The genetic material dates to the Middle Pleistocene era, which ran from roughly 780,000 to 120,000 years ago, and raises the specter of some odd kissing cousins.
In a scientific tour de force, researchers analyzing a scrap of bone from an ancient human have extracted DNA at least 300,000 years old, more than double the age of the next-oldest genetic material from a member of our tribe.
The achievement could set off a rush to find more human DNA of a similar age, which some experts had once regarded as a nearly impossible goal. The DNA itself suggests an unexpected link between the Spanish human it came from and ancient humans in Siberia, creating further confusion about a period in human evolution known as "the muddle in the middle."
Rather than clarifying the family tree from half a million years ago, the new result "has complicated it," says human-origins expert Chris Stringer of London's Natural History Museum, who was not involved in the study. It also "opens up that door to look (for more) and to know that potentially, there really is DNA there, which would've been disputed before this."
Other than the parts of the world covered by permafrost, there's hardly a better place to look for antique DNA than Sima de los Huesos, Spanish for "Pit of the Bones," a dank, still chamber some 100 feet below the Earth's surface in the mountains of northern Spain. It might not sound like an attractive final resting spot, but conditions there apparently made it nearly perfect for preserving DNA. Scientists turned to a leg bone excavated from the cave in the 1990s to find out whether they could coax DNA from such an ancient fossil.
"There were many places where they could've given up," says evolutionary geneticist Ed Green of the University of California-Santa Cruz, who was not involved in the new study. "They stuck with it. … Technically, it's quite an achievement."
The DNA purified from the bone was a particular form that is passed only from mother to child, rather than the "nuclear" DNA inherited from both parents. In this week's edition of Nature, researchers say their analysis shows the maternal DNA from Sima is roughly 400,000 years old and must be at least 300,000 years old, based on geological evidence from the site. The next-oldest genetic material from an ancient human is 100,000-year-old DNA from a Neanderthal, says lead researcher Matthias Meyer of Germany's Max Planck Institute for Evolutionary Anthropology.
Researchers already have a good understanding of human evolution in Europe and Asia over the past 100,000 years, thanks to ample genetic data, Meyer says. But this DNA dates to the Middle Pleistocene era, which ran from roughly 780,000 to 120,000 years ago. The identity and relationships of the ancient humans of this period are the subject of contention.
And the new DNA only deepens the mystery. Many scientists, though not all, think the Sima people were early Neanderthals because their fossils have Neanderthal-like features in their teeth, skulls and elsewhere. But the Sima maternal DNA isn't Neanderthal-like. Instead, it shows a close affinity to the DNA of the Denisovan people, who lived about 3,000 miles away in Siberia. Early humans didn't roam much, Green says, so the link "is very surprising. That demands an explanation."
The unexpected relationship has led to a welter of speculations. Meyer, who led the team behind the new research, says the Sima species may have interbred with Homo erectus, an even earlier human. The Sima people then gave rise to both Denisovans, who got the Homo erectus DNA, and the Neanderthals, who didn't. Stringer and Green have alternative explanations.
Meyer concedes that his interbreeding scenario is "speculation for sure. … It's just not clear where this happened, when it happened, with whom it happened." He and other researchers are eager to examine the Sima person's nuclear DNA, which is a more reliable record of relationships than maternal DNA. It will be a stiff challenge to extract nuclear DNA from the Sima bone, Meyer says, but he's optimistic his team will recover some portion of that material, which may settle the debate.
"No matter which of these various scenarios is true, it will reshape what we think about human evolution in this Middle Pleistocene era," says Green. "I'm sure people will be talking about this for some time."