Ancient DNA has been recovered from human bones in the past. In 1997 Svante P¿¿bo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and his colleagues extracted and sequenced DNA from a Neandertal bone. The team worked with DNA from the mitochondria, the cell's energy-producing organelles, which have their own DNA that is passed along from mother to child. Because cells have multiple mitochondria, mitochondrial DNA (mtDNA) is far more common than nuclear DNA in fossilized remains and thus easier to recover in sufficient amounts for analysis. But there are limits to what mtDNA can reveal about extinct species. Nuclear DNA is what researchers have been waiting for, though many have doubted the feasibility of obtaining it.
In the new study, Edward M. Rubin of the U.S. Department of Energy Joint Genome Institute and a team of collaborators (including P¿¿bo) sampled 42,000- and 44,000-year-old bones of the cave bear Ursus spelaeus. This relative of modern brown bears and polar bears lived at the same time as the Neandertals and its remains are plentiful. Rather than first replicating the DNA with the polymerase chain reaction (PCR) as is typical, the investigators cut to the chase and sequenced the ancient DNA directly, using computing technology developed for modern genome projects. Much of what they sequenced turned out to be microbial contaminants, as expected, but 6 percent was cave bear nuclear DNA, including bits of 21 genes.
Having demonstrated the validity of their technique on cave bear DNA, the team is hoping to apply it to other remains. "Next we would like to access and evaluate genomic information about other hominid species, Neandertals in particular, as they represent probably our closest prehistoric relative," Rubin comments. The recently discovered Homo floresiensis (a.k.a. "the hobbit") might also yield to this method of DNA analysis. Dinosaur fossils, however, are almost certainly too old to preserve viable DNA, he notes. A report detailing the findings appears in today's Science.