Invasion of the continents: cyanobacterial crusts to tree-inhabiting arthropods

The colonization of continental environments (land and fresh water) has focused historically on a major event during the mid-Paleozoic Era characterized by the relatively sudden emergence of megascopic embryophytes, fungi, arthropods and tetrapods. A significant earlier phase of Precambrian (Archean and Proterozoic Eons) terrestrialization extends to the first 80% of the history of life and records the colonization of subaerial soils or rock surfaces predominantly by cyanobacterial mats and crusts. These two phases are separated by a 90-million-year early Paleozoic interlude of minimal terrestrial colonization. Trophically modern ecosystems appeared during the Late Silurian–Middle Devonian (425–375 million years ago), consisting of complex symbiotic, parasitic and other trophic associations, including detritivory and limited herbivory. The integration of these two historically disparate fields (Precambrian microorganisms and their biochemical and sedimentological signatures, and the paleoecology of mid-Paleozoic ecosystems) has resulted in a wider perspective on terrestrialization. Here, I present an ecological and evolutionary context for the emergence of terrestrial ecosystems and examine associations among organisms, from the endosymbiotic capture of organelles by eukaryotes to modes of metazoan nutrition on land. Such studies now enable the tracking, in ecological detail, of the invasion of continental environments during the past 3.5 billion years of life.