According to my paleontology teacher, biggest difference between any two humans on the planet today is less than the difference between a neanderthal and a modern human.
There are many differences in the skeleton alone, and there could have been many more in physiology.
On the other hand, on rare occasions even two different species can produce offspring, usually sterile.
For instance, almost every combination of big cat can hybridize.
HOWEVER:
A liger, resulting from a female lion and male tiger,
can't reproduce with other ligers, but it CAN reproduce with members of one of the parent species. Get it?
Even if neanderthals and humans where different species, they may still have been able to smuggle a few genes back into the human gene pool.
Which does not mean it happened per se.
Yes, but just keep in mind that we do regard lions and tigers as separate species, even though they can produce offspring. Likewise with horses and donkeys, as another example.
As I have maintained throughout, my position is not that sexual pairings of Neanderthal and Cro Magnon never took place, or even that they could not result in progeny, but rather that my judgment at this point in time is that the weight of the evidence overwhelmingly points against Neanderthal/Cro Magnon admixture as any type of consequential phenomenon, and that indeed the weight of the evidence suggests that the two were fully distinct species.
I am not a paleontologist, and I hate to disagree with one, so I will let other paleontologists do so instead. In post #211 I linked three relevant journal articles. One of these goes directly to what I suspect your teacher alluded to:
The Neanderthal taxonomic position: models of intra- and inter-specific craniofacial variationThe morphological distances between Neanderthals and modern humans, and between Neanderthals and Late Paleolithic/early anatomically modern specimens, are consistently greater than the distances among recent human populations, and greater than the distances between the two chimpanzee species. Furthermore, no strong morphological similarities were found between Neanderthals and Late Paleolithic Europeans. This study does not find evidence for Neanderthal contribution to the evolution of modern Europeans. Results are consistent with the recognition of Neanderthals as a distinct species. [bold print added]
The second, although I don't think it's precisely what your teacher had in mind, is a difference between modern humans and Neanderthals that is without question greater than the difference between any two modern humans: Surprisingly rapid growth in Neanderthals. What this study finds is that Neanderthals reached full maturity (adulthood) by about age 15. No modern human reaches full maturity by age 15.
Finally, there is the implicit distinction of the FOXP2 allele that exercises a crucial function in human speech and language. This mutation is dated to hundreds of thousands of years after mtDNA dates the last common ancestor between Neanderthals and modern humans. Every modern human relies on this mutation to speak properly; it is implicit that this mutation was absent among Neanderthals.
But, as has been noted over and over, there is sharp disagreement in paleoanthropology circles about all of this.