““Collectively, the veritable zoo of mobile genetic elements comprises a remarkable 40 percent of our genome. The retroviral sequences littered across all vertebrate genomes are termed endogenous retroviral elements (ERVs). Endogenous retroviral sequences themselves are so plentiful that they take up more space in our genomes than genes encoding human proteins. Their existence is evidence of waves of retroviral infection and germline infiltration throughout vertebrate evolution. We have only recently begun to realize the powerful influence that retroviruses wielded over the evolution of vertebrate genomes and the identity of our species. They were catalysts of genetic instability that fueled evolutionary change; today they are vestiges of their former selves, fossils of viruses that once preyed on vertebrate hosts. Their remains are evidence of pyrrhic victories of sorts in many wars and arms races that have taken place between retroviruses and hosts. Most endogenous retroviruses have long been silenced by host cell restriction mechanisms. Associated with no phenotype, and under no selective pressure they become nonviable after millions of years of mutational drift, resulting in the accumulation of mutations and deletions in their coding sequences and control elements.””
