Viral genome remnants indicate that the mysterious Denisovans, not humans, are Neanderthals’ closest cousins

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Humans and Neanderthals are close cousins. So close, in fact, that some researchers argue the two hominids might actually be members of the same species. But a few years ago, anthropologists discovered a mysterious new type of hominid that shook up the family tree. Known only from a finger fragment, a molar tooth and the DNA derived from both, the Denisovans lived in Asia and were contemporaries of Neanderthals and modern humans. And they might have been Neanderthals’ closest relatives. A recent study of virus “fossils” provides new evidence of this relationship.

Hidden inside each, embedded in our DNA, are the genetic remnants of viral infections that afflicted our ancestors thousands, even millions of years ago. Most known virus fossils are retroviruses, the group that includes HIV. Consisting of a single strand of RNA, a retrovirus can’t reproduce on its own. After the retrovirus invades a host cell, an enzyme reads the RNA and builds a corresponding strand of DNA. The virus-derived DNA then implants itself into the host cell’s DNA. By modifying the host’s genetic blueprints, the virus tricks the host into making new copies of the retrovirus. These virus fossils have distinct genetic patterns that scientists can identify during DNA analyses. After the Human Genome Project was finished in 2003, researchers estimated that about 8 percent of human DNA is made up of virus DNA.

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