How life colonised land in the Palaeozoic

“Life moved from the sea to the land” sounds like a single journey. The Palaeozoic record shows a much slower and more complicated history. Plants, arthropods and vertebrates entered terrestrial environments at different times, and each lineage faced distinct problems: avoiding dehydration, supporting a body against gravity, exchanging gases, reproducing away from open water and finding food. Some early land colonists left abundant fossils; others are known mainly from indirect traces.

What counts as evidence of life on land?

A fossil found in a terrestrial rock is not automatically evidence that its maker lived on land. Rivers can carry marine shells inland, and storms can move land remains into the sea. Researchers combine anatomy with the rock around a fossil. Sediment structures, soil horizons, root traces, spores, trackways and the position of a specimen help establish whether an organism lived in the environment where it was buried.

Different evidence answers different questions. A spore can show that a plant was reproducing, but it may travel far from its source. A fossil stem records body form, while a root system preserved in place links a plant more directly to a soil. A footprint records movement across a surface but may not identify the trackmaker. The landward transition is reconstructed from these overlapping clues rather than from one “first land animal.”

Plants changed the ground before forests appeared

Microscopic spores with features associated with land plants occur in Ordovician rocks, before large plant bodies are common in the fossil record. Their distribution suggests that small plants were established on land by this time, although isolated spores are difficult to assign to a precise plant form. Silurian fossils preserve more recognisable stems and simple vascular tissues. These plants were low-growing and lacked the deep roots and branching canopies of later forests.

During the Devonian, plants became taller and more structurally varied. Vascular tissues moved water and nutrients through the body; branching stems raised photosynthetic surfaces above the ground; roots anchored plants and accessed water below it. Rooted landscapes began to hold sediment, create soils and alter how water moved through valleys. By the Late Devonian, forests included trees with substantial trunks and deep root systems. Their influence reached beyond the plants themselves, changing rivers, weathering and the habitats available to animals.

These changes were regional and gradual. The first appearance of a fossil plant in one basin is not the date when vegetation appeared everywhere. Preservation favours some habitats, and many early plant communities left only fragmentary records. The story is better described as increasing ecological complexity than as a single botanical revolution.

Arthropods occupied land in several waves

Arthropods were among the earliest animals to use terrestrial habitats. Trackways and body fossils indicate that some were on land by the Silurian, while later deposits preserve a much wider range of millipedes, arachnids and insects. Their external skeleton already provided support, but living on land still required changes in gas exchange, water balance and reproduction. Those solutions evolved more than once across arthropod branches.

Fossil footprints can predate the body fossils of the animals that made them. A trackway proves that an animal crossed a surface, but its shape may not reveal whether the maker was an early vertebrate, an arthropod or another group unless distinctive details are preserved. This is why claims about the “first” land animal often change as new traces and better-dated rocks are studied.

From fish fins to weight-bearing limbs

Late Devonian rocks record several early tetrapods and close relatives with combinations of fish-like and limb-like anatomy. Fossils such as Tiktaalik preserve a mobile neck and robust fin bones that could bear some weight in shallow-water settings. They are evidence for a transition within aquatic and shoreline environments, not a modern fish abruptly walking onto a dry plain. Early tetrapods retained gills or other aquatic adaptations, and many lived in water.

Trackways add a different view. Some Devonian footprints may be older than the best-known body fossils, but their age and interpretation remain debated. A sequence of prints can show that a limbed animal moved along a bottom or exposed surface. It cannot by itself establish how often that animal lived fully on land or how it reproduced.

Amniotes loosened the tie to water

By the Carboniferous, early amniotes had evolved reproductive anatomy that allowed the embryo to develop within protective membranes, reducing dependence on open water for egg laying. This was one important adaptation, not a complete escape from environmental limits. Early amniotes still needed suitable moisture, food and shelter, and the fossil record shows them alongside amphibian relatives in varied wetland and upland settings.

The Permian inherited forests, rivers and terrestrial food webs built over a long period. Drying climates in some regions favoured plants and animals better suited to seasonal conditions, while other habitats remained humid. The transition onto land was not a ladder toward a predetermined modern ecosystem. It was a branching sequence of experiments, local extinctions and new adaptations.

For the wider chronology, see the Palaeozoic overview and the accounts of the first Devonian forests and Tiktaalik.