Yikes! That's a tough question. Seeing as how we can't even handle getting along with other races, cultures and religions I think if there were different species we'd have all killed each other off.

Society would be two tiered, rulers and labourers. There would be class distinctions within the tiering, and quite possibly interbreeding, creating a pseudo level of outcasts. These outcasts would be embittered toward both sides, and work at undermining the prosperity, destroying both civilisations, and claiming the one rule as their own. After this, there would be infighting, and as the question of pure-breeding arises, segregation of society would occur, leading to a schism that would tear the newly formed rulers apart. Their rule would be quashed by pure-bred insurgents from both sides, and a new, three tiered socity would be created.

They were perfectly interbreedable. Examples have been found of mixed individuals. However, it seems that both avoided the other as much as possible (maybe they were not, on the whole, attracted to each other). However, if things had gone on, they might have interbred more, and produced a slightly different looking human race. After all, Neanderthals were just as intelligent as Cro-Magnon, and had similar technology for a long while until CM made his breakthroughs. They could speak, use fire, make clothing, hunt effectively and had elaborate rituals (as can be seen from their burials).

The earth could certainly sustain both, but could they tolerate each other? It is still argued whether the Neanderthals died out from warfare with the newly arrived Homo sapiens (Cro-Magnon) or simply could not compete. They were contemporaries in Europe for several thousand years.

The Earth already sustains more than two distinct species..there are the optimists and the pessimists...the selfish and the selfless...the accepters and the rejectors...the arrogant and the humble...the rigid mindsets and the open- to -the- world mindsets...the self-proclaimed "good people" who go to church and pray and those who are good, do good whether they pray or go to church or not!

The earth can hardly (equitably) sustain the varieties of homo sapien nowadays! So, no. ;-)

Homo sapiens is an aggressive race that hates itself and is extremely prone to genocide. Examples of genocide in the 20th century not withstanding, there is quite a lot of archaeological evidence of genocide of entire races of Homo sapiens in prehistory (in South America, for instance, when the Asia-originating nomads that crossed the Alaskan land bridge into the Americas that would become known as "Native Americans" systematically wiped out the more peaceful indigenous people, that had settled in South America, most likely by accident, and were closely related to modern Australian aborigines). We hate ourselves so much that we come up with every single way imaginable to divide ourselves - race, religion, culture, political parties (which many of the U.S. founding fathers openly warned would destroy the integrity of democracy if ever allowed to truly define American politics... oops!). Even with our religions, we give ourselves away. The most successful religions (in terms of conversions and spreading-power) in the world, the Judaeo-Christian-Islamic triad, all speak to the fundamental sin and evil and inadequacy of us as a species, and that even a literally infinitely good and merciful god can't help but condemn at least some of us to eternal suffering because we suck so much - some of us, apparently, infinitely so. It takes some serious self-loathing to buy into that way of thinking. But that's basic human nature. But at the same time, we want to feel special. We may hate ourselves, but like most self-loathing people, we're also extremely self-absorbed and self-serving. We want our own self-hatred to be special in the grand scheme of things. The existence of other animal species that remind us of ourselves (1) makes us hate them as we hate ourselves and (2) makes us hate them for robbing us of the delusion of feeling special. So I strongly doubt that Neanderthals (species Homo neanderthalensis) would ever have survived our own species' genocidal proclivities. A leading theory of how Homo neanderthalensis died out is that our Homo sapiens ancestors acted genocidally against all hominids that we co-existed with at the time. The only reason the other, still living great apes survived is because they seemed sufficiently different from early Homo sapiens so that he could make that nice but illusionary line between "human" and "animal" that we still have engrained deeply in the various world cultures and languages, despite all scientific evidence to suggest that this line is entirely fabricated and that there is not one uniquely "human" mental quality. Despite sharing a ridiculous percentage of DNA in common with the other great apes, and a great deal of obvious physical and mental similarities, it was only mere years ago that we classified humans and our ancestors in an entirely different family - Hominidae, and the reclassification of the great apes into our taxonomical family is still considered a matter of dispute in some circles. Reliable DNA-divergence based dating suggests humans and the other great apes parted ways evolutionarily much later than many paleontologists feel comfortable with, even today. With the incredible amount of DNA we share in common with chimps, in addition to the obvious physical and mental similarities, it's absurd they're classified in a different genus - certainly there other many species that are classified in the same genus which sometimes share far less common DNA or even appearance in common. The fact that the great apes can clearly be taught to express themselves linguistically at the level of human children, and demonstrate complex reasoning skills even in the absence of human training still does not prevent us from performing cruel experiments on them and not seeing them as having any moral value, or, at least, nothing comparing to the moral value of a human of similar or less intelligence. Their similarity actually encourages our experimentation on them, both physical and psychological, because they make good "models" of human physical and mental disease (and somehow the point that this should invalidate their use in medical experiments and give them the same "human rights" we claim we value for each other is lost in translation). But the only reason humans don't test on humans is because it's illegal... we only need to look as far as Nazi Germany to see how quickly humans turn to other humans as experimental "models" when they're allowed to do so. All of the above should be good supporting evidence that we (1) could never co-exist with Neanderthals and (2) that the theory that we actively sought out genocide on them and other hominid relatives has some decent character evidence given the behaviour of modern humans.

rhetorical question, they didnt die out, they continued their evolutiuon through various ways to become what is today..........