The genetic sample came from a 400,000-year-old thigh bone pulled from the cold, damp depths of a Spanish cave called Sima de los Huesos, or “Pit of Bones.” Researchers surmised that it belonged to an extinct species of hominin known as Homo heidelbergensis, a direct ancestor of Neanderthals, and they expected it to resemble DNA extracted from of a handful of Neanderthal bones found in Spain, Croatia and other sites in Europe.
They were wrong.
“This really raises more questions than it answers really,” said biologist Svante Paabo of the Max Planck Institute for Evolutionary Anthropology in Germany, a pioneer in the quest to decode ancient DNA.
Paabo and his colleagues published a report in the journal Nature.
The DNA is the oldest known genetic material ever recovered from an early human, beating the previous record by roughly 300,000 years. It was taken from cell mitochondria.
Mitochondrial DNA is passed down virtually unchanged from mothers to children. Therefore, if the mystery leg bone belonged to an ancestor of Neanderthals, it should have been quite similar.
Instead, the gene sleuths discovered a much closer match with the Denisovans, a little-known group of Neanderthal relatives that lived in Siberia.
The study’s authors, however, remain at a loss to explain the link between an early European hominin and another population in Asia.
One possibility is that the bone belonged to a member of a group that was ancestral to both Neanderthals and Denisovans and that Neanderthals acquired different mitochondrial DNA after the two groups diverged.
“Another alternative is that this ancestral group actually interbred with something much older, something like Homo erectus, and obtained its mitochondrial DNA from them,” Paabo said.
Modern humans, or Homo sapiens, are not the descendants of Neanderthals or Denisovans, although they did live as contemporaries at one time and interbred, according to scientists. At some point about 30,000 years ago, all other species of the Homo genus became extinct, leaving only us.
Answers to this evolutionary mystery might emerge if the team is able to recover nuclear DNA from cells in the femur. Paabo said scientists are already working on it.