The History Inside Us

Every day our DNA breaks a little. Special enzymes keep our genome intact while were alive, but after death, once the oxygen runs out, there is no more repair. Chemical damage accumulates, and decomposition brings its own kind of collapse: membranes dissolve, enzymes leak, and bacteria multiply. How long until DNA disappears altogether? Since the delicate molecule was discovered, most scientists had assumed that the DNA of the dead was rapidly and irretrievably lost. When Svante Pbo, now the director of the Max Planck Institute for Evolutionary Anthropology in Germany, first considered the question more than three decades ago, he dared to wonder if it might last beyond a few days or weeks. But Pbo and other scientists have now shown that if only a few of the trillions of cells in a body escape destruction, a genome may survive for tens of thousands of years.

In his first book, Neanderthal Man: In Search of Lost Genomes, Pbo logs the genesis of one of the most groundbreaking scientific projects in the history of the human race: sequencing the genome of a Neanderthal, a human-like creature who lived until about 40,000 years ago. Pbos tale is part heros journey and part guidebook to shattering scientific paradigms. He began dreaming about the ancients on a childhood trip to Egypt from his native Sweden. When he grew up, he attended medical school and studied molecular biology, but the romance of the past never faded. As a young researcher, he tried to mummify a calf liver in a lab oven and then extract DNA from it. Most of Pbos advisors saw ancient DNA as a quaint hobby, but he persisted through years of disappointing results, patiently awaiting technological innovation that would make the work fruitful. All the while, Pbo became adept at recruiting researchers, luring funding, generating publicity, and finding ancient bones.

Eventually, his determination paid off: in 1996, he led the effort to sequence part of the Neanderthal mitochondrial genome. (Mitochondria, which serve as cells energy packs, appear to be remnants of an ancient single-celled organism, and they have their own DNA, which children inherit from their mothers. This DNA is simpler to read than the full human genome.) Finally, in 2010, Pbo and his colleagues published the full Neanderthal genome.

That may have been one of the greatest feats of modern biology, yet it is also part of a much bigger story about the extraordinary utility of DNA. For a long time, we have seen the genome as a tool for predicting the future. Do we have the mutation for Huntingtons? Are we predisposed to diabetes? But it may have even more to tell us about the past: about distant events and about the network of lives, loves, and decisions that connects them.

Empires

Long before research on ancient DNA took off, Luigi Cavalli-Sforza made the first attempt to rebuild the history of the world by comparing the distribution of traits in different living populations. He started with blood types; much later, his popular 2001 book Genes, Peoples, and Languages explored demographic history via languages and genes. Big historical arcs can also be inferred from the DNA of living people, such as the fact that all non-Africans descend from a small band of humans that left Africa 60,000 years ago. The current distribution across Eurasia of a certain Y chromosomewhich fathers pass to their sonsrather neatly traces the outline of the Mongolian Empire, leading researchers to propose that it comes from Genghis Khan, who pillaged and raped his way across the continent in the 13th century.

But in the last few years, geneticists have found ways to explore not just big events but also the dynamics of populations through time. A 2014 study used the DNA of ancient farmers and hunter-gatherers from Europe to investigate an old question: Did farming sweep across Europe and become adopted by the resident hunter-gatherers, or did farmers sweep across the continent and replace the hunter-gatherers? The researchers sampled ancient individuals who were identified as either farmers or hunters, depending on how they were buried and what goods were buried with them. A significant difference between the DNA of the two groups was found, suggesting that even though there may have been some flow of hunter-gatherer DNA into the farmers gene pool, for the most part the farmers replaced the hunter-gatherers.

Looking at more recent history, Peter Ralph and Graham Coop compared small segments of the genome across Europe and found that any two modern Europeans who lived in neighboring populations, such as Belgium and Germany, shared between two and 12 ancestors over the previous 1,500 years. They identified tantalizing variations as well. Most of the common ancestors of Italians seem to have lived around 2,500 years ago, dating to the time of the Roman Republic, which preceded the Roman Empire. Though modern Italians share ancestors within the last 2,500 years, they share far fewer of them than other Europeans share with their own countrymen. In fact, Italians from different regions of Italy today have about the same number of ancestors in common with one another as they have with people from other countries. The genome reflects the fact that until the 19th century Italy was a group of small states, not the larger country we know today.

In a very short amount of time, the genomes of ancient people have facilitated a new kind of population genetics. It reveals phenomena that we have no other way of knowing about.

Significant events in British history suggest that the genetics of Wales and some remote parts of Scotland should be different from genetics in the rest of Britain, and indeed, a standard population analysis on British people separates these groups out. But this year scientists led by Peter Donnelly at Oxford uncovered a more fine-grained relationship between genetics and history. By tracking subtle patterns across the genomes of modern Britons whose ancestors lived in particular rural areas, they found at least 17 distinct clusters that probably reflect different groups in the historic population of Britain. This work could help explain what happened during the Dark Ages, when no written records were madefor example, how much ancient British DNA was swamped by the invading Saxons of the fifth century.

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The History Inside Us