A possible new link in human lineage -- all from a little finger
DNA from a 40,000-year-old pinkie finger, belonging to a child and found in a cave in Siberia, indicates that the bone is from a previously unknown family of human relatives that lived among Neanderthals and modern humans, German researchers reported Wednesday.
The discovery, if confirmed by research already underway, would mark the first time that an entirely new species of hominid has been identified solely on the basis of DNA sequencing, the team reported online in the journal Nature. It also suggests that other currently unknown species could be similarly identified.
With the recent, and still controversial, discovery of the Hobbit-like species Homo floresiensis that survived in Indonesia until about 13,000 years ago, the evidence now indicates that at least four species of human-like creatures walked the Earth at the same time. The finding suggests that “40,000 years ago, the planet was more crowded than we thought,” wrote evolutionary biologist Terence A. Brown of the University of Manchester in an editorial accompanying the report.
The new species shared a common ancestor with both modern humans and Neanderthals about 1 million years ago, based on the DNA sequences, according to the team led by anthropologists Johannes Krause and Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. That is about 500,000 years older than the last common ancestor shared by Neanderthals and modern humans.
“I like it because it makes us sort of a normal mammal,” said anthropologist Todd R. Disotell of New York University’s Center for the Study of Human Origins, who was not involved in the research. “At every other phase of evolutionary history, there has always been multiple species at any one time.”
The DNA sequences are from mitochondria, the tiny organelles within cells that provide power for all the activities of life. They thus provide a useful indicator of lineage, but say little about the physical characteristics of the whole organism. And because the researchers have only the finger bone, found in the Altai Mountains in 2008, they are unable to draw any broader conclusions about its identity.
But Krause and his colleagues are now working feverishly to sequence the much-more-difficult-to-analyze DNA in the nucleus of cells in the hopes that the information will provide a much better picture of the new form of life they appear to have discovered.
Until the team has a complete sequence of nuclear DNA, “we are not saying this is a new species,” Pääbo said at a news conference. But all the evidence so far suggests it is, he added.
Meanwhile, other researchers are speculating about the kinship of the bone -- known in the laboratory as “X Woman” because mitochondria are passed down through mothers -- guessing that it might belong to species such as Homo heidelbergensis or Homo antecessor, both of which split off from the human lineage in the same general window of time but were thought to have gone extinct much earlier.
The “idea that this is an unknown species is a little bit premature,” added anthropologist Richard Potts of the Smithsonian Institution’s National Museum of Natural History. There are a couple of candidates that could have survived in isolated pockets longer than researchers had thought, he said.
The pinkie bone, from a child age 7 to 9 but of unknown gender, was found in the Denisova Cave in southern Siberia. The location is critical primarily for climatic reasons. While most hominid fossils have been found in equatorial regions, the heat and humidity of such places rapidly degrade DNA, making sequencing problematic, if not impossible. But DNA is preserved much better at higher altitudes and in colder climates.
The Denisova Cave shows signs of being occupied by humans and their relatives periodically for at least 125,000 years. The X Woman’s pinkie was found in a layer of soil and debris dating from 30,000 to 48,000 years ago.
Krause’s lab developed techniques to sequence DNA from Neanderthals and “their work is absolutely the best in the world,” Disotell said. Krause said at a news conference that the lab was sequencing mitochondrial DNA from several samples when he discovered one that was very unusual. “The sequence was similar to humans, but very distinct from humans,” he said. After performing many tests to ensure the results were not an artifact, “I called Svante on his mobile [phone] and told him to sit down.”
Pääbo, who was in the U.S. at the time, said, “It was absolutely amazing. At first I didn’t believe him. I thought he was pulling my leg. . . . This was some new creature that has not been on our radar screens so far.”
The team compared X Woman’s sequence to that of six Neanderthals and 57 modern humans and concluded that her lineage branched off about 1 million years ago while that remote ancestor was still living in Africa. She is thus not a direct ancestor of humans, but a cousin of sorts.
Similar genetic information about H. floresiensis could help to settle the controversy about whether it is simply a dwarf modern human or a distinct species. Unfortunately, the climate in Indonesia probably precludes obtaining any H. floresiensis DNA.
Now the question is becoming whether researchers will find other new species as they look at more DNA-containing fossils from other high-altitude and cold sites. “I wouldn’t be surprised if we find multiple lineages,” Disotell concluded.