Missing Links

In the summer of 1981, a Swedish graduate student named Svante Paabo filled a laboratory at the University of Uppsala with the stench of rotting liver. Paabo was supposed to be studying viruses, but he had become secretly obsessed with a more exotic line of research: extracting DNA from Egyptian mummies. No one at the time had any idea if the desiccated flesh of pharaohs still contained any genetic material, so Paabo decided to run an experiment. He bought a piece of calf’s liver and put it in a lab oven at about 120 degrees for a few days to approximate mummification. In the dried, blackened lump of meat, he succeeded in finding scattered fragments of DNA.

It was the start of what has turned out to be an extraordinary scientific career. Paabo went on to find DNA in a 2,400-year-old mummy and then from much older animals, like extinct cave bears and ground sloths. In 2010, he became world-famous when he and his colleagues unveiled the Neanderthal genome.

Neanderthals have puzzled scientists ever since their fossils first emerged in a German quarry in 1856. They were clearly ancient (their fossils span a range from about 200,000 to 30,000 years ago) and had distinctive anatomical differences from living humans, such as a thick brow ridge. But Neanderthals had brains as big as ours; they could make sophisticated tools and hunt large mammals. Precisely how they were related to modern humans became the source of a debate that rolled on for decades.

In “Neanderthal Man” Paabo offers a fascinating account of the three decades of research that led from a secret hobby to a scientific milestone. The book follows the style of two previous memoirs by pioneering geneticists — James D. Watson’s “The Double Helix” (1968) and J. Craig Venter’s “A Life Decoded” (2007). In “The Double Helix,” Watson described discovering the structure of DNA. In “A Life Decoded,” Venter told how he led a team that developed new ways to read DNA and eventually assembled a rough draft of the entire human genome. Paabo now recounts his success in recovering a human genome that has been sitting in fossils for tens of thousands of years.

All three books are stories of scientific triumphs, but they’re presented more like memoirs. At their best, memoirs are sustained drilling expeditions, in which writers mine their experiences to gain a deeper understanding of themselves. For all sorts of reasons, scientists are often unwilling to dig so far. “Neanderthal Man” is a case in point.

Three-quarters of the way through the book, for example, Paabo abruptly reveals a startling piece of information: “I had grown up as the secret extramarital son of Sune Bergstrom, a well-known biochemist who had shared the Nobel Prize in 1982.” We readers reply, “Do go on.”

But Paabo abruptly cuts to a new chapter on sequencing technologies. The book contains many such fragments of a complicated, fascinating life. The reader is left to assemble them into a full picture, just as Paabo assembles the bits of DNA he finds in fossils.

Paabo’s genetic jigsaw-puzzle work, on the other hand, wins his sustained attention. You can practically hear the 20-year-old lab notebooks creak open as Paabo works his way systematically through his experiments. The narrative occasionally slows down when Paabo gets deep in the weeds about a scientific journal reviewing one of his papers, or the political intricacies of obtaining a chunk of a fossil from another country. For the most part, though, “Neanderthal Man” is a revealing history of a new scientific field.

What emerges most clearly is just how vexingly hard it is to get an accurate reconstruction of an extinct organism’s genes. Paabo has continually seized on the newest technology for sequencing DNA and hacked it to suit his own research. He’s had to be obsessive about cleanliness, since scientists can unwittingly drench fossils in their own DNA, making it difficult to identify the fragments of ancient genes. Paabo built “clean rooms,” labs dedicated to handling ancient DNA, and instituted strict hygiene protocols. Once Paabo’s team deciphered millions of segments of DNA in the fossils, they used statistical methods to set aside any living human contamination. Bacteria that invade fossils also leave their own DNA behind, and Paabo and his colleagues learned to recognize their genes too.

In the 1990s, Paabo and his colleagues proved that these methods could let them accurately read the DNA of animals dead for tens of thousands of years. But he was after bigger game. “I longed to bring a new rigor to the study of human history by investigating DNA sequence variation in ancient humans,” he writes.

When Paabo and his colleagues sequenced the Neanderthal genome, they found evidence of a complex kinship between modern humans and Neanderthals. We share a common ancestor that lived some 600,000 years ago. Neanderthals spread out across a territory that stretched from Spain to Siberia, and endured until about 30,000 years ago. Our own ancestors evolved in Africa and strode out into the Old World, where they most likely encountered Neanderthals and, as the research of Paabo and his colleagues has shown, had children with them. Some of us carry traces of those encounters in our DNA today.

When the Neanderthal genome is finally published, Paabo is justifiably proud. We can’t begrudge him the opportunity to regale us about the news conferences and honors. But readers may start to wonder what exactly the payoff was for those many years of struggle. Reconstructing a Neanderthal genome was a tour de force, we can all agree, but why does it matter?

Paabo spends only a little time directly addressing this question. He argues that the Neanderthal genome can serve as a counterpoint to our own. It enables Paabo and his colleagues to draw up a list of mutations that our ancestors acquired after they split from Neanderthals. Among those mutations may be changes that led to our capacity for language, symbolic thought or the other traits that make us uniquely human.

Unfortunately, the list for now is just a catalog of names. Neither Paabo nor any other scientist can yet clearly link our mutations to our human nature. For most of “Neanderthal Man,” the goal of sequencing the Neanderthal genome feels like a distant finish line that Paabo is moving toward with the steady pace of a long-distance runner. But when he snaps the finish-line tape at last, we realize that we’ve actually reached the starting line of a much longer race.