From ancient catalytic folds to retrotransposons and retroviruses

This review constitutes an evolutionary journey that begins with the emergence of reverse transcriptase and other ancient catalytic folds that shape retroelements (Class I transposable elements) and ends with the appearance of retroviruses and their subsequent or simultaneous endogenization. This path comprises milestones such as the formation of prokaryotic retroelements, their appropriation by eukaryotes, which resulted in the occurrence of non-long terminal repeat (non-LTR) retroelements, the mosaic formation of LTR retroelements by utilizing components of non-LTR retroelements and DNA transposons, and the acquisition of new domains that led to the emergence of retroviruses. We present here a comprehensive recounting of all evolutionary events that took place from the formation of primordial enzymes to the origin of retroviruses and their endogenization.