Notice the ape-like barrel shaped torso and ribs; human torsos are much more elongated.
The Neanderthal sequences were substantially different from modern human mtDNA. Researchers compared the Neanderthal to modern human and chimpanzee sequences. Most human sequences differ from each other by on average 8.0 substitutions, while the human and chimpanzee sequences differ by about 55.0 substitutions. The Neanderthal and modern human sequences differed by approximately 27.2 substitutions
Half of 55 is 27.5. Don't take my word for it, try it on your own calculator.
That explains the previously big mystery described by James Shreeve in his 96 Discover article "Neanderthal Peace" as to why there had never been any real evidence of human/Neanderthal interbreeding despite the two groups living in close proximity for long periods of time, i.e. despite there being substantial reason to expect such evidence to be common and easily found.
From Shreve's article:
Whatever the tools suggest, the skeletons of moderns and Neanderthals look different, and the pattern of their differences is too consistent to dismiss. As anthropologist Erik Trinkaus of the University of New Mexico has shown, those skeletal differences clearly reflect two distinct patterns of behavior, however alike the archeological leavings may be. Furthermore, the two physical types do not follow one from the other, nor do they meet in a fleeting moment before one triumphs and the other fades. They just keep on going, side by side but never mingling. In his behavioral approach to bones, Trinkaus purposely disregards the features that might best discriminate Neanderthals and moderns from each other genetically. By definition, these traits are poor indicators of the effects of life-style on bone, since their shape and size are decided by heredity, not by use. But there is one profoundly important aspect of human life where behavior and heredity converge: the act that allows human lineages to continue in the first place.Humans love to mate. They mate all the time, by night and by day, through all the phases of the female’s reproductive cycle. Given the opportunity, humans throughout the world will mate with any other human. The barriers between races and cultures, so cruelly evident in other respects, melt away when sex is at stake. Cortés began the systematic annihilation of the Aztec people--but that did not stop him from taking an Aztec princess for his wife. Blacks have been treated with contempt by whites in America since they were first forced into slavery, but some 20 percent of the genes in a typical African American are white. Consider James Cook’s voyages in the Pacific in the eighteenth century. Cook’s men would come to some distant land, and lining the shore were all these very bizarre-looking human beings with spears, long jaws, browridges, archeologist Clive Gamble of Southampton University in England told me. God, how odd it must have seemed to them. But that didn’t stop the Cook crew from making a lot of little Cooklets.
Project this universal human behavior back into the Middle Paleolithic. When Neanderthals and modern humans came into contact in the Levant, they would have interbred, no matter how strange they might initially have seemed to each other. If their cohabitation stretched over tens of thousands of years, the fossils should show a convergence through time toward a single morphological pattern, or at least some swapping of traits back and forth.
But the evidence just isn’t there, not if the TL and ESR dates are correct. Instead the Neanderthals stay staunchly themselves. In fact, according to some recent ESR dates, the least Neanderthalish among them is also the oldest. The full Neanderthal pattern is carved deep at the Kebara cave, around 60,000 years ago. The moderns, meanwhile, arrive very early at Qafzeh and Skhul and never lose their modern aspect. Certainly, it is possible that at any moment new fossils will be revealed that conclusively demonstrate the emergence of a Neandermod lineage. From the evidence in hand, however, the most likely conclusion is that Neanderthals and modern humans were not interbreeding in the Levant.
Of course, to interbreed, you first have to meet. Some researchers have contended that the coexistence on the slopes of Mount Carmel for tens of thousands of years is merely an illusion created by the poor archeological record. If moderns and Neanderthals were physically isolated from each other, then there is nothing mysterious about their failure to interbreed. The most obvious form of isolation is geographic. But imagine an isolation in time as well. The climate of the Levant fluctuated throughout the Middle Paleolithic--now warm and dry, now cold and wet. Perhaps modern humans migrated up into the region from Africa during the warm periods, when the climate was better suited to their lighter, taller, warm-adapted physiques. Neanderthals, on the other hand, might have arrived in the Levant only when advancing glaciers cooled their European range more than even their cold-adapted physiques could stand. Then the two did not so much cohabit as time-share the same pocket of landscape between their separate continental ranges.
While the solution is intriguing, there are problems with it. Hominids are remarkably adaptable creatures. Even the ancient Homo erectus- -who lacked the large brain, hafted spear points, and other cultural accoutrements of its descendants--managed to thrive in a range of regions and under diverse climatic conditions. And while hominids adapt quickly, glaciers move very, very slowly, coming and going. Even if one or the other kind of human gained sole possession of the Levant during climatic extremes, what about all those millennia that were neither the hottest nor the coldest? There must have been long stretches of time--perhaps enduring as long as the whole of recorded human history--when the Levant climate was perfectly suited to both Neanderthals and modern humans. What part do these in-between periods play in the time-sharing scenario? It doesn’t make sense that one human population should politely vacate Mount Carmel just before the other moved in.
If these humans were isolated in neither space nor time but were truly contemporaneous, then how on earth did they fail to mate? Only one solution to the mystery is left. Neanderthals and moderns did not interbreed in the Levant because they could not....
Racial differences amongst present humans are subspecies differences. The difference between modern humans and Neanderthals is a species difference.