MUT’s “Science News of the Day”. DNA studies adds new member to the human family – neither modern man nor Neanderthal.
In the Altay Mountains of southern Siberia, some 200 miles from where Russia touches Mongolia, China, and Kazakhstan, nestled under a rock face about 30 yards above a little river called the Anuy, there is a cave called Denisova. It has long attracted visitors. The name comes from that of a hermit, Denis, who is said to have lived there in the 18th century. Long before that, Neolithic and later Turkic pastoralists took shelter in the cave, gathering their herds around them to ride out the Siberian winters. Thanks to them, the archaeologists who work in Denisova today, surrounded by walls spattered with recent graffiti, had to dig through deep layers of goat dung to get to the deposits that interested them. But the cave’s main chamber has a high, arched ceiling with a hole near the top that directs shimmering shafts of sunlight into the interior, so that the space feels holy, like a church.
In the back of the cave is a small side chamber, and it was there that a young Russian archaeologist named Alexander Tsybankov was digging one day in July 2008, in deposits believed to be 30,000 to 50,000 years old, when he came upon a tiny piece of bone. It was hardly promising: a rough nubbin about the size and shape of a pebble you might shake out of your shoe. Later, after news of the place had spread, a paleoanthropologist I met at Denisova described the bone to me as the “most unspectacular fossil I’ve ever seen. It’s practically depressing.” Still, it was a bone. Tsybankov bagged it and put it in his pocket to show a paleontologist back at camp.
The bone preserved just enough anatomy for the paleontologist to identify it as a chip from a primate fingertip—specifically the part that faces the last joint in the pinkie. Since there is no evidence for primates other than humans in Siberia 30,000 to 50,000 years ago—no apes or monkeys—the fossil was presumably from some kind of human. Judging by the incompletely fused joint surface, the human in question had died young, perhaps as young as eight years old.
Anatoly Derevianko, leader of the Altay excavations and director of the Institute of Archaeology and Ethnography in Novosibirsk, thought the bone might belong to a member of our own species, Homo sapiens. Sophisticated artifacts that could only be the work of modern humans, including a beautiful bracelet of polished green stone, had previously been found in the same deposits. But DNA from a fossil found earlier in a nearby cave had proved to be Neanderthal, so it was possible this bone was Neanderthal as well.
Derevianko decided to cut the bone in two. He sent one half to a genetics laboratory in California; so far he has not heard from that half again. He slipped the other half into an envelope and had it hand-delivered to Svante Pääbo, an evolutionary geneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. It was there that the case of the Denisovan pinkie bone took a startling turn.
Pääbo, a transplanted Swede, is arguably the world’s leading expert in ancient DNA, especially human DNA. His milestones are many. In 1984 he became the first person to isolate DNA from an Egyptian mummy. In 1997 he accomplished the same feat for the first time with a Neanderthal, a kind of human that vanished more than 25,000 years before the Egyptian pharaohs. That secured his scientific reputation.
When Pääbo received the package from Derevianko, his team was hard at work producing the first sequence of the entire Neanderthal genome—another feat that had once seemed impossible and that was occupying most of his attention. His lab also had a backlog of other fossils to test from all parts of the globe. So it wasn’t until late 2009 that the little Russian finger bone drew the attention of Johannes Krause, at the time a senior member of Pääbo’s team. (He’s now at the University of Tübingen.) Like everyone else, Krause assumed the bone was from an early modern human. He had developed a method for distinguishing the DNA of such a fossil from that of the archaeologists, museum workers, and anyone else who might have handled and therefore contaminated it.
Krause and his student Qiaomei Fu extracted the finger bone’s mitochondrial DNA (mtDNA), a small bit of the genome that living cells have hundreds of copies of and that is therefore easier to find in ancient bone. They compared the DNA sequence with those of living humans and Neanderthals. Then they repeated the analysis, because they couldn’t believe the results they’d gotten the first time around.
On a Friday afternoon, with Pääbo away at a meeting at Cold Spring Harbor Laboratory on Long Island, Krause called a meeting of the lab staff and challenged anyone to come up with a different explanation for what he was seeing. No one could. Then he dialed Pääbo’s cell. “Johannes asked me if I was sitting down,” Pääbo remembers. “I said I wasn’t, and he replied that I had better find a chair.”
Krause himself recalls that Friday as “scientifically the most exciting day of my life.” The tiny chip of a finger bone, it seemed, was not from a modern human at all. But it wasn’t from a Neanderthal either. It belonged to a new kind of human being, never before seen.