Small Things Considered

Abstract

By now, many of us are aware that ∼8% of the human genome is retroviral, compared to only ∼1.5% that codes for human proteins. These sequences are the result of ancient infections of germ cells by retroviruses, which insert into DNA as part of their life cycle. Upon insertion into the genome, such endogenous retroviruses (ERVs) evolve neutrally and are eventually inactivated by mutations and by host repression of their expression. Therefore, these viruses are no longer viruses but a collection of broken parts. Considering that we carry out ∼10,000 billion cell divisions in a lifetime, the maintenance of this viral junk heap requires considerable resources. So why do we keep it around? Probably because of weak purifying selection combined with the hunch that “this stuff might come in handy someday.” In fact, it appears that an important evolutionary innovation—the placenta—has depended on parts pulled from this “genomic junk drawer.”