Evolutionary terrestrialization scenarios for soil invertebrates

It is generally assumed that the soil environment played an important role in the evolution of life on land, as a transition medium between water and air. This idea was already formulated by M.S. Gilharov in 1949 but it has never been tested systematically against modern phylogenetic reconstructions from DNA data. In this paper I review the phylogenies of soil invertebrate lineages that have been published in the last decade, as a guide to test Ghilarov's hypothesis. I propose to distinguish between major and minor terrestrializations and six different evolutionary scenarios. The various lineages of soil invertebrate (nematodes, tardigrades, crustaceans, apterygotes, insects, myriapods, chelicerates, flatworms, earthworms, potworms and snails) show different pathways of terrestrialization. Some lineages seem to have conquered the land directly from the marine environment, others through intermediate habitats (intertidal, caves and freshwater). The review illustrates that the study of soil invertebrates holds many promises for enriching evolutionary sciences.