An analysis of the oldest DNA has created more questions than answers about the origin of humans.
Scientists took DNA from a 400,000-year-old femur, from fossils excavated in the 1990s from a deep cave in northern Spain called Sima de los Huesos, or "pit of bones," Ewen Callaway wrote in the science journal Nature
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The remains in the cave were previously thought to be from early Neanderthals who lived in Europe until about 30,000 years ago. However, DNA results determined that the bones were closely linked to Denisovans, a little-known population from southwestern Siberia.
"That's not what I would have expected; that’s not what anyone would have expected," Chris Stringer, a paleoanthropologist at London’s Natural History Museum, told Nature.
The Sima de los Huesos research sequenced most of the femur's mitochondrial genome, made up of DNA from the cells' energy-producing structures and passed down the maternal line.
The analysis showed branches in evolutionary history that placed the DNA closer to that of Denisovans than to Neanderthals or modern humans.
"This really raises more questions than it answers," Svante Pääbo, a molecular geneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, told Nature.
Juan Luis Arsuaga, professor of paleontology at the Complutense University of Madrid and co-author on the study, told PBS.org
that the Sima de los Huesos hominids were average height for the time and had rounder skulls than the Neanderthals, but they still had a prominent brow ridge.
"They foreshadow some of the characteristics of Neanderthal morphology," Arsuaga said. "We thought they would be related."
Scientists said that the Denisovans remain a mystery. Researchers said that only two Denisovan teeth and a finger have ever been identified. Matthias Meyer at the Max Planck Institute and lead author of the study said it stunned them to find traces of Denisovan DNA in the Neanderthal-looking skeleton, thousands of miles and years from their known whereabouts.
"To be quite honest, we were puzzled. We're still trying to figure it out," Meyer told PBS.org. "It's kind of strange; this piece of DNA going around Europe and Asia, and it pops up at two different times and places."
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