The Denisovan gets its name from the Denisova Cave, tucked away in the Altai Mountains in Siberia, near where the borders of Russia, Mongolia, and China intersect. The Denisovan pinky bone was found in 2008. (The teeth were found eight years before that but weren’t initially identified.) Archaeologists from the Institute of Archaeology and Ethnology of Novosibirsk stumbled upon these fragments as they searched for Neanderthal tools. Despite the presence of tools from the mid-Stone Age Mousterian Neanderthal culture, the finger bone found in Denisova showed mitochondrial DNA (mtDNA) different from Neanderthals or humans. Mitochondrial DNA is exclusively derived matrilineally—that is, tracing descent through the mother’s genetic line—with no information about any admixture deriving from males anywhere along the line. Further analysis of the nuclear genome showed Denisovans are more closely tied to Neanderthals, splitting off some 400,000 years ago.

“The mDNA is only one line of descent; it’s only one part of one’s ancestry, so It’s not a reflection of all your ancestors,” Reich, who worked on the Denisova project, says. “It’s your mother’s mother’s mother’s mother’s mother and you have many ancestors. And your mother’s mother’s mother’s mother’s mother could have lived in a very different population or had a very different history than your mother’s father’s father’s mother’s mother’s mother.”

In other words, mtDNA is only part of the story.

Reich had previously worked with Denisovan project lead Svante Pääbo of the Max Planck Institute tracing the DNA of Neanderthals. Pääbo and his team knew there was something different about the original mtDNA extracted, which showed 1 million years separation from Neanderthals and modern humans. Reich found more clues in the nuclear genome.

The genome is extracted from the nucleus of the gene rather than the mitochondria organelles, and shows traits inherited from male ancestors—a process that provides a more complete genetic picture of our distant cousin species. As researchers zeroed in on this genome, a picture began to emerge of a separate human lineage. With that, we had a new hominin cousin species hiding in plain sight in East Asia.

“Denisovans are an example of—in my mind—how mitochondrial DNA can lead you wrong, and only the nuclear genome tells the full story,” Pääbo said. “The mtDNA of the Denisovans diverge before modern human and Neandertal mtDNA [break off] from each other, yet the nuclear genome shows that they share a common ancestor with Neandertals, but far back in time. Perhaps they even got the mtDNA by gene flow from some other hominin in Asia.”

So the mother’s mother’s mother (ad infinitum) of the Denisovan showed a more ancient lineage than the nuclear genome, which revealed a hominin with a more recent branch of evolution with Neanderthals. The Denisovans split off from the lineage of Neanderthals 200,000 years after humans had already had their split from the species.