We did in fact interbreed. Some modern human populations are as much as 5% neanderthal in their DNA. That much gene content to me suggest that this population was absorbed; a one off breeding here or there would not persist to 5% of the genetic material millions of years ago today unless hybridization was occurring frequently.

Neanderthals and humans co-existed for some 5,000 years. Think of that in terms of our modern history, and how many ethnical groups was there in that time span - and almost all of them perished completely, either destroyed or assimilated by others (usually a combination of both). Thinking of it, neanderthals actually lasted for quite long time, probably thanks to the overall scarcity of hominid populations and vast territory available to them.

We also lived in caves for several tens of millenia across multiple stretches of time, without written language. There may have been some sort of ephemeral symbolic transfer (knots, beads, wood carvings,) but it looks like those cultures used primarily oral traditions. We know many non literate cultures develop memory palace techniques, so rigorous oral traditions can be incredibly durable and stable, with lots of information passed down accurately. All that to say, human tribes could have had detailed and specific knowledge of interactions with Neanderthals over millenia, and developed peaceful tradition that maximizes cooperation, or at least borders that kept conflict minimized. Neanderthals could likely do the same memory tricks.

5000 years is a blip - if humans find a sufficiently habitable environment, we apparently can settle for a pretty low quality of life.

It only works until it doesn't, though. Neanderthals or humans could have initiated an incident, in which traditions were distrusted or discarded or lost on either side, resulting in hostility, or they lost their elders to disease and the knowledge to survive.

Just like megafauna extinction, "bad humans were bad" probably doesn't come close to what actually happened.

Neanderthals were likely as or more intelligent than humans, implying susceptibility to all the same conflicts, biased thinking, and bad incentives that affect human cultures. It could be environmental or biological or a pure fluke that Neanderthals didn't outlive humans, but whatever the reason they're gone, it's going to be just as convoluted and nuanced as the rise and fall of millenia old human cultures.

Nobody ever lived in caves.

Artefacts were left in caves because they're the only place where the artifacts can persist for hundreds and thousands of years, and ancient man knew this just like the modern man.

Cavemen are a trope, for sure, but there were cave dwellers, known as troglodytes. There were extensive periods of time where caves represented a really good solution to environmental and climate changes over the last 300,000 years.

Troglodytes were relatively rare, and most prehistoric humans were probably of the nomadic or small village types of hunter gatherer cultures.

https://www.smithsonianmag.com/smart-news/ancient-humans-wea... - the toba catastrophe was one such event that likely forced humans to live in caves extensively.

https://en.m.wikipedia.org/wiki/Pinnacle_Point

https://en.m.wikipedia.org/wiki/Blombos_Cave

There are plenty of caves that show evidence of long-term use of fire and burials, a good indication of habitation. However, it's true that caves were never a primary mode of habitation, just one that was convenient when it could be found... and pretty much the only one that we can see now due to differential preservation.

> since the alternative is so unthinkable

In the scientific parts of the ethnographic record (there is a split in anthro between scientists and non-scientists, see e.g. Napoleon Chagnon[0]; basically, he pointed out that the Yanomamo were breeding themselves for violence and was punished for it) the idea that our M.O. in small-scale societies when encountering outsiders, or when invading other groups, is to kill the men and take the women and children (often as slaves) is not all that controversial. See e.g. http://www.nature.com/articles/nature19758

0: https://en.wikipedia.org/wiki/Napoleon_Chagnon

It is interesting to read David Reich’s wonderful book “Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past”. He deals carefully and cautiously with the processes of human population “replacements”. So cautiously that the word “genocide” is not even in the index.

But with few exceptions, when I read between his lines, I see that ugly word tacitly, time and time again. Exceptions include some Celtic migration events, but in most cases the DNA results indicate major breaks in DNA haplotypes that are consistent with an unhappy end to one group.

And of course recent “sapiens” history gives us our own backyard examples—unrelenting!

If you read Napolean Chagnon’s “Nobel Savages” or Polly Wiessner’s and Akii Tumu’s “Historical Vines” you can get a good=bad gut feeling for what “replacement” probably meant operationally in pre-literate cultures. Sadly not much different since the invention of Greek orthography, Gutenberg’s printing press, TV and the internet. Us versus the Other is apparently the name of the human game.

If we could get everyone to absorb Richard Rorty’s pragmatic approach to a life well lived starting in grade school, now that would be real progress.

The genetic data is clear that interbreeding took place, and neanderthals contribute a few % of modern human DNA in many Eurasian populations. The lack of surviving neanderthal y chromosomes could be suggestive of genocide though, but it's also possible that the neanderthal y chromosome lineages in humans disappeared due to subsequent expansions by newer lineages in human populations.

For an example of non-genocide, look at African bushmen. While their relations with other humans aren't always great, African bushmen populations haven't really experienced any genocidal decline. But if/when they fully interbreed with other humans and get incorporated into the gene pool (after evolving as a separate branch from mainline homo sapiens for over a hundred thousand years), their genes will be an even smaller fraction of the human gene pool than the neanderthal genes.

> While their relations with other humans aren't always great, African bushmen populations haven't really experienced any genocidal decline.

They are confined to a few small areas in South Africa and a few small areas much, much further north than that. The obvious implication is that they used to cover the area in between, too.

This doesn't make for a very compelling example of non-genocide.