This fellowship was broadly aimed at examining accelerated protein evolution by microorganisms in subsurface environments. A variety of bacteria, archaea, and their viruses are capable of generating massive protein variation for specific genes, to a degree that is unparalleled in other forms of life, yet this phenomenon has not been investigated in the subsurface biosphere. A combination of molecular and bioinformatic techniques enabled identification of gene-diversifying elements across a multitude of microbial and viral genomes. Genomes bearing this trait were found in three subsurface systems: a terrestrial aquifer, coastal marine sediments, and subsurface crustal fluids. As a product of this fellowship, papers have been published describing the abundance and functional diversity of hypervariable genes in groundwater metagenomes, and, separately, on the structural characterization of a hypervariable protein from nanoarchaea. An additional manuscript is in preparation, which highlights the importance of localized hypermutation in manipulating host-parasite interactions for archaeal viruses from crustal fluids. Through new collaboration, research on this phenomenon has also extended beyond subsurface microorganisms, to address the importance of accelerated evolution in structuring host-microbe-virus interactions in the human microbiome.