The first colonization of land began during the Archean Eon, more than 3 billion years ago, when microbial mats spread across bare continental surfaces. But the large-scale colonization most people picture, with plants and animals establishing themselves on land, took place during the Paleozoic Era, roughly 480 to 360 million years ago. This wasn’t a single event. It unfolded over hundreds of millions of years, with microbes arriving first, then plants, then invertebrates, and finally vertebrates.
Microbes Came First, Billions of Years Ago
Long before any plant or animal existed, photosynthetic microbes formed thin, crusty mats on exposed rock and soil. Evidence from ancient soils and rock weathering patterns points to microbial life on the continents during the Archean Eon, which spans roughly 4 to 2.5 billion years ago. These organisms were similar to modern cyanobacteria, and they were already producing oxygen through photosynthesis by at least the Mesoarchean Era (around 3.2 to 2.8 billion years ago).
These microbial mats were sparse. Researchers estimate that covering just 0.02% of the late Archean land mass with photosynthetic mats would have been enough to produce a measurable sulfur flux into the atmosphere and oceans. At 1% coverage, these mats could sustain a sulfur cycle comparable to modern levels. So even a thin microbial presence had outsized chemical effects, linking land and ocean biogeochemistry billions of years before anything resembling a plant existed. These “terrestrial microbial jungles” dominated the continents through the entire Proterozoic Eon, until plants finally displaced them around 470 million years ago.
Plants Reached Land in the Ordovician
The leap from aquatic algae to land plants is one of the most important transitions in Earth’s history. Molecular clock estimates suggest the ancestors of land plants (embryophytes) originated around 500 million years ago during the Cambrian, but for decades, the oldest plant fossils came from the Devonian, about 420 million years ago. That gap narrowed significantly with the discovery of fossilized spores in Ordovician deposits in Australia dating to approximately 480 million years ago. These spores show features intermediate between confirmed land plant spores and earlier algal forms, providing a bridge between aquatic ancestors and true terrestrial plants.
By the Middle Ordovician, dispersed microfossils confirm that a cryptic flora of small, simple plants had established itself on land. These early colonizers were non-vascular, resembling modern liverworts and mosses. They lacked roots and grew low to the ground, relying on direct contact with moist surfaces to absorb water. The oldest accepted land plant megafossils, small rhyniophytoid plants, appear in Silurian rocks from the Wenlock age, around 430 million years ago.
Fungi played a critical role in this transition. Fossil evidence shows that fungi closely associated with liverworts existed from the Ordovician onward. These partnerships helped early plants extract nutrients from bare rock and soil. One famous organism from the Early Devonian, called Prototaxites, could reach over 8 meters long and more than a meter in diameter. It was likely formed from rolled-up mats of liverworts with fungal and cyanobacterial partners rather than being a single giant fungus, as once thought. In a world with no trees, these structures towered over everything else on land.
Key Adaptations That Made Land Life Possible
Water supports organisms in ways that air does not. It prevents drying out, shields against ultraviolet radiation, and provides buoyancy. Moving to land required solving all of those problems at once.
Plants evolved a waxy outer coating called a cuticle that sealed their surfaces against water loss. They also developed stomata, tiny adjustable pores on their surfaces that could open to absorb carbon dioxide for photosynthesis and close to conserve moisture. The fossil record shows stomata-like pores on land plant surfaces over 400 million years ago. Together with root-like anchoring structures and eventually internal plumbing (vascular tissue) to transport water upward, these innovations let plants grow taller and spread into drier environments.
Atmospheric conditions also had to cooperate. By the Late Silurian, around 420 million years ago, oxygen levels had risen to at least 15 to 17% of the atmosphere. We know this because fossil charcoal first appears at that time, and natural fires cannot sustain themselves below roughly that threshold. Before oxygen rose high enough, the ozone layer would have been too thin to block lethal ultraviolet radiation, keeping complex life confined to water or to sheltered, damp habitats on land.
Arthropods Were the First Land Animals
The earliest animals on land were not fish dragging themselves ashore. They were arthropods: millipede-like creatures, early arachnids, and other invertebrates with exoskeletons that already provided some protection against drying out.
Molecular and fossil evidence suggests that the three major groups of terrestrial arthropods (myriapods, hexapods, and arachnids) may have invaded land as early as the Cambrian or Ordovician. The oldest fossil assemblage preserving clearly terrestrial arthropods comes from the latest Silurian, around 420 million years ago, at a site called Ludford Lane in Shropshire, England. That assemblage includes centipedes, early relatives of millipedes called arthropleurids, and a spider-like arachnid called a trigonotarbid. These animals lived among the low-growing plants and decomposing organic matter of the earliest land ecosystems.
Vertebrates Followed in the Devonian
Vertebrates were latecomers to land. The Devonian period, sometimes called the “Age of Fishes,” saw the transition from lobe-finned fish to the first four-legged land animals (tetrapods). The most famous transitional fossil is Tiktaalik roseae, which lived about 385 million years ago. It had fins, not legs, but its skeleton shows features that foreshadow walking: a sturdy pelvis nearly as large as its shoulder girdle, expanded hip bones, and specialized sacral ribs that likely connected to the pelvis through ligaments, providing rear-body support.
Tiktaalik also had increased mobility at the junction between its skull and spine, meaning it could move its head independently of its body, something fish in open water don’t need to do. These adaptations for supporting body weight, buttressing the hind fins, and freeing head movement all evolved while the animal still had fins, before true limbs appeared. The first fully terrestrial tetrapods, animals that spent their entire lives on land, arrived in the early Carboniferous period, roughly 360 to 340 million years ago.
How Land Life Reshaped the Planet
The colonization of land was not just a biological milestone. It fundamentally changed Earth’s atmosphere and climate. Early land plants, even shallow-rooted species without deep root systems, accelerated the chemical breakdown of continental rock. This process pulled carbon dioxide out of the atmosphere and locked it into ocean sediments.
Before forests appeared around 385 million years ago, atmospheric CO2 sat at roughly 525 to 715 parts per million, already low enough that Earth may have been partially glaciated. Earlier in the Paleozoic, CO2 levels were likely 10 times higher than today’s concentration or more. As plants spread and diversified through the Devonian and into the Carboniferous, they drove a long decline in CO2 that pushed Earth from greenhouse conditions into an icehouse state, culminating in the vast Permo-Carboniferous glaciation. The very organisms that conquered the land ended up cooling the planet that made their existence possible.