There they lay for some 50 years until they were examined by a team led by Eske Willerslev of the University of Copenhagen. Dr. Willerslev, an expert in analyzing ancient DNA, was seeking to understand the peopling of the Americas by searching for possible source populations in Siberia. He extracted DNA from bone taken from the child’s upper arm, hoping to find ancestry in the East Asian peoples from whom Native Americans are known to be descended.

But the first results were disappointing. The boy’s mitochondrial DNA belonged to the lineage known as U, which is commonly found among the modern humans who first entered Europe about 44,000 years ago. The lineages found among Native Americans are those designated A, B, C, D and X, so the U lineage pointed to contamination of the bone by the archaeologists or museum curators who had handled it, a common problem with ancient DNA projects. “The study was put on low speed for about a year because I thought it was all contamination,” Dr. Willerslev said.

His team proceeded anyway to analyze the nuclear genome, which contains the major part of human inheritance. They were amazed when the nuclear genome also turned out to have partly European ancestry. Examining the genome from a second Siberian grave site, that of an adult who died 17,000 years ago, they found the same markers of European origin. Together, the two genomes indicate that descendants of the modern humans who entered Europe had spread much farther east across Eurasia than had previously been assumed and occupied Siberia during an extremely cold period starting 20,000 years ago that is known as the Last Glacial Maximum.

The other surprise from the Mal’ta boy’s genome was that it matched to both Europeans and Native Americans but not to East Asians. Dr. Willerslev’s interpretation was that the ancestors of Native Americans had already separated from the East Asian population when they interbred with the people of the Mal’ta culture, and that this admixed population then crossed over the Beringian land bridge that then lay between Siberia and Alaska to become a founding population of Native Americans.