Evolutionary events in the long history of microbial life may explain contemporary patterns in species distribution and community structure.  Soil moisture has been suggested as a master variable that shapes bacterial community assembly due to the variable tolerances of different organisms to wet and dry conditions.  The phylogenetic distribution of moisture preference may therefore underlie modern-day patterns in soil bacterial community structure, but the history and evolutionary dynamics of moisture preference in terrestrial bacteria are not well defined.  We determined the phylogenetic distribution of moisture preference in Bacteria and its effects on soil microbial diversity and structure across several different ecosystems with vastly different soil moisture conditions.  We reconstructed the ancestry of moisture preferences of 2112 different bacterial taxa over more than 3 billion years of evolution, and analyzed rates and patterns of diversification for bacteria that flourish in high and low moisture soils.

Results/Conclusions

We demonstrate that ancestral soil bacteria developed adaptations to prosper in dry conditions relatively late in evolutionary history, likely triggered by the evolution of oxygenic photosynthesis and subsequent development of a planetary ozone shield.  Dry-adapted bacteria have since evolved into a surprisingly low diversity of lineages.  Consequently, the variety of soil bacteria presently found in dry ecosystems is less than in wet ecosystems such as rainforests.  These results provide a basis for forecasting the response of complex soil bacterial communities to climate or land-use changes that alter soil moisture.