In a cave in Northern Spain, researchers have discovered clues to the identity of the victims of a mass murder committed 49,000 years ago. The butchered bones of 12 men, women, and children protruding from the floor may be the remains of an extended Neandertal family that were killed and eaten by their fellow Neandertals. Now, DNA analysis of the bones is providing rare clues into the family structure of these close cousins of modern humans.
Researchers have long wondered why Neandertals went extinct. Some think they lacked the genetic diversity to survive deadly viruses or other challenges. Others have proposed that their social groups were smaller and less sophisticated than those of modern humans; if so, their networks for trading food, tools, or information critical for survival would not have been as reliable. It's been hard to test such hypotheses with fossils, but new methods to study ancient DNA  are starting to produce clues.
The latest insight comes from a "tunnel of bones" in a cave in El Sidrón, Spain. Here, a team of Spanish researchers has extracted and analyzed mitochondrial DNA and fragments of Y chromosomes from the remarkably well-preserved bones of 12 Neandertals. The bones were cut by stone tools and smashed open for marrow, suggesting that the Neandertals were cannibalized before the ground collapsed beneath their remains and buried them soon after their death, 49,000 years ago. The researchers found that the individuals in the group were very similar genetically, confirming earlier reports  that Neandertals had less genetic diversity than modern humans.
The team also found that three adult males, two teenage males, and one child carried the same lineage of mitochondrial DNA, which is inherited only from one's mother, suggesting a close relationship on the maternal side. By contrast, the three adult females carried mtDNA from three different lineages, showing that they were less closely related to each other maternally, says lead author Carles Lalueza-Fox of the Institute of Evolutionary Biology (CSIC-UPF) in Barcelona, Spain. The other Neandertals in the group included a teenager, a child, and an infant, all of whom carried a type of mtDNA found in one adult female, suggesting that they were her offspring or close relatives.
"This looks like a family," says Lalueza-Fox, whose team reports its findings, online today in Proceedings of the National Academy of Sciences. "It's similar to what you would find if you went to a wedding and sampled the people in the wedding party. If you sample 12 people in the street, you would never find so many people with the same mtDNA."
He thinks that the similarity in mtDNA among the males suggests that they lived in small groups of closely related males and that females moved in from other clans, a social system called patrilocality. That's similar to modern humans: about 70% of living hunter-gatherers live in patrilocal groups. "The world of the Neandertal was a very small world," says Lalueza-Fox. "They were in these small family groups. When they met each other, things could go from exchanging females to killing each other—even eating each other." In the case of this particular group, he says, it appears that they were the meal for other very hungry Neandertals since they cracked open every tiny bit of bone for marrow and smashed the skulls for brain parts.
Paleoanthropologist Jean-Jacques Hublin of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, says the evidence that the group was patrilocal and that females moved more than males is "rather convincing." But molecular anthropologist Linda Vigilant, also of Max Planck, cautions that a study of a single group of 12 Neandertals isn't enough to generalize about the social structure of the species. Researchers need to know how often individuals who are not close relatives share mtDNA, as well as to check the findings with nuclear DNA to be sure of their conclusions, she says.