Life moved from the ocean to dry land in stages that began billions of years earlier than most people assume, and once established, terrestrial organisms fundamentally reshaped the planet’s atmosphere, soils, water cycles, and climate. The story is not simply one of creatures crawling ashore; it is a story of microbes, fungi, plants, and animals building an interconnected system that now regulates conditions for nearly all life on Earth. Understanding how land life evolved is inseparable from understanding why it matters, because the very ground we stand on, the air we breathe, and the rain that falls are all, in large part, biological products.
Microbes Came First, and They Came Early
Long before any plant sent roots into soil or any animal dragged itself onto a mudflat, microbial communities were already living on land surfaces. Thin, photosynthetic communities of cyanobacteria, microalgae, and early cryptogams formed what scientists now call biological soil crusts. These living films along the boundary between soil and atmosphere are among the oldest forms of terrestrial life, and their ancestors may have existed as far back as 3.2 billion years ago.1Nature Communications. Possible nitrogen fertilization of the early Earth Ocean by microbial continental ecosystems That date places rudimentary land life deep in the Archean eon, well before oxygen accumulated in the atmosphere.
These ancient crusts were not passive bystanders. By fixing atmospheric nitrogen and converting it into forms usable by other organisms, they began shaping global nutrient cycles when the continents were still largely barren rock. Modeling work suggests that even if microbial communities covered only a few percent of the land surface that exists today, their nitrogen output could have rivaled that of modern continents, feeding nutrients into rivers and eventually into the ocean.1Nature Communications. Possible nitrogen fertilization of the early Earth Ocean by microbial continental ecosystems Biological soil crusts still exist today in deserts and other sparsely vegetated environments, performing many of the same functions: stabilizing surfaces, cycling nutrients, and hosting complex microbial communities that ecologists only recently began studying in earnest.2PubMed. The Microbiology of Biological Soil Crusts
How Plants Conquered Dry Land
Plants did not colonize land alone. Molecular clock studies show that fungi made the transition to land first, roughly 720 million years ago, and that the earliest land plants followed later, with the first true embryophytes diversifying around 482 million years ago.3Nature Communications. Contemporaneous radiations of fungi and plants linked to symbiosis Arbuscular mycorrhizal fungi, the same group that still forms symbiotic relationships with the roots of most living plants, appear to have been present at the very start of plant radiation on land. In other words, the partnership between plants and fungi is not a later add-on; it is foundational. Fungi likely helped early plants absorb minerals from rock and retain water in an environment that was brutally dry compared to the ocean.
Surviving on land demanded biological innovations that aquatic algae never needed. The evolution of a waxy cuticle, a hydrophobic layer covering the outer surface of all land plants, was one of the most important. This coating protects against water loss and shields cells from ultraviolet radiation and other environmental stresses.4PubMed Central. The Formation and Function of Plant Cuticles Equally critical were changes at the molecular level. Streptophyte algae, the closest living aquatic relatives of land plants, already possessed families of water-transport proteins and stress-response proteins that became more elaborate as the lineage moved toward terrestrial life.5PubMed Central. Desiccation tolerance in streptophyte algae and the algae to land plant transition The toolkit for tolerating desiccation was assembled gradually, not invented from scratch the moment a green organism touched soil.
Animals Follow, Front Legs First
Arthropods were the first animals confirmed on land. The fossil record clearly places them there by the Silurian and Devonian periods, roughly 440 to 360 million years ago. But molecular dating hints at something more dramatic: the three major land arthropod groups (myriapods, arachnids, and hexapods, the lineage that includes insects) may have invaded land as early as the Cambrian or Ordovician, potentially a hundred million years before the oldest fossil evidence of complex terrestrial ecosystems.6PubMed Central. Was There a Cambrian Explosion on Land? The Case of Arthropod Terrestrialization Molecular clock analyses support an origin for these lineages somewhere between the Cambrian and the Silurian, with myriapods in particular appearing to have moved onto land well before any trace or body fossils record them doing so.7PubMed Central. A molecular palaeobiological exploration of arthropod terrestrialization If confirmed, this would mean that animal life on land has a much deeper history than textbooks have traditionally taught.
The transition of vertebrates from water to land followed a different path. Early tetrapods did not simply grow legs and walk out of a pond. Their forelimbs evolved first, initially for interacting with the substrate while the animals were still largely aquatic. Adaptations of the humerus and the breathing apparatus came before the evolution of digits, and true weight-bearing specializations appeared later.8Annual Review of Earth and Planetary Sciences. The Fin to Limb Transition: New Data, Interpretations, and Hypotheses from Paleontology and Developmental Biology The hindlimbs lagged behind: compared to the forelimbs, the pelvic appendages of the earliest tetrapods had fewer muscles, especially in the lower leg.9PubMed. Evolution of Hindlimb Muscle Anatomy Across the Tetrapod Water-to-Land Transition, Including Comparisons With Forelimb Anatomy The picture that emerges is of a creature caught between two worlds, with a forelimb locomotion style unlike any living animal’s, shaped by competing selective pressures from aquatic and terrestrial environments.10PubMed Central. Evolution of forelimb musculoskeletal function across the fish-to-tetrapod transition
Plants Rewrote the Atmosphere
Once vascular plants became widespread and evolved into trees during the Devonian period, their impact on the atmosphere was profound. The expansion of deeply rooted forests accelerated the weathering of rock, pulling phosphorus and other nutrients into rivers and oceans. That nutrient pulse fed marine algae, which produced organic matter that, when buried in ocean sediments, drew carbon dioxide out of the atmosphere and released oxygen as a byproduct. Modeling of the Late Devonian shows that this chain of events dropped atmospheric CO₂ while pushing oxygen levels up toward the modern level of about 21%.11Communications Earth & Environment. The expansion of land plants during the Late Devonian contributed to the marine mass extinction
Recent geochemical work broadly supports this timeline, with evidence that the first heavily wooded forests in the Late Devonian were the key driver that raised atmospheric oxygen to modern concentrations and oxygenated the deep ocean.12PubMed Central. Mid-Devonian ocean oxygenation enabled the expansion of animals into deeper-water habitats This is a remarkable fact: the air composition that most complex animal life depends on is, in significant part, a product of trees that evolved hundreds of millions of years ago. But the process had a dark side. The same nutrient runoff that oxygenated the planet also triggered ocean anoxic events, starving marine life of oxygen in deep waters and contributing to mass extinctions in the sea. Land plants gave the planet breathable air while simultaneously helping to suffocate parts of the ocean.
Building Soil From Rock
Soil is not just ground-up rock. It is a biological product, and the rapid formation of the soil we recognize today began about 400 million years ago when vascular plants first evolved roots.13European Journal of Soil Science. RUSSELL REVIEW Are plant roots only “in” soil or are they “of” it? Roots, soil formation and function Roots and their associated microbes break down minerals through a cascade of chemical and biological processes, and they supply carbon deep into the ground where it can persist for centuries. As trees evolved and grew larger during the Late Devonian, root penetration deepened and extended into drier upland areas. This likely caused a permanent increase in the thickness and area of deeply weathered soil profiles across the continents.14GSA Today. Late Devonian oceanic anoxic events and biotic crises: Rooted in the evolution of vascular land plants?
Today, the link between living vegetation and soil health is strikingly direct. A global dataset of nearly 700 paired-site observations found that converting native forest to plantations, grasslands, or croplands reduces carbon storage, slows nutrient cycling, and shifts soil fungal communities away from beneficial symbionts toward pathogens.15PubMed Central. Deforestation impacts soil biodiversity and ecosystem services worldwide Forest and wetland soils tend to carry significantly more organic matter than cropland, and their microbial networks, while simpler than those in farmland, are more stable.16PubMed Central. Impacts of Land Use on Soil Nitrogen-Cycling Microbial Communities The process that land plants set in motion 400 million years ago, converting rock into living soil, can be reversed in a matter of decades by deforestation.
The Pollination Partnership
When flowering plants appeared, they did not spread by wind alone. Angiosperms were ancestrally insect-pollinated, and insects have served as their pollinators for roughly 86% of angiosperm evolutionary history.17PubMed. Insect pollination for most of angiosperm evolutionary history Fossil pollen from the mid-Cretaceous shows that about three-quarters of the angiosperm species at the time relied on animal pollination, with adaptations for generalist insects, specialized pollen-collecting insects, and other pollinators already well represented.18PubMed Central. Early steps of angiosperm pollinator coevolution
The relationship cut both ways. Angiosperms played a dual role in insect diversification: during the Cretaceous, they helped buffer insect lineages against extinction, and during the Cenozoic (after the dinosaur-killing impact), they actively promoted the origination of new insect lineages. A significant radiation of new pollinator groups began as angiosperm diversity increased, becoming especially pronounced after about 50 million years ago.19PubMed Central. The angiosperm radiation played a dual role in the diversification of insects and insect pollinators This reciprocal feedback, where more plant species create niches for more insect species, which in turn pollinate more plant species, is one of the engines behind the extraordinary biodiversity of terrestrial ecosystems today.
Underground Networks That Connect Entire Forests
The fungal partnerships that helped plants colonize land in the first place never went away. Today, most land plants are still connected to mycorrhizal fungi, and those fungi link individual plants to one another through extensive underground networks. These connections are not just passive nutrient pipelines. Experiments have shown that when one plant is attacked by a pathogen or herbivore, other plants connected to the same fungal network can up-regulate their own defenses.20PubMed Central. The evolution of signaling and monitoring in plant-fungal networks
In one well-studied example, bean plants attacked by aphids began producing a volatile chemical that repels aphids and attracts their natural enemies, parasitoid wasps. When the attacked plant was connected to a naïve, unattacked plant through an unbroken mycorrhizal network, the naïve plant began producing the same defensive compound, essentially mimicking a response to an attack it had not experienced. Breaking the fungal connection eliminated the effect.21Journal of Experimental Botany. Harnessing plant-to-plant signalling via common mycorrhizal networks for enhanced community-level resistance in crops How widespread and ecologically significant this warning system is in wild ecosystems is still debated, but the existence of the mechanism is well established. It reframes the way we think about forests: not as collections of competing individuals, but as communities wired together underground.
Carbon Storage on Land
Terrestrial ecosystems are enormous carbon reservoirs, and the biggest stores are not always where you would expect. In boreal forested peatlands, the peat layers beneath the trees hold far more carbon than the trees themselves. Across multiple study sites, peat contained between 4 and nearly 25 times more carbon per square meter than the combined aboveground and belowground tree biomass. Even when the comparison was restricted to carbon accumulated over the same roughly 200-year window, the upper peat layers still held about two and a half times more carbon than the trees growing on top of them.22Scientific Reports. Peat deposits store more carbon than trees in forested peatlands of the boreal biome
Globally, northern peatlands cover roughly 4% of the land surface yet store about 30% of the world’s soil carbon, accumulating it at a long-term average rate of about 24 grams per square meter per year. Under the right conditions, short-term accumulation rates can spike far higher.23Environmental Reviews. Impact of global change and forest management on carbon sequestration in northern forested peatlands How these peatlands are managed matters enormously. On drained peatlands in Finland, shifting from conventional clear-cutting to continuous-cover forestry, which avoids exposing the peat layer, produced a higher carbon sink and lower greenhouse gas emissions throughout the simulation period.24Scientific Reports. Potential of continuous cover forestry on drained peatlands to increase the carbon sink in Finland Peatland management is, in effect, a decision about whether to lock up or release thousands of years of accumulated carbon.
Predators, Prey, and the Nutrient Pool
The influence of land life on planetary systems is not limited to plants and microbes. Animals, including top predators, play a surprisingly direct role in maintaining soil health. In Australia, researchers compared areas where dingoes were common with areas where they were rare. Where dingoes were scarce, kangaroo populations boomed and grazed heavily, stripping vegetation. Fencing experiments confirmed the cascade: excluding kangaroos from overgrazed areas led to measurable increases in vegetation cover and in soil carbon, nitrogen, and phosphorus. Where dingoes naturally kept kangaroo numbers in check, these effects were negligible because the vegetation was never overgrazed in the first place.25PubMed Central. Removal of an apex predator initiates a trophic cascade that extends from herbivores to vegetation and the soil nutrient pool A single predator’s presence or absence can ripple all the way down to the chemistry of the dirt. And because dingoes roam across vast territories, this effect operates at an enormous spatial scale.
Biodiversity as a Buffer Against Extreme Weather
One of the strongest practical arguments for preserving terrestrial biodiversity is that diverse ecosystems handle extreme weather better. A broad synthesis of grassland experiments found that communities with only one or two species saw their productivity swing by about 50% during climate events like droughts or unusually wet years. Communities with 16 to 32 species were substantially more resistant, with productivity shifting by only about 25%.26Nature. Biodiversity increases the resistance of ecosystem productivity to climate extremes The stabilizing effect held across a wide range of climate events, whether wet or dry, moderate or extreme, brief or prolonged. This is not an abstract ecological principle. It means that as climate extremes become more frequent, simplified landscapes with few species are more likely to suffer large disruptions to the services people depend on, from food production to flood control.27Earth’s Future. Biodiversity and Climate Extremes: Known Interactions and Research Gaps
Mass Extinction and Recovery on Land
Land ecosystems have been devastated before. The end-Permian extinction, roughly 252 million years ago, was the most catastrophic event in the fossil record, wiping out an estimated 81% of marine species and savaging terrestrial plant communities as well.28PubMed Central. Mass extinctions and land plant evolution The causes were primarily volcanic: massive eruptions in what is now Siberia released greenhouse gases and toxic compounds that destabilized climate worldwide. Terrestrial plant life recovered, but not quickly. New lineages replaced old ones, and the ecological structure of land communities shifted in ways that took millions of years to stabilize. The lesson is that while life on land is resilient over geological timescales, the path back from catastrophic loss is slow, unpredictable, and produces a different world than the one that existed before.
Deforestation Unravels the System Fast
Tropical deforestation continues at a net rate of about 5.5 million hectares per year, and the damage to soil functions extends far beyond the obvious loss of trees. Changes to soil properties continue for decades after clearing, eventually reaching deep subsoils and affecting nutrient storage, carbon reserves, erosion resistance, water drainage, and greenhouse gas emissions.29Nature Reviews Earth & Environment. Deforestation and reforestation impacts on soils in the tropics In some converted croplands, soil quality indices drop to 62 to 79% of their forested values, especially at the surface.30CATENA. Evaluating forest soil quality after deforestation and loss of ecosystem services using network analysis and factor analysis techniques
What makes this particularly consequential is that soil is not a renewable resource on any human timescale. The thick, biologically active soils that took hundreds of millions of years to develop through the interplay of roots, microbes, and weathering can lose much of their functional capacity within decades of forest removal. Reforestation can recover some of this capacity, but the trajectory is slow and depends heavily on what replaces the original forest.
Humans Transformed the Land Far Earlier Than You Think
A global archaeological assessment found that the planet was largely transformed by human land use, through hunting, gathering, farming, and herding, by about 3,000 years ago. That is considerably earlier than the dates used in most land-use reconstructions that Earth scientists rely on for climate modeling.31PubMed. Archaeological assessment reveals Earth’s early transformation through land use This matters because it means the “natural” baseline that modelers assume existed before industrial civilization was already heavily shaped by people. Forests had been cleared, grasslands had been managed with fire, and soils had been reworked by agriculture for thousands of years before anyone started measuring atmospheric carbon. The planetary systems that land life built were being altered by one of its own species much earlier than commonly appreciated.
Life in Rocks and Deserts
Terrestrial life is not confined to the green, visible surface. Microbial communities thrive deep in the Earth’s crust, in a realm scientists call the deep subterranean biosphere. These organisms participate in weathering, mineral precipitation, and the cycling of carbon, nitrogen, sulfur, and metals, many of the same processes that surface life drives, but carried out in total darkness under enormous pressure.32Earth-Science Reviews. The deep subterranean biosphere Even in deep, hydrothermally altered granite in semi-arid environments, microbial communities actively reduce iron-bearing minerals, contributing to rock weathering far below the soil surface.33Geo-Bio Interfaces. Microbial weathering of iron-bearing minerals in deep hydrothermally altered granitic rock of a semi-arid environment (Chilean Coastal Cordillera)
At the other extreme, some land plants have evolved to tolerate conditions that would kill nearly any crop. The desert moss Syntrichia caninervis can lose virtually all its water content and shut down photosynthesis entirely, yet recover to 83% of its normal water content and 90% of its photosynthetic capacity within just 30 minutes of rehydration.34PubMed Central. Exploring the mechanisms of desert plant adaptation to arid climates: a multi-omics analysis of dehydration and rehydration responses in Syntrichia caninervis Full recovery occurs within 12 hours. These organisms hint at the deep evolutionary roots of desiccation tolerance, the same trait that allowed the ancestors of all land plants to leave the water in the first place, pushed here to an extreme that borders on the science-fictional. Life on land, it turns out, is far more tenacious, far more deeply embedded in the planet’s functioning, and far older than most of us realize.