Editor's Note: Excerpted with permission from Neanderthal Man: In Search of Lost Genomes, by Svante Pääbo. Available from Basic Books, a member of The Perseus Books Group. Copyright © 2014.
Late one night in 1996, just as I had dozed off in bed, my phone rang. The caller was Matthias Krings, a graduate student in my laboratory at the Zoological Institute of the University of Munich. All he said was, “It’s not human.”
“I’m coming,” I mumbled, threw on some clothes, and drove across town to the lab. That afternoon, Matthias had started our DNA sequencing machines, feeding them fragments of DNA he had extracted and amplified from a small piece of a Neanderthal arm bone held at the Rheinisches Landesmuseum in Bonn. Years of mostly disappointing results had taught me to keep my expectation low. In all probability, whatever we had extracted was bacterial or human DNA that had infiltrated the bone sometime in the 140 years since it had been unearthed. But on the phone, Matthias had sounded excited. Could he have retrieved genetic material from a Neanderthal? It seemed too much to hope for. In the lab, I found Matthias along with Ralf Schmitz, a young archaeologist who had helped us get permission to remove the small section of arm bone from the Neanderthal fossil stored in Bonn. They could hardly control their delight as they showed me the string of A’s, C’s, G’s, and T’s coming out of one of the sequencers. Neither they nor I had ever seen anything like it before.
What to the uninitiated may seem a random sequence of four letters is in fact shorthand for the chemical structure of DNA, the genetic material stored in almost every cell in the body. The two strands of the famous double helix of DNA are made up of units containing the nucleotides adenine, thymine, guanine, and cytosine, abbreviated A, T, G, and C. The order in which these nucleotides occur makes up the genetic information necessary to form our body and support its functions. The particular piece of DNA we were looking at was part of the mitochondrial genome—mtDNA, for short—that is transmitted in the egg cells of all mothers to their children.
Several hundred copies of it are stored in the mitochondria, tiny structures in the cells, and it specifies information necessary for these structures to fulfill their function of producing energy. Each of us carries only one type of mtDNA, which comprises a mere 0.0005 percent of our genome. Since we carry in each cell many thousands of copies of just the one type, it is particularly easy to study, unlike the rest of our DNA—a mere two copies of which are stored in the cell nucleus, one from our mother and one from our father. By 1996, mtDNA sequences had been studied in thousands of humans from around the world. These sequences would typically be compared to the first determined human mtDNA sequence, and this common reference sequence, in turn, could be used to compile a list of which differences were seen at which positions. What excited us was that the sequence we had determined from the Neanderthal bone contained changes that had not been seen in any of those thousands of humans. I could hardly believe that what we were looking at was real.
As I always am when faced with an exciting or unexpected result, I was soon plagued by doubts. I looked for any possibility that what we saw could be wrong. Perhaps someone had used glue produced from cowhide to treat the bones at some point, and we were seeing mtDNA from a cow.
No: we immediately checked cow mtDNA (which others had already sequenced) and found that it was very different. This new mtDNA sequence was clearly close to the human sequences, yet it was slightly different from all of them. I began to believe that this was, indeed, the first piece of DNA ever extracted and sequenced from an extinct form of human.
We opened a bottle of champagne kept in a fridge in the lab’s coffee room. We knew that, if what we were seeing was really Neanderthal DNA, enormous possibilities had opened up. It might one day be possible to compare whole genes, or any specific gene, in Neanderthals to the corresponding genes in people alive today. As I walked back home through a dark and quiet Munich (I’d had too much champagne to drive), I could hardly believe what had happened. Back in bed, I couldn’t sleep. I kept thinking about Neanderthals, and about the specimen whose mtDNA it seemed we had just captured.