Origins of Land Animals

Abstract

The emergence of plants and animals from the sea on to land—terrestrialization—was one of the major advances in the history of life on Earth. Evidence for early land colonization is sparse, but the fossil record reveals that by mid-Paleozoic times complex terrestrial ecosystems had become established. The earliest evidence of excursions onto land by aquatic animals comes from trace fossils of arthropods crossing damp sediment in late Cambrian times. These animals were unlikely to have been truly terrestrial. The first animals living on land may have been those capable of being blown by the wind into damp terrestrial habitats (i.e., possibly tardigrades) but there is no fossil evidence for this. The earliest body fossil evidence of land animals comes from millipedes from the Silurian of Scotland (c. 428Ma). Early terrestrial ecosystems were dominated mainly by detritivorous and predatory arthropods. Tetrapods appeared in the late Devonian, and were terrestrial by early Carboniferous times. It was not until the latest Paleozoic that modern-type terrestrial ecosystems, with abundant herbivores in the food chain, had evolved.