Most Collembola (springtails) species live in the soil but a few are semiaquatic. Which came first: soil dwelling Collembola or aquatic species? C. A. D’Haese* collected DNA evidence to construct a phylogeny (a family tree) that would address the question.
The DNA evidence could be interpreted in several ways, but all interpretations supported the hypothesis that soil dwelling collembola came first, and aquatic life came later. Species of Collembola from multiple branches of the phylogenetic tree later became semi-aquatic.
In the colonization of land over 400 million years ago, Collembola ancestors transitioned from obtaining oxygen from the water to obtaining oxygen from the air. In modern Collembola, oxygen dissolves in the moist cuticle and thence distributed to the internal tissues. Cuticle permeable to oxygen is also permeable to water leaving Collembola at risk of dehydration. Some soils have high water content even to the point of flooding. Soils provide high humidity and protection and protection from wind and heat that can promote desiccation. The early terrestrial arthropods were closely associated with soil environments. Moist terrestrial soils could provide a “bridge” between the aquatic environment and the harsh dry surface of the land.
*D’Haese, Cyrille A. “Were the first springtails semi-aquatic? A phylogenetic approach by means of 28S rDNA and optimization alignment.” Proceedings of the Royal Society B: Biological Sciences 269.1496 (2002): 1143.