There are two lengths of DNA that originated from viruses and now reside in the genomes of humans and other primates, for instance, without which—an astonishing fact—successful pregnancy would be impossible. There’s viral DNA, nestled among the genes of terrestrial animals, that helps package and store memories in tiny protein bubbles. Still other genes co-opted from viruses contribute to the growth of embryos, regulate immune systems, resist cancer—important effects only now beginning to be understood. Viruses, it turns out, have played crucial roles in launching major evolutionary
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