^^^^^ CB: I can't totally answer your question, but briefly, as far as I know "genetic comparisons" can't determine if two individuals are of the same species, unless the genetic matches are identical. It can be used to say two individuals are not of the same species. In other words, I don't know if it is possible to say a difference in the DNA of a Neanderthal fossil is so small from a human that the Neanderthals could interbreed and produce fertile offspring. I believe, the only way to tell if two individuals are of the same species is to have them have sex that produces a fertile offspring result ( if you wanted to know if two female individuals are of the same species , you'd have to have the brother of one and the other attempt to produce fertile offspring). We don't have two living individuals, one Neanderthal , one homo sapiens, so we can't say for sure that they are the _same_ species. I think biologists _have_ used genetic information ( mitochondria dna ) to somehow determine that we and Neanderthals are _not_ the same species. The genetic difference was so great, I guess. See below ( when I started out majoring in anthro, there was a hypotheses that Neanders and humans were the same species, but that changed with the more recent mitochondria experiment)
^^^^^^
Neandertal DNA July 29, 1997 by Mark Rose
According to most paleoanthropologists, Homo heidelbergensis gave rise to modern humans in Africa and Neandertals in Europe. Fossils such as the Mauer mandible and the skull from Tautavel place this divergence before 500,000 years ago. Differences between Neandertal DNA, recently recovered by scientists working in Germany and the United States, and DNA of modern humans place the separation between 690,000 and 550,000 years ago. (Amélie A. Walker) [LARGER IMAGE]
For the first time, DNA of a premodern human has been recovered. Svante Pääbo of the University of Munich and colleagues in Germany and the United States successfully extracted the DNA from a right humerus (upper arm bone) of a Neandertal. Their findings, presented in the July issue of the journal Cell, provide important information about when Neandertals and modern humans diverged from a common ancestor, the nature of interaction between Neandertals and modern humans, and the ultimate fate of the Neandertals.
(continued at http://www.archaeology.org/online/news/dna.html )
Other discussions
http://www.talkorigins.org/faqs/homs/mtDNA.html