Denisova cave poses yet another big problem for biblical literalists, for as recently as 30,000 years ago it appears that homo sapiens were very, very far from alone, but had at least five non-extinct and closely related human cousin species that we shared the planet with. This today in the New York Times:
“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 recently 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.
Challenging the order of nature? Imagining alternative futures? Killing off our competitors? Hmm.
Prometheus Unbound 
The article suggests that our ancestors survived while related species like homo erectus and neanderthals did not is that our species were better at processing information. If I were a cynic, I might think of another, more sinister explanation: given the historical tendency of homo sapiens to express tribal identity through ethnic cleansing against those who are different, maybe our ancestors just didn’t like the looks of those neanderthals and just killed them off.
I know I’m not the first to suggest this, of course.
Mystical:
Your point is taken, and the NY Times put it euphemistically: “In fact, the tree has kept generating new branches that get cut off, presumably by the sole survivor.”
In other words, we killed them off.
—Santi