The emergence of forests, oxygen surges, and vertebrate evolution – limbs and lungs adapted for life on land
A world surrounded by change
The Late paleozoic era was marked by significant changes in Earth's biosphere and climate. The Devonian (419–359 million years ago), also called the “Age of Fishes,” saw jawed fishes and reefs flourish in the oceans, while land plants rapidly spread from small, simple forms to tall trees. Following this, the Carboniferous (359–299 million years ago) was characterized by lush coal forests and abundant oxygen, with not only plants but also early amphibians and giant arthropods thriving on land. These transformations laid the foundations of modern terrestrial ecosystems and demonstrate how biological innovations and environmental feedback can radically alter Earth's surface.
2. Devonian environment: plants conquer the land
2.1 Early vascular plants and the first forests
In the Early Devonian, small vascular plants (e.g., rhiniophytes, zosterophylls) colonized the land. Moving into the Middle–Late Devonian, larger and more complex plants evolved, such as Archaeopteris, considered one of the first true “trees.” Archaeopteris had woody trunks and broad, leaf-like appendages. In the Late Devonian, these trees already formed primary true forests, sometimes reaching over 10 m in height, strongly affecting soil stability, carbon cycling, and climate [1], [2].
2.2 Soil formation and atmospheric change
With the establishment of plant roots and the accumulation of organic deposits, true soil (paleosols) began to form, accelerating the weathering of silicate rocks, reducing atmospheric CO2 levels, and accumulating organic carbon. This increase in terrestrial productivity likely caused the decrease of CO2 in the atmosphere and promoted planetary cooling. At the same time, increased photosynthesis gradually raised oxygen levels. Although this was not as dramatic as the Carboniferous oxygen surge, the changes in the Devonian period paved the way for later oxygen increases.
2.3 Marine extinctions and geological crises
The Devonian is also known for several extinction pulses, including the Late Devonian extinction (~372–359 million years ago). The spread of terrestrial plants, changes in ocean chemistry, and climate fluctuations may have triggered or intensified these extinction events. Reef-building corals and some fish groups were affected, reshaping marine ecosystems but leaving evolutionary niches open for other species.
3. The first tetrapods: fish step onto land
3.1 From fins to limbs
In the Late Devonian, some lobe-finned fish (Sarcopterygii) lineages developed stronger, more developed pectoral and pelvic fins with massive internal bones. Famous transitional fossil examples like Eusthenopteron, Tiktaalik, and Acanthostega show how fin structures evolved into limbs ending with digits in shallow or swampy waters. These proto-tetrapods could live in shallow water or deltaic environments, combining aquatic swimming with early stages of terrestrial movement.
3.2 Why venture onto land?
Hypotheses for why fish evolved into tetrapods include:
- Escape from predators / new niches: Shallow waters or temporary ponds forced adaptation.
- Food resources: New dietary sources from terrestrial plants and arthropods.
- Oxygen deficiency: Warm Devonian waters may have been hypoxic, so surface or partial air breathing provided an advantage.
By the end of the Devonian, true “amphibian-like” tetrapods already had four weight-bearing limbs and lungs for breathing air, although many still depended on water for reproduction.
4. Beginning of the Carboniferous: the age of forests and coal
4.1 Carboniferous climate and coal swamps
The Carboniferous period (359–299 million years ago) is often divided into the Mississippian (early Carboniferous) and Pennsylvanian (late Carboniferous) subperiods. During this time:
- Giant lycopsids and fern forests: Lepidodendron, Sigillaria (clubmosses), horsetails (Calamites), seed ferns, and early conifers thrived in humid equatorial lowlands.
- Coal formation: Thick layers of accumulated plant material in oxygen-poor swamps turned into large coal deposits (hence the name “Carboniferous”).
- Oxygen increase: Extensive organic burial activity apparently raised atmospheric O2 concentration to ~30–35% (much higher than the current 21%), allowing the formation of gigantic arthropods (e.g., meter-long centipedes) [3], [4].
4.2 Tetrapod radiation: the rise of amphibians
With abundant swampy lowlands and oxygen surplus, early terrestrial vertebrates (amphibians) widely spread:
- Temnospondyls, anthracosaurians, and other amphibian-like clades diversified in semi-aquatic habitats.
- Limbs were adapted for walking on solid ground, but reproduction still required water, so they remained tied to moist habitats.
- Some lineages that later evolved into amniotes (reptiles, mammals) acquired more advanced reproductive strategies (amnion egg) by the end of the Carboniferous, further securing adaptation to fully terrestrial life.
4.3 Giant arthropods and oxygen
The Carboniferous oxygen surplus is linked to giant insects and other arthropods, e.g., Meganeura (a dragonfly-like insect with ~65–70 cm wingspan) or the giant Arthropleura millipede. The high partial pressure of O2 enabled more efficient tracheal respiration. This ended as climate changed in later periods and O2 levels dropped.
5. Geological and paleoclimatic shifts
5.1 Continental configurations (formation of Pangaea)
During the Carboniferous, Gondwana (southern supercontinent) drifted northward, merging with Laurasia, and by the end of the late Paleozoic began forming Pangaea. This collision built huge mountain ranges (e.g., Appalachian–Variscan orogeny). Changing continental arrangements influenced climate by redirecting ocean currents and atmospheric circulation.
5.2 Glaciations and sea level change
Late Paleozoic glaciations began in southern Gondwana (late Carboniferous – early Permian, the "Karoo" glaciation). Large ice sheets in the southern hemisphere caused cyclic sea level changes, affecting coastal coal-swamp habitats. The interaction of glaciations, forest expansion, and plate tectonics shows how complex connections govern the Earth system.
6. Fossil evidence of terrestrial ecosystem complexity
6.1 Plant fossils and coal macerals
Carboniferous coal beds richly preserve plant remains. Trunk impressions (Lepidodendron, Sigillaria) or large leaves (seed ferns) testify to multilayered forests. Microscopic organic remains in coal (macerals) show how dense biomass, under oxygen deficiency, transformed into thick coal – later becoming the "fuel" of industrial revolutions.
6.2 Early amphibian skeletons
Abundantly preserved early amphibian (temnospondyl and others) skeletons show hybrids of aquatic and terrestrial adaptations: sturdy limbs, but often with primitive teeth or other features linking fish and later developed terrestrial traits. Some paleontologists call these intermediate forms "basal amphibians", connecting Devonian tetrapods with the first Carboniferous crown amphibians [5], [6].
6.3 Giant insects and arthropod fossils
Prominent finds of insect wings, arthropod exoskeletons, or tracks confirm gigantic terrestrial arthropods in these swampy forests. Oxygen excess allowed them larger body sizes. These fossils directly reveal Carboniferous ecological interactions where arthropods were important herbivores, decomposers, or smaller vertebrate predators.
7. Towards the Late Carboniferous
7.1 Climate change, oxygen decline?
Towards the end of the Carboniferous, with intensifying glaciations in southern Gondwana, ocean circulation changed. The shifting climate may have reduced the spread of coastal swamps, ultimately weakening large-scale organic burial that caused the oxygen peak. As the Permian (~299–252 million years ago) progressed, the Earth system reorganized again, droughts deepened in some equatorial zones, and large arthropods declined.
7.2 Foundations of amniotes
In the Late Carboniferous, some tetrapods developed the amniotic egg, freeing them from water-dependent reproduction. This innovation (leading to reptiles, mammals, birds) marks another major step toward vertebrate terrestrial dominance. Synapsids (mammal lineage) and sauropsids (reptile lineage) began to diverge, eventually outcompeting older amphibian groups in many niches.
8. Significance and legacy
- Terrestrial ecosystems: By the end of the Carboniferous, Earth's land areas were already densely covered with plants, arthropods, and various amphibian groups. This is the first true "terrestrialization," creating the basis for future terrestrial biospheres.
- Oxygen and climate feedback: Massive organic burial in swamps raised atmospheric O2 levels, regulating climate. This shows the direct impact of biological processes (forests, photosynthesis) on the planetary atmosphere.
- Vertebrate evolution stage: From the Devonian fish-tetrapod transition to the Carboniferous amphibians and the dawn of amniotes – this period laid the foundation for the later evolution of dinosaurs, mammals, and ultimately ourselves.
- Economic resources: Carboniferous coal deposits – still an important energy source, paradoxically driving current anthropogenic CO2 emissions. Understanding the formation of these deposits aids geological studies, paleoclimate reconstructions, and resource management.
9. Connections with current ecosystems and exoplanet lessons
9.1 Ancient Earth as an exoplanet analog
Devonian–Carboniferous transition analysis can help astrobiology understand how widely spread photosynthetic life, large biomass, and changing atmospheric composition could arise on a planet. "O2 excess" – such a phenomenon could be visible in spectral signals if a similar scale boom of forests or algae occurred on some exoplanet.
9.2 Significance for the present
Current discussions about the carbon cycle and climate change echo Carboniferous processes – then massive carbon accumulation (coal), now rapid carbon release. Understanding how ancient Earth maintained or shifted climate states, burying carbon abundantly or experiencing glaciations, can aid current climate models and decision-making.
10. Conclusion
The period from Devonian to Carboniferous is pivotal in Earth's history, transforming our planet's terrestrial environments from sparsely vegetated areas to dense, swampy forests that created an oxygen-rich atmosphere. At the same time, vertebrates overcame the water–land barrier, opening the way for amphibians and later reptiles or mammals. Abundant geosphere and biosphere changes – plant expansion, oxygen fluctuations, large arthropods, amphibian dispersal – show how life and environment can astonishingly converge over tens of millions of years.
Consistent paleontological discoveries, new geochemical methods, and improved modeling of ancient environments allow a deeper understanding of these distant transformations. Today we look at the early "green" ages of Earth, connecting the watery Devonian world with the coal-rich Carboniferous swamps, completing the picture of a planet full of complex terrestrial ecosystems. Important shared lessons are thus visible on how global environmental changes and evolutionary innovations can determine the fate of life across epochs, and perhaps beyond Earth.
Links and further reading
- Algeo, T. J., & Scheckler, S. E. (1998). "Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events." Philosophical Transactions of the Royal Society B, 353, 113–130.
- Clack, J. A. (2012). Gaining Ground: The Origin and Evolution of Tetrapods, 2nd ed. Indiana University Press.
- Scott, A. C., & Glasspool, I. J. (2006). "The diversification of Paleozoic fire systems and fluctuations in atmospheric oxygen concentration." Proceedings of the National Academy of Sciences, 103, 10861–10865.
- Gensel, P. G., & Edwards, D. (2001). Plants Invade the Land: Evolutionary & Environmental Perspectives. Columbia University Press.
- Carroll, R. L. (2009). The Rise of Amphibians: 365 Million Years of Evolution. Johns Hopkins University Press.
- Rowe, T., et al. (2021). "The complex diversity of early tetrapods." Trends in Ecology & Evolution, 36, 251–263.