From a piece by Faye Flam in none other than Bloomberg, comes this wonderfully succinct nugget that expresses something that readers of this blog know I ascribe to:
"Scientists have also revised their view of Neanderthal extinction – long
attributed to some deficit on their part. Maybe nothing dramatic
happened at all, said Hawks. They would have made up a small fraction of
the world’s population, and when larger groups of modern humans joined
them in Europe they might have simply been absorbed."
This is what I coined the "Demography not Drama" explanation.
It is likely the Neandertal population was tiny, and when modern humans entered Europe, they simply absorbed them, perhaps even absorbed multiple sub-populations (which the genetics data now supports too).
With each generation, there is a great chance that a male line or a female line will disappear. All it takes is for a man to have only daughters, or a woman to have only sons. Older lines (which have been around for more generations) face longer odds of appearing to have survived, because each generation increases the chances a line will appear to have died out. The patrilines and matrilines from a group starting with a smaller population size will also appear to have died out over time.
We have seen this occur in the modern world, both in the example of surnames on isolated islands (the families didn't die out, but the surnames eventually greatly reduced in numbers because of the randomness of males having male children) and with thoroughbreds (the original thoroughbred founding population included 30+ male horses, but only 3 sirelines (akin to surnames or Y-chromosome haplogroups) have survived.
This doesn't mean the others "died out." Like Neandertals, their genes live on among us.