were the first to colonize land?
Arthropods were the first animals to evolve the ability to live on land around 450 million years ago. These organisms possessed external skeletons and belonged to the group that includes modern insects, arachnids, and crustaceans. Their structural features provided protection and support necessary for terrestrial survival.
Q6: How did tetrapods transition from water to land?
Early tetrapods evolved around 380 million years ago from lobe-finned fish ancestors that possessed lungs and stronger limb bones. These features enabled movement from aquatic to terrestrial environments. Tetrapods later diversified into amphibians, reptiles, birds, and mammals, exploiting abundant food sources provided by plants that had already colonized land.
Q7: What role did plants play in early terrestrial food webs?
Plants that colonized land formed the foundation of early terrestrial food webs, supporting both arthropods and tetrapods. As primary producers, plants provided essential energy and nutrients for herbivorous animals. This established food web structure enabled the diversification and dominance of animal species on land, demonstrating the evidence for evolution and common ancestor relationships among terrestrial organisms.