I've always found it curious that the languages spoken by an overwhelming proportion of the human population can be traced to a small number of proto-languages that were each spoken by only a small population of people, at some point circa a few thousands years ago.
For example, the Indo-European languages all descend from the language of the proto-Indo-European tribes in central Asia/eastern Europe, Chinese and all the Sino-Tibetan languages descend from the language of a relatively small group of Sino-Tibetans, and the direct ancestor of all the Austronesian languages was at one point confined to Taiwan.
But it is not the case that modern day Indo-European/Sino-Tibetan/Austronesian speaking people are all descended from the proto Indo-Europeans/proto-Sino-Tibetans/ancient Taiwanese etc. It seems that what happened was that throughout the world, over the past few thousand years, a select group of "linguistically successful" peoples (e.g. the proto-Indo Europeans, proto Sino-Tibetans) expanded to large areas via migration and conquest, and absorbed and intermarried with the local peoples, eventually replacing the local languages with their own. (In Europe this seemed to have happened by the time of the Roman Empire. In China this was happening as recently as the first few centuries AD, as the Chinese dynasties conquered and assimilate the various non-Sino-Tibetan speaking peoples in southern China.)
So the question is: is this a recent phenomenon that only emerged in the past few millennia, or has this probably happened many times before? For example, if we had a time machine and traveled 10,000 years to the past, would we see the same pattern as we see today, with several large language families (each of them descended from a proto-language from ~15,000 years ago)? Or will instead we just see lots of small families and language isolates? (I.e. in terms of the linguistic map, would the entire inhabited world, tens of thousands of years ago, just look like Papua New Guinea today?)
And for example, could many of the so called Pre-Indo-European_languages actually be related to each other, being descended from an even earlier wave of migration/conquest out of somewhere?
PS: I understand this question is highly speculative. Perhaps academic linguistics has little to say on the matter. If that's the case 1) I'd still like to hear from an expert why that's the case, and 2) please share your speculations, whatever they may be.