Terrestrialization

Terrestrialization is the evolutionary process by which organisms adapt to life on land, moving from aquatic or moist environments to terrestrial habitats. It encompasses morphological, physiological, and reproductive changes that reduce dependence on standing water and enable survival in air, on desiccating surfaces, and under fluctuating temperatures. Because water is essential for many early life processes, terrestrialization occurred independently in several lineages and at different times, rather than as a single, uniform event.

In plants, terrestrialization involved the transition from simple nonvascular forms to vascular plants with xylem and phloem, the development of a waxy cuticle and stomata to reduce water loss, roots for anchorage and absorption, and reproductive innovations such as seeds and pollen. Later, the rise of lignified tissues, wood, and, in angiosperms, flowers and fruits further stabilized life on land and enabled widespread colonization of diverse habitats. Among animals, terrestrialization began with arthropods and other invertebrates colonizing dry land in the Paleozoic; key adaptations included waterproof exoskeletons, sensory and locomotor innovations. The vertebrates followed with tetrapods—amphibians adapting to both water and land, and later amniotes (reptiles, birds, mammals) developing the amniotic egg, dry-resistant skin or scales, and efficient lungs, enabling fully terrestrial life and longer-distance dispersal.

Terrestrialization reshaped ecosystems by enabling new niches and interactions, such as soil formation, nutrient cycling, pollination, and predator–prey dynamics. It has continued through to modern times, with many lineages retaining aquatic stages or returning to water, while others establish long-standing terrestrial lineages. The study of terrestrialization integrates paleontology, comparative physiology, and ecology to understand how life forms independently solved similar environmental challenges.