Panspermia: An early cellular origin of viruses

The data-driven studies of origin of viruses are challenging. Traditional alignment-dependent methods may be ill suited to study virus origins. Over long evolutionary timescales, DNA substitutions may completely erase the evolutionary signal, especially in the case of fastevolving RNA and retro-transcribing viruses. Other complications such as limited choice of orthologous gene markers conserved across a broad range of organisms further complicate the recovery of a reliable alignment to use for evolutionary inferences. Under such circumstances, it can be better to focus on characters that are better conserved in evolution (such as protein structures) or to explore other alignmentindependent alternatives. Here we share our experience of using alternative molecular and morphological characters to study virus origins and evolution. We discuss how our phylogenomic data-driven insights agree with an early cellular origin of viruses and strongly refute the classical perceptions about viruses as mere byproducts of cellular evolution. We also propose a phylogenomic-data inspired model of virus origination that transforms primordial vesicles of horizontal genetic exchange into full-fledge viruses capable of establishing complex virus-host life cycles.