We can't interbreed with chimps despite all the planet of the apes stuff,
Pan troglodytes is a great ape, technically so are we, but the lines split million of years ago.
That time is crucial; the longer the divide the less the possibility of successful breeding as the differences grow with subsequent generations.
We know of four different generae within the Hominids.........us, gorillas, chimps and orangutans.
We are classed as Homo sapiens sapiens
Chimps are Homo Panina pan troglodytes
Bonobos are Homo Panina pan paniscus
Orangutans are Homo pongo pygmaeus
Think of each species as a gene pool where individuals share enough in common that they can successfully interbreed, we have no evidence that any of these four surviving homonidae families can. We are just too different despite the fact that if we go far enough back we are all descended from a common ancestor population.
If they are declared a seperate species, then the gene pool is different enough that interbreeding does not generally happen.
It's that 'generally' that causes the query with the Neanderthals
Hominoidae Homo sapiens sapiens
Hominoidae Homo sapiens neanderthalis
Different species............but just how long ago was the divide ? Is the second out of Africa wave our ancestors and the first wave theirs ? If so then the divide and time of the divide is probably too great. If not
On balance, I think the divide is too great. I suspect that archaeology will find more and more evidences for a complex society within Neanderthal populations, just as it will find more evidence for the lives of HSS ones.
Found a nice simple explanation of classification on Wikipedia
http://en.wikipedia.org/wiki/Species
cheers,
Toddy