A previously unknown kind of human group disappeared from the world so completely that it has left behind the merest piece 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 removed DNA from the bone and reported that it differed obviously from that of both modern humans and of Neanderthals , living in Europe until the arrival of modern humans on the continent some 44,000 years ago.      The child carrying the DNA line 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 unearthed in 2008 from a place known as the Denisova cave.      Researchers are careful not to call the Denisova child a new human species, though it may prove to be so, because the evidence is initial.      But the genetic material removed from the bone, found in a layer laid down on the cave floor between 48,000 and 30,000 years ago, belonged to a distinct human line that traveled out of Africa at a different time from the two known ancient human species. Homo erectus , found in East Asia, left Africa two million years ago, and the ancestor of Neanderthals moved away some 500,000 years ago. The numbers of differences found in the child's DNA indicate that its ancestors left Africa about one million years ago.      The region was inhabited by both Neanderthals and modern humans at that time. Counting the new human line, three human species may have lived together.  The standard view has long been that there were three human resettlements 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 small humans who lived on the Indonesian island of Flores until 13,000 years ago, causing a serious problem to this view. The new line is the second such challenge.  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 for a new species to be recognized on the basis of DNA alone.  In new diggings starting this summer, archaeologists will look for remains more analytical 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 new line. Evidence from the bone of the child shows that   _  .
its ancestor moved to Europe 1,000,000 years ago the habitat of its ancestor was in Africa the Denisova child belonged to Neanderthals the Denisova child is a new human species