Neanderthals (Homo sapiens neanderthalensis) thrived in Europe for about 300 thousand years, only going extinct some 30 thousand years ago. Cro-Magnons, also called Early Modern Humans (Homo sapiens sapiens) entered the European continent from Africa and the Middle east about 80 thousand years ago, coexisting with the Neanderthals for some 50 thousand years.
There are three basic theories on how the two sub-species or “breeds” might have interacted.
The first and oldest of these theories is the violent encounter theory. In short, in this hypothesis states that whenever the two sub-species would meet, they would fight. The Cro-Magnons with their superior weapons, such as spear-thrower also called an atlatl, would generally win these battles. This constant low grade warfare eventually drove the Neanderthals on to marginal land and also reduced their population to a tipping point where it could not recover. Although given the low population density of both sub-species, encountering each other in any way was a very rare event. But that being said, according to one anthropologist, at least one time a Neanderthals was killed and eaten by Cro-Magnons and then the victim’s teeth was worn as a necklace. This is based on the discovery of a Neanderthal jawbone in 2009, that seems to have been processed for food and its teeth removed.
The next theory is the rare meeting theory. In which the two rarely met, but when they did the interaction was in the main, peaceful. But even then these encounters did not turn out well for the Neanderthals, as the two groups would exchange not only goods but also microbes. The Neanderthals would have had no resistance to these new diseases and the diseases would have run rampant through the Neanderthal population.
The last theory was the inter-breeding theory. A recent discovery stated that modern humans have about four percent Neanderthal genes in their genome. Even before this discovery some scientists had speculated that the two groups were not in fact separate species, but rather were more like dog “breeds”, that is to say that while Cro-Magnons and Neanderthal looked somewhat different, they were in fact capable of producing fertile offspring. This theory has now been borne out by the genetic evidence.
The truth most likely falls somewhere in the middle of all these theories; encounters were very rare but they did happen. These meetings would sometimes no doubt be violent, especially if the meeting took place by accident and involved some resource such as food or flint. Other encounters would be peaceful and clearly there was also some sexual interaction given recent genetic discovery.
Jared Diamond, The Third Chimpanzee: The Evolution and Future of the Human Animal, Harper Perennial, 2001.