Overview

Changes in the environment of the early Earth drove the evolution of organisms. As prokaryotic organisms in the oceans began to photosynthesize, they produced oxygen. Eventually, oxygen saturated the oceans and entered the air, resulting in an increase in atmospheric oxygen concentration, known as the oxygen revolution approximately 2.3 billion years ago. Therefore, organisms that could use oxygen for cellular respiration had an advantage. More than 1.5 years ago, eukaryotic cells and multicellular organisms also began to appear. Initially, all of these species were restricted to the oceans of Earth.

The first organisms to live on land were photosynthetic prokaryotes that inhabited moist environments near ocean shores. Despite the lack of water, terrestrial environments offered an abundance of sunlight and carbon dioxide for photosynthesis. Around 500 million years ago, the ancestors of nowadays plants were able to colonize drier environments, but they required adaptations to prevent dehydration. They developed methods for reproduction that did not depend on water and protected their embryos from drying out. These early plants furthermore evolved a vascular system that included roots to acquire water and nutrients and a shoot to obtain sunlight and carbon dioxide.

Plants and fungi appear to have colonized land at the same time. Their coevolution onto land is the result of the mutually beneficial relationship between many plants and fungi, seen in both modern organisms and some of the earliest plant fossils; Fungi aid in the absorption of nutrients and water while benefiting from the nutrients provided by the plant. 

Arthropods were the first animal species to colonize land, around 450 million years ago. The first tetrapods later evolved to live on land as well, finding an abundance of food in the plant species that had colonized the land. Amphibians dominated terrestrial animal life for 100 million years. Later, dinosaurs and then mammals would become the most abundant terrestrial animals.

Procedure

Over three billion years ago, lifeforms could only survive in the oceans. Some prokaryotes had begun to capture carbon dioxide to produce sugars, a process called photosynthesis. The photosynthetic organisms released oxygen, which accumulated in the atmosphere.

How did the aquatic organisms evolve to inhabit land? Single-celled photosynthetic prokaryotes first populated wet terrestrial areas over two billion years ago. In contrast to the environment of the ocean, terrestrial environments provided plenty of sunlight and carbon dioxide, two crucial components for photosynthesis. Multi-cellular plants and fungi gradually evolved the ability to live on land by the Ordovician Era around 485 million years ago. One of the key adaptations for terrestrial life was the specialization of cells, initially separating a chute that acquires light from root-like structures that anchor the plant and facilitate the absorption of water and nutrients. Additional structural changes contributed to the success of land-dwelling plants.

For instance, a waxy cuticle limited evaporation. A cell wall and vascular tissue provided structural support and facilitated water transport within 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 were the predecessors to today's insects and crustaceans. Tetrapods, animals with four legs, evolved approximately 400 million years ago. The evolution of lungs and an internal skeleton in their fish ancestors were crucial to the massive spread of this group that eventually comprised amphibians, reptiles, birds, and mammals. Both tetrapods and arthropods found an abundance of food resources in the plants that inhabited land at that time. In this sequential process, plants, fungi, and animals colonized the landmasses of the earth, and eventually evolved into the species that we know today.