New DNA discovery: totally new outlook on evolution
The fossil found in Spain, had previously seemed to many experts to belong to a forerunner of Neanderthals and as the early hominins looked a little like Neanderthals, researchers expected their DNA to share a common ancestor When Dr. Meyer and his colleagues drilled into the femur, they found ancient human DNA inside, just as they had hoped. However, after analyzing mitochondrial DNA extracted from the bone, it was found that early Europeans share a common ancestor with another mysterious species – eastern Eurasian sister group to the Neanderthals, the Denisovans. "Our expectation was that it would be a very early Neanderthal," Dr. Meyer said.
First discovered in Siberia in 2010, the Denisovans were a genetically different group of people who are known only by a pinkie bone and a tooth. "Everybody had a hard time believing it at first," Dr. Meyer said. "So we generated more and more data to nail it down."
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.
"Right now, we’ve basically generated a big question mark," said Matthias Meyer, a geneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and a co-author of the new study.
The mismatch between the anatomical and genetic evidence surprised the scientists, who are now rethinking human evolution over the past few hundred thousand years.
Finding such ancient human DNA was a major advance, said David Reich, a geneticist at Harvard Medical School. "That’s an amazing, game-changing thing," he said.
"This would not have been possible even a year ago," said Juan Luis Arsuaga, a paleoanthropologist at Universidad Complutense de Madrid and a co-author of the paper.
Since the 1970s, Spanish scientists have excavated 28 nearly complete skeletons of humans from the cave. Based on the anatomy of the fossils, Dr. Arsuaga has argued that they belonged to ancestors of Neanderthals, which lived in western Asia and Europe from about 200,000 to 30,000 years ago but the new finding give a totally different picture of human evolution especially as Denisovans were believed to be limited to East Asia, and they were not thought to look so Neanderthal-like.
As The New York Times writes, based on previously discovered ancient DNA and fossil evidence, scientists generally agreed that humans’ direct ancestors shared a common ancestor with Neanderthals and Denisovans that lived about half a million years ago in Africa. In 2006, a team of French and Belgian researchers obtained a fragment of Neanderthal DNA dating back 100,000 years, which until now was the oldest human DNA ever found, Science Direct says.
Around the same time as that discovery, Russian scientists sent the Max Planck team 80,000-year-old Denisovans’ fossils they had found in a cave in Siberia. When the German scientists sequenced the entire genome from the finger bone of a girl, it turned out to be neither human nor Neanderthal.
This made the scientists assume that their shared ancestors split off from humans’ lineage and left Africa, then split further into the Denisovans and Neanderthals about 300,000 years ago. The evidence suggested that Neanderthals headed west, toward Europe, and that the Denisovans moved east. Humans’ ancestors, meanwhile, stayed in Africa, giving rise to Homo sapiens about 200,000 years ago. Humans then expanded from Africa into Asia and Europe about 60,000 years ago. They then interbred not only with Neanderthals, but with Denisovans, too. Later, both the Denisovans and Neanderthals became extinct.
Dr. Arsuaga doubts that Denisovans were spread out across so much of the Old World, from Spain to Siberia, so one alternative explanation could be 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.
Beth Shapiro, an expert on ancient DNA at the University of California, Santa Cruz, assumes that the humans of Sima de los Huesos belong to another branch of humans, which might have been a species called Homo erectus and have originated about 1.8 million years ago having become extinct within the last few hundred thousand years.
"The more we learn from the DNA extracted from these fossils, the more complicated the story becomes," Dr. Shapiro said.
This new genetic link has baffled experts from around the world.
Thus, Professor Chris Stringer from London's Natural History Museum says it shattered the previous record of 100,000 year old DNA. "With the DNA from the rest of the genome, we really can build up a very full story of these earlier stages of human evolution which so far we haven't been able to do," he said.
Professor Allan Cooper, the Director of the Australian Centre for Ancient DNA at Adelaide University said the discovery is "quite remarkable and interestingly we're starting to find similar patterns when we look at other animals and we use ancient DNA and go back in time. We find that species are not sticking to their own boundaries."
Professor Cooper added the discovery has turned human evolution theory on its head. "If all, you know, humans are humans and they don't breed to anything else. Then we find out humans have got Neanderthal and Denisovan in them, so they must have interbred with both of them. Okay, that's complicated enough. Now we've got Denisovans interbreeding with Neanderthals, we've got this group in Spain which looks like Denisovan but should be Neanderthal."
Dr. Meyer hopes that he and his colleagues will be able to get more DNA from the Spanish fossil, as well as other fossils from the site, to help solve the puzzle.