IMO it's not that there 'were' multiple species and only we survived; we are still multiple species, but we don't point that out for cultural correctness. Yes, the extent of our differences has narrowed as we passed through survival bottlenecks, but we never became a single species.
To be pedantic about the fuzziness of the "species" delineation, not every human is capable of producing viable, non-sterile offspring (breeding partner notwithstanding).
The fact that some individuals are sterile has nothing whatsoever to do with species delineation. If there were entire pairs of population groups, say the Ainu and the Xhosa, which can no longer produce viable offspring with each other due to generic distance, then we could begin a conversation about speciation of homo sapiens. But there aren't, so we don't.
Given that there is a single species of human today and most of us have some percentage of Neanderthal DNA, it seems that we are one species today and we were also one species in the past (since we could and did interbreed)
That is tautologically true, considering how Neanderthal is an offshoot of Sapiens.
The question maybe worth asking is just how far they split off, compared to how far we are split off today (or maybe in the pre-modern era, I would expect us to be merging fast since then).
This is complete bullshit. We've sequenced neanderthal DNA and can tell exactly how much modern people have in common.
If you want to promote some scientifically-accurate racism, the only "pure-blooded" homo sapiens come from central and southern Africa, everyone else is just a little bit neanderthal.
Right? Arthur Clarke has a story in which our long lost cousins finally come to rescue us from the shipwreck millennia ago, and offer also to correct the genetic mutation that reduces skin pigment...
