The role of symbiosis in the transition of some eukaryotes from aquatic to terrestrial environments

Symbiosis may have played a far greater role in biological evolution than was previously thought. The symbiosis that made the colonization of land by plants possible was as a consequence of the development of arbuscular mycorrhizae. However, the present review draws attention to the role of lichens in assisting in this transition and to the phenomenon of lichenization. The recent discovery of lichen fossils in marine phosphorites in China and molecular clock estimates indicate that lichenized fungi were already present in Precambrian seas and, like contemporary species, played a role as pioneers in occupying new habitats. There is evidence that the holistic properties of associations between fungi and cyanobacteria and/or green algae facilitated the transition onto land and the subsequent colonization of terrestrial habitats. A key role in this process was played by poikilohydry. The algal components of delichenized fungi, along with lichens and photosynthetic aquatic organisms all contributed to the increase in atmospheric oxygen. Lichens, fungi and cyanobacteria settling on land were undoubtedly important in the formation of soils and thereafter enhancing their fertility. It is suggested that vascular and other green plants were able grow on these primitive soils that were stabilized by the growth of lichens, algae and cyanobacteria in a similar way to those which play a role in desert crusts at the present time.