by Jamie Henzy — Al­though it's a reg­u­lar oc­cur­rence for bac­te­ria to ac­quire new genes whole cloth, by hor­i­zon­tal gene trans­fer, such events are rather rare for com­plex eu­kary­otes like us. Eu­kary­otes are vastly more in­clined to take ad­van­tage of gene du­pli­ca­tion events. An ex­cep­tion to this rule is the pres­ence in ver­te­brate genomes of nu­mer­ous retro­vi­ral se­quences (en­doge­nous retro­viruses, or ERVs, for short) that were ac­quired...