The human genome contains traces of ancient viruses that infected our ancestors millions of years ago — and some of those viral leftovers were later repurposed into genes that help make human pregnancy possible

Both halves of this claim are well supported, and neither rests on a single study. About 8 percent of the human genome, by the standard estimate, consists of sequences left behind by ancient retroviruses. And among those sequences are a small number of genes that the body now uses to build the placenta. The viral [...] The post The human genome contains traces of ancient viruses that infected our ancestors millions of years ago — and some of those viral leftovers were later repurposed into genes