Unearthed by an ivory carver from a Siberian riverbank, a man's 45,000-year-old thigh bone reveals when people first mated with Neanderthals, an international genetics team reports Wednesday. 
 
 The Ust'-Ishim man's thigh bone is the oldest human bone found so far outside of Africa and the Middle East, according to the report in the journal Nature. It's nearly twice as old as the next oldest from a modern human, which comes from a boy who died elsewhere in Siberia some 24,000 years ago. 
 
 Scientists collected DNA from the bone and analyzed the ancient man's complete genetic map, or genome. The DNA narrows down the time when mating first brought Neanderthal genes into the human gene pool: from 50,000 to 60,000 years ago. 
 
 "It's really exciting that we now have a really high-quality genome sequence of an early modern human that is this old," says study author and genetics expert Janet Kelso of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. 
 
 Recent DNA studies led by Max Planck's Svante Pääbo, another author of the new study, have found traces of Neanderthal in modern people. Typically about 1.6 to 2.1 percent of the genes in people of Eurasian descent are Neanderthal in origin. (Related: "Neanderthals Died Out 10,000 Years Earlier Than Thought, With Help From Modern Humans.") 
 
 Early Liaisons 
 
 Archaeological finds show that Neanderthals and modern humans overlapped in the Middle East as long as 100,000 years ago, says paleoanthropologist John Hawks of the University of Wisconsin in Madison. But the new DNA findings seem to rule out mating taking place until much later. 
 
 Previous studies put the timing of the earliest human-Neanderthal mating anywhere from 86,000 to 37,000 years ago. 
 
 The researchers narrowed that range to 50,000 to 60,000 years ago by calculating the loss of Neanderthal genes over time since the gene swapping occurred. The Ust'-Ishim man had about 2.3 percent Neanderthal genes, but modern people typically have less than 2.1 percent. 
 
 Using the mutation rate as a genetic "clock," the researchers extrapolated back to determine the era when modern humans picked up genes from Neanderthals. 
 
 "I think the paper is pretty convincing on this," Hawks says. But he cautions that the idea of a single time of human mating with Neanderthals "almost certainly is an oversimplification. The contacts could have extended over a longer period." 
 
 A possible second, more recent, episode may explain slightly higher numbers of Neanderthal genes common today in East Asians, according to the study. 
 
 Asian Migration 
 
 The femur shaft turned up on the banks of the Irtysh River near Ust'-Ishim, Russia, in 2008. A Russian ivory carver and historian named Nikolay Peristov collected the bone after it eroded from a bluff above the river in western Siberia. It was identified as human, based on its teardrop-shaped cross section, in 2010. 
 
 Photograph by Bence Viola, MPI EVA 
 
 View Images Ust'-Ishim man's thigh bone. Photograph by Bence Viola, MPI EVA 
 
 The age of the femur confirms the timing suggested by artifacts of the "first foragers" in Siberia, early hunter-gatherers who spread into Europe and Asia within the past 60,000 years, Kelso says. 
 
 "We now show that there were indeed modern humans in the area," she says. A slightly warmer climate in Siberia more than 45,000 years ago may have enticed modern people to migrate to the region. 
 
 Genetically, the thigh bone's owner appears equally related to modern-day Asians and Native Americans. Surprisingly, he appears to be about as closely related to them as to the 24,000-year-old Siberian boy or Stone Age European hunter-gatherers dated in other ancient DNA studies, notes paleoanthropologist Henry Harpending of the University of Utah in Salt Lake City, who was not part of the study. 
 
 Of modern groups, the Ust'-Ishim man is less closely related to Europeans, perhaps because today's Europeans owe some of their ancestry to farmers who migrated there from the Middle East more than 10,000 years ago. 
 
 The ancient man's DNA does not appear to make him a direct ancestor of any modern people. He may have belonged to a branch of Stone Age people who migrated into Europe and Central Asia long ago, only to die out in an ice age. 

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 Scientists have reconstructed the genome of a man who lived 45,000 years ago, by far the oldest genetic record ever obtained from modern humans. The research, published on Wednesday in the journal Nature, provided new clues to the expansion of modern humans from Africa about 60,000 years ago, when they moved into Europe and Asia. 
 
 
 
 
 
 
 
 
 
 And the genome, extracted from a fossil thighbone found in Siberia, added strong support to a provocative hypothesis: Early humans interbred with Neanderthals. 
 
 “It’s irreplaceable evidence of what once existed that we can’t reconstruct from what people are now,” said John Hawks, a paleoanthropologist at the University of Wisconsin who was not involved in the study. “It speaks to us with information about a time that’s lost to us.” 
 
 The discoveries were made by a team of scientists led by Svante Paabo, a geneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Over the past three decades, Dr. Paabo and his colleagues have developed tools for plucking out fragments of DNA from fossils and reading their sequences. 
 
 Early on, the scientists were able only to retrieve tiny snippets of ancient genes. But gradually, they have invented better methods for joining the overlapping fragments together, assembling larger pieces of ancient genomes that have helped shed light on the evolution of humans and their relatives. 
 
 In December, they published the entirety of a Neanderthal genome extracted from a single toe bone. Comparing Neanderthal to human genomes, Dr. Paabo and his colleagues found that we share a common ancestor, which they estimated lived about 600,000 years ago. 
 
 Recently, Dr. Paabo and his colleagues got an opportunity to test their new methods on an exceptional human bone. 
 
 In 2008, a fossil collector named Nikolai V. Peristov was traveling along the Irtysh River in Siberia, searching for mammoth tusks in the muddy banks. Near a settlement called Ust'-Ishim, he noticed a thighbone in the water. Mr. Peristov fished it out and brought it to scientists at the Russian Academy of Sciences. 
 
 The Russian researchers identified the bone as a modern human, not a Neanderthal. To determine its age, they sent samples to the University of Oxford. Scientists there measured the breakdown of radioactive carbon and determined the bone was about 45,000 years old — making it the oldest modern human fossil ever found outside of Africa and the Near East. 
 
 In 2012, Dr. Paabo and his colleagues took samples from the bone to search for DNA. To their surprise, it held a number of genetic fragments. 
 
 “This is an amazing and shocking and unique sample,” said David Reich, a geneticist at Harvard Medical School and co-author of the new study. 
 
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 The researchers used the DNA fragments to create a high-resolution copy of the man’s complete genome. A Y chromosome revealed that the thighbone belonged to a man. 
 
 The scientists then compared the genome of the so-called Ust'-Ishim man to those of ancient and living people. 
 
 They found that his DNA was more like that of non-Africans than that of Africans. But the Ust'-Ishim man was no more closely related to ancient Europeans than he was to East Asians. 
 
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 He was part of an earlier lineage, the scientists concluded — a group that eventually gave rise to all non-African humans. 
 
 Homo sapiens, our own species, appeared in Africa around 200,000 years ago. Previous studies, both on genes and on fossils, have suggested that they then expanded through the Near East to the rest of the Old World. 
 
 The Ust'-Ishim man’s genome suggests he belonged to a group of people who lived after the African exodus, but before the split between Europeans and Asians. 
 
 Dr. Paabo and his colleagues also found that the Ust'-Ishim man had pieces of Neanderthal DNA in his genome, just as living non-Africans do. But his Neanderthal DNA had some important differences. 
 
 Fossils indicate that Neanderthals spread across Europe and Asia before becoming extinct an estimated 40,000 years ago. Today, the Neanderthal DNA in each living non-African human is broken up into short segments sprinkled throughout the genome. 
 
 Dr. Paabo and his colleagues have hypothesized that this arrangement is a result of how cells divide. 
 
 During the development of eggs and sperm, each pair of chromosomes swaps pieces of their DNA. Over the generations, long stretches of DNA get broken into smaller ones, like a deck of cards repeatedly shuffled. 
 
 Over thousands of generations, the Neanderthal DNA became more fragmented. Dr. Paabo and his colleagues predicted, however, that Neanderthal DNA in the Ust'-Ishim man’s genome would form longer stretches. 
 
 And that’s exactly what they found. “It was very satisfying to see that,” Dr. Paabo said. 
 
 By comparing the Ust'-Ishim man’s long stretches of Neanderthal DNA with shorter stretches in living humans, Dr. Paabo and his colleagues estimated the rate at which they had fragmented. They used that information to determine how long ago Neanderthals and humans interbred. 
 
 Previous studies, based only on living humans, had yielded an estimate of 37,000 to 86,000 years. Dr. Paabo and his colleagues have now narrowed down that estimate drastically: Humans and Neanderthals interbred 50,000 to 60,000 years ago, according to the new data. 
 
 The findings raised questions about research suggesting that humans in India and the Near East dated back as far as 100,000 years ago. Some scientists believe that humans expanded out of Africa in a series of waves. 
 
 But Christopher Stringer, a paleoanthropologist at the Natural History Museum, said that the new study offered compelling evidence that living non-Africans descended from a group of people who moved out of Africa about 60,000 years ago. 
 
 Any humans that expanded out of Africa before then probably died out, Mr. Stringer said.
– It's no surprise that modern humans and Neanderthals used to get it on—most people of Eurasian descent are, genetically, 1.6% to 2.1% Neanderthal. The question has long been when they did, with a wide estimate putting it between 37,000 and 86,000 years ago based on the DNA of people alive today. But now the complete genetic sequencing of a man's thigh bone, which is itself roughly 45,000 years old, suggests that we took a dip in the Neanderthal gene pool 50,000 to 60,000 years ago, reports National Geographic. The bone was found in 2008 in western Siberia; it's the oldest human bone to have been recovered beyond Africa and the Middle East. "This is an amazing and shocking and unique sample," one geneticist and co-author of the study—published this week in the journal Nature—tells the New York Times. Because long stretches of DNA break into smaller ones over many generations, the researchers predicted that the Neanderthal DNA in the 45,000-year-old bone would be less fragmented than it is in us today, and they were right. By estimating the rate at which those DNA strands fragment, they came up with the interbreeding estimate of 50,000 to 60,000 years ago. (Earlier this year, better radiocarbon dating suggested Neanderthals died out earlier than we thought.)