(THE ATLANTIC) ED YONG, March 3, 2016 — “Evolution, it turns out, is really good at irony.”

Hundreds of millions of years ago, prehistoric viruses inserted their genes into the genomes of our ancestors. They found their way into eggs or sperm, and then into embryos. As they passed down from one generation to the next, they picked up mutations that disabled their ability to infect new cells. Eventually, they became permanent fixtures of our genomes, as much a part of our DNA as our own genes.

Today, these ‘endogenous retroviruses’ or ERVs make up 8 percent of our genome. They are genetic fossils—remnants of our viral ancestors, and records of epidemics past.

ERVs are not passive stowaways. They provided raw material for evolution, in the form of viral genes that our ancestors could tinker with and put to new use. For example, syncytin, a gene that’s essential for creating the placenta, came from a virus; in the words of Carl Zimmer, “If not for a virus, none of us would ever be born.”

Source: How Viruses Infiltrated Our DNA and Supercharged Our Immune System – The Atlantic