Why the line between land and water animals is often a matter of judgment

Nature · Published 29 September 2026 Swipe more like this →

In short: Terrestrial animals are not a single, unified group, but a diverse collection of lineages that independently conquered land. From microscopic water bears to desert snails, these creatures evolved ingenious ways to survive without buoyancy, yet many remain tethered to moisture, blurring the boundary between aquatic and land-dwelling life.

Terrestrial animals do not form a single clade; rather, they are a polyphyletic group united only by their existence on land. This transition from water to land occurred independently across many animal phyla, primarily during the Paleozoic and Mesozoic eras. Because there is no universally accepted criterion for defining a terrestrial animal, the label is often a matter of judgment. Many species, such as penguins or mosquitoes, spend significant portions of their life cycles in water, while others, like fiddler crabs, exist in a semi-terrestrial state, retreating to burrows during high tides.

To survive outside the buoyancy of water, terrestrial animals evolved robust skeletons for structural support and specialized respiratory organs like lungs or cutaneous systems for gas exchange. They also developed homeostatic mechanisms, such as impermeable cuticles to prevent fluid loss and excretory systems that process nitrogenous waste as urea or uric acid. These adaptations allowed diverse groups—including arthropods, mollusks, and chordates—to colonize environments ranging from lush forests to arid deserts. For instance, land snails like Xerocrassa seetzeni have successfully adapted to extreme heat and aridity.

Some groups remain transitional, relying on moist environments to thrive. Annelids, velvet worms, and flatworms are restricted to damp habitats, often breathing through their skin. Others, such as rotifers, gastrotrichs, and tardigrades, are microscopic and depend on transient films of water. These organisms often survive desiccation by entering states of dormancy or suspended animation, allowing them to remain ubiquitous in terrestrial ecosystems despite their need for moisture. This evolutionary flexibility highlights that terrestrialization is not a single event, but a continuous, multifaceted process of adaptation.

The history of this invasion is deep, with fossil evidence suggesting arthropods made forays onto land as early as 530 million years ago, likely to mate or lay eggs away from predators. By the late Devonian, around 375 million years ago, vertebrates like Tiktaalik roseae had developed muscular, weight-bearing limbs and lungs, establishing a permanent foothold on land. This ongoing expansion into terrestrial niches remains one of the most significant events in the history of life.

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