Scientists have found the oldest DNA evidence yet of humans' biological history. But instead of neatly clarifying human evolution, the finding is adding new mysteries.
In a paper in the journal Nature, scientists reported Wednesday that they had retrieved human DNA from a fossil dating back about 400,000 years, shattering the previous record of 100,000 years.
The fossil, a thigh bone found in Spain, had previously seemed to many experts to belong to a forerunner of Neanderthals. But its DNA tells a very different story.
It most closely resembles DNA from an enigmatic lineage of humans known as Denisovans. Until now, Denisovans were known only from DNA retrieved from 80,000-year-old remains in Siberia, 4,000 miles east of where the new DNA was found.
The mismatch between the anatomical and genetic evidence surprised the scientists, who are now rethinking human evolution over the past few hundred thousand years.
It is possible, for example, that there are many extinct human populations that scientists have yet to discover. They might have interbred, swapping DNA. Scientists hope that further studies of extremely ancient human DNA will clarify the mystery.
Hints at new hidden complexities in the human story came from a 400,000-year-old femur found in a cave in Spain called Sima de los Huesos ("the pit of bones" in Spanish). The scientific team used new methods to extract the DNA from the fossil.
The finding is hard to reconcile with the picture of human evolution that has been emerging. Denisovans were believed to be limited to East Asia, and they were not thought to look so Neanderthal-like.
One alternative explanation is that the humans of Sima de los Huesos were not true Neanderthals but belonged to the ancestors of both Denisovans and Neanderthals.
It is also possible that the newly discovered DNA was passed to both Neanderthals and Denisovans but eventually disappeared from Neanderthals, replaced by other variants.
In 2006, a team of French and Belgian researchers obtained a fragment of Neanderthal DNA dating back 100,000 years, which until now held the record for the oldest human DNA ever found.
The paper was co-authored by Matthias Meyer of Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and Juan Luis Arsuaga of Universidad Complutense de Madrid.