A Subsurface Stepping Stone Hypothesis for the Conquest of Land by Arthropods

The conquest of land by arthropods is commonly believed to be a surface phenomenon associated with the arrival of photosynthetic plants, atmospheric oxygenation, and an ozone shield in the mid-Paleozoic Era. However, recent molecular and fossil evidence suggests terrestrial fauna may have first appeared in the Cambrian, before the proliferation of plants and ozone, which are thought to be essential for survival. This raises the question—how could arthropods survive on land without established plants and an ozone shield? We propose a hypothesis that chemolithoautotrophic cave ecosystems, independent of photosynthesis, may have served as a subsurface stepping stone, providing a possible explanation for the land invasion enigma. Chemolithoautrophic caves have offered abundant food and radiation protection, enabling ancient arthropods to evolve strategies to adapt to new frontiers through gradual dispersion from the sea to shielded cave waters, then to cave hygropetric margins of cave waters, and, finally, to the surface.