Artist's drawing of what the ancient humans found at Sima de los Huesos may have looked like. Courtesy: Javier Trueba/Madrid Scientific Films
By tracing ancient human DNA, scientists are learning that prehistoric humans may have immigrated and diversified earlier and more frequently -- and there were a lot more of them -- than we previously realized. An analysis of the oldest known human genetic material found that a new collection of 400,000-year-old hominins -- ancient humans -- may be a common ancestor to Neanderthals and Denisovans. The findings were published in the journal Nature this week and they give us new clues about the origins of modern humans.
Archaeologists crawl through caverns at Sima de los Huesos to uncover ancient human fossils. Video by Javier Trueba/Madrid Scientific Films. (No audio)
Archaeologists found 28 hominin skeletons in the "pit of bones" at Sima de los Huesos, an archaeological site in the Atapuerca mountains of Spain. Cavers had to crawl through tunnels to reach the fragile bones buried more than 100 feet below the surface. With no air circulation, the chilly cave hasn't changed in thousands of years, making it the perfect "refrigerator" to preserve fossils, said Matthias Meyer at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and lead author of the study.
Skeleton of a Homo heidelbergensis found at Sima de los Huesos in Northern Spain. Photo by Javier Trueba/Madrid Scientific Films
The skeletons were originally identified as Homo heidelbergensis, and looked a bit like Neanderthals, an ancient human relative that lived in Europe 200,000 to 28,000 years ago, but these bones were actually 200,000 years older. 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, explained Juan Luis Arsuaga, professor of paleontology at the Complutense University of Madrid and co-author on the study. They had dense, heavy bones, and wide, stout torsos, so they must have been very muscular and incredibly strong, he said.
"They foreshadow some of the characteristics of Neanderthal morphology," Arsuaga said. "We thought they would be related."
But a genetic analysis of the bones told a different story. Meyer found traces of Denisovan DNA, a Neanderthal cousin that lived around 40,000 years ago in east Asia. Scientists don't know what they looked like; only two Denisovan teeth and a finger bone have ever been identified. Finding traces of their DNA in a Neanderthal-looking skeleton, thousands of miles and years from their known whereabouts, was surprising, Meyer said.
"To be quite honest, we were puzzled. We're still trying to figure it out," Meyer said. "It's kind of strange, this piece of DNA going around Europe and Asia, and it pops up at two different times and places."
This discovery has left archaeologists with more questions than answers, Meyer said. These bones may be a new species in human evolution, a common ancestor of Neanderthals and Denisovans, which will help us understand when these groups split from our ancestors, Homo heidelbergensis and Homo erectus, Arsuaga proposed.
But if that is the case, then the bones should show traces of Neanderthal DNA too, Meyer said, which they didn't. And if further study of these bones finds Neanderthal DNA, that means there must have been some interbreeding with other archaic people, he explained.
It's possible that these fossils are ancestors to all three groups, said Alan Cooper, director of the Australian Centre for Ancient DNA. These bones are a first glimpse into the Middle Pleistocene era, a period where a lot happened in human evolution, but fewer well-preserved fossils survived, he said. Ultimately, this mixture of Denisovan DNA and Neanderthal features shows us that when it comes to evolution, "we're complete mongrels," he said. "Everybody was bonking everybody else."
And that inter-breeding may have given humans the traits they needed to survive.
"As we look at more groups of (ancient humans), we see more signs of hybridization. It may have been an adaptation strategy," he said. "When that happens, when the DNA mixes, trying to time the point where species separated gets complicated."
Scientists believe that modern humans, Neanderthals and Denisovans overlapped in history. There are several theories to explain why the Neanderthals and Denisovans died out: competition, changing climate, disease. But ancient DNA tells us that wherever our Homo sapien ancestors went, they bred with other human populations, said John Hawks, associate professor of anthropology at University of Wisconsin-Madison.
"Every time we've looked at the DNA of (ancient humans) we've found something we didn't expect," Hawks said. "It's just not divergence and connecting ancestors as we go back in the past. We're connecting across populations and there are more populations than we thought there were."
We carry pieces of these extinct species with us today, Meyer said. The average person's DNA is 2 percent Neanderthal, and in aboriginal people of Australia and some Indonesians 4 to 6 percent of their genome is Denisovan.
Ancient DNA is drawing a new family tree for archaeologists, Hawks said, revealing new branches and connections between ancient human populations and ourselves. Five years ago, scientists believed only two species of human existed 400,000 years ago -- Homo erectus and Homo heidelbergensis. But discoveries like this and the "hobbit", another early human ancestor recently discovered in Indonesia, reveal that there have been numerous human populations around the world, and they crossed paths.
"There were always populations adapted to the edges, the unusual habitats, and they were always in danger, from changing climates and food scarcity and competition. And in the long run, they became extinct," Hawks said. "But they contributed something to modern populations."