Homo Sapiens and Neanderthals would have to have been pretty closely related to interbreed. This also shows that the term "species," while useful, also has some inherent difficulties.
The problem is that evolutionary forces weren't reading human taxonomic charts and graphs while they were happening.
I attended a four day National Science Teacher's Association (NSTA) conference this summer. Ken Miller, the author of the popular high school textbook Biology published by Pearson was a guest lecturer.
Among other things, he made the intriguing comment that Aristotle's taxonomy was getting a major reworking in light of many recent discoveries in genetics and paleontology. Although he didn't say when this new taxonomy would be introduced into future textbooks and high school curriculum in the United States, he hinted it would be within the next year or so (I know, that sounds a lot like "Soon!").
For those that are interested, here is a link to his website:
There are tons of excellent resources on their website!
Enjoy,
Oubliette