In 1856, while quarrying limestone in the Neander Valley, workers uncovered the remains of a previously unknown kind of animal, some long bones and a skull fragment with heavily ridged eyebrows. Darwin's controversial “Origin of Species” was published three years later in 1859 and over the next century, “Neanderthal Man” was posited as a link in the evolutionary development of Homo Sapiens, modern man. Developments in genetics and paleontology since 1970, now indicate that this group of hominids were almost certainly evolutionary cousins, not ancestors, and that the two species may have branched from a common ancestor approximately 700,000 years ago.
There are no known living Neandthals. The last traces of their species that have been found, date to a period between 30,000 and 18,000 years ago when it is now thought that they became extinct. This is a difficult time to decipher for paleontologists because of the rapid changes to climate, evidenced by ice core samples, while the glaciers receded towards the poles and sea levels became a few hundred feet deeper. Current understanding of events is that humans migrated into Europe and Asia during this period and Neanderthals disappeared.
Peering into prehistory is a balance of detective work and speculation. Based on the fossil evidence, Neanderthals are believed to have been primarily hunters, reached sexual maturity a few years earlier than humans and lived in smaller groups. Humans are believed to have relied heavily on vegetable supplementation to their diet and been very aggressive to other tribes of humans, but this is a derivative assessment based on probabilities and not conclusive evidence. Both species shared tool-making, personal adornment and other complex social behavior but that does not provide an argument for perpetual hostility on a vast land mass over a period of several thousand years.
But the coincidence of human migration during this period should be considered as having, at least, some causal factor in their demise. Nothing is known, concretely, about Neanderthal social organization, but human organizations are only limited by food supply and effective coordination, which is a likely advantage if Neanderthal communities were numbered to a few dozen individuals. If opportunity presented, humans would have most certainly been capable of an eradication of a rival species.
What is more likely is that the two species did not have frequent contact, that small differences in species development and social adaptability gave humans a survival edge in a rapidly fluctuating climate that made niche hunting a precarious living when so many species died out. Animal domestication and early cereal farming on a small scale may have been the difference to human survival in EurAsia and Neanderthal extinction. The only argument seems to be how large a role human competition played in this and that, so far, is not conclusively clear.
Within the last decade, there has been serious consideration (some of it argued from Old Testament references) that Homo Sapiens and Neanderthals may have interbred. Advances in genetics are giving a strong indication that this was likely true. While their distinct species and social groups may be gone, humans may have inadvertently been responsible for preserving some of the genetic uniqueness that was the Neanderthals.