By NICHOLAS WADE

A previously unknown kind of human group vanished from the world so completely that it has left behind the merest wisp of evidence that it ever existed — a single bone from the little finger of a child, buried in a cave in the Altai mountains of southern Siberia.

Researchers extracted DNA from the bone and reported on Wednesday that it differed conspicuously from that of both modern humans and of Neanderthals, the archaic human species that inhabited Europe until the arrival of modern humans on the continent some 44,000 years ago.

The child who carried the DNA lineage was probably 5 to 7 years old, but it is not yet known if it was a boy or a girl. The finger bone was excavated by Russian archaeologists in 2008 from a place known as the Denisova cave.

The researchers, led by Johannes Krause and Svante Paabo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, are careful not to call the Denisova child a new human species, though it may prove to be so, because the evidence is preliminary.

But they say the genetic material extracted from the bone, an element called mitochondrial DNA, belonged to a distinct human lineage that migrated out of Africa at a different time from the two known archaic human species. Homo erectus, found in East Asia, left Africa two million years ago, and the ancestor of Neanderthals emigrated some 500,000 years ago. The number of differences found in the child’s DNA indicate that its ancestors left Africa about one million years ago, the researchers say. Their report is published online in the journal Nature.

Dr. Paabo, a pioneer in decoding ancient human DNA, said in a news conference that before asserting the Denisova child was a new species, he needed to rule out the possibility that it belonged to a population formed by interbreeding between the new lineage and a known species. He said he was analyzing the rest of the child’s DNA, from the main or nuclear genome, to test this possibility.

“Back at the time this lineage came out of Africa, it had to have been a distinct group, perhaps a distinct species,” he said. “But whether or not this individual was distinct species, we have to wait for the nuclear DNA.”

The finger bone was found in a layer laid down on the cave floor between 48,000 and 30,000 years ago, according to radiocarbon dating. At that time, toward the end of the Pleistocene Ice Age, which ended 10,000 years ago, the climate was probably much colder. The people of the new lineage presumably wore clothes, Dr. Krause said, because chimpanzees and gorillas cannot withstand much cold, suggesting that fur alone is inadequate protection.

The artifacts found in the cave in the same layer as the finger bone include ornaments and a bracelet that are typical of modern human sites from the Upper Paleolithic age in Europe. These are puzzling artifacts to be found with a nonmodern human species. But bones can move up and down in archaeological sites, and it is hard to know if the finger bone is truly associated with these artifacts, Dr. Krause said, even though there is little sign of mixing in the cave’s layers.

The valley beneath the Denisova cave 30,000 years ago would have been mostly a steppe, or treeless grassland, according to pollen analysis, and it was roamed by ice-age species like the woolly mammoth and woolly rhino, Dr. Krause said.

The region was inhabited by both Neanderthals and modern humans at that time. Counting the new human lineage, three human species may have lived together in proximity. “So the picture of the humans around in the late Pleistocene gets a lot more complex and a lot more interesting,” Dr. Paabo said.

The standard view has long been that there were three human migrations out of Africa — those of Homo erectus; of the ancestor of Neanderthals; and finally, some 50,000 years ago, of modern humans. But in 2004, archaeologists reported that they had found the bones of miniature humans who lived on the Indonesian island of Flores until 13,000 years ago, posing a serious problem for this view. The new lineage is the second such challenge, and it suggests that human migrations out of Africa, though far from continuous, were more frequent than supposed.

“We are learning more and more what a luxuriant evolutionary tree humans have had,” said Ian Tattersall, a paleoanthropologist at the American Museum of Natural History in New York. The tree during evolutionary time has kept sprouting new branches, all but one of which die off, before the process is repeated.

As little as 30,000 years ago, it now appears, there were five human species in the world: Homo erectus, the little Floresians, Neanderthals, modern humans and the new lineage from the Denisova cave. This is similar to the situation two million years ago, when four hominid species are known to have lived in the Turkana Basin of Kenya, Dr. Tattersall said.

“We think it’s normal to be alone in the world as we are today,” Dr. Tattersall said, and to see human evolution as a long trend leading to Homo sapiens. In fact, the tree has kept generating new branches that get cut off, presumably by the sole survivor. “The fossil record is very eloquent about this, and it’s telling us we are an insuperable competitor,” Dr. Tattersall said. Modern humans’ edge over other species probably emerged from their ability to process information: “We can invent alternatives in our heads instead of accepting nature as it is,” Dr. Tattersall said.

If the nuclear DNA of the Denisova child should differ as much as its mitochondrial DNA does from that of Neanderthals and modern humans, the case for declaring it a new species would be strengthened. But it would be unusual, if not unprecedented, for a new species to be recognized on the basis of DNA alone.

In new excavations starting this summer, archaeologists will look for remains more diagnostic than the finger bone. Researchers will also begin re-examining the fossil collections in museums to see if any wrongly assigned bones might belong instead to the species of the new lineage, Dr. Krause said.

 

http://www.nytimes.com/2010/03/25/science/25human.html

 

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Precisely the reasons we're unlikely to find the "missing link" because the evolution of humans is a tree with many branches.

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