The rise and fall of language diversity through the Holocene
science.org
science.org
If we think about pre-agriculture human societies, you have many small groups occupying different economic niches who may trade, fight, or intermarry with one another but are generally constrained in size and power by a lack of surplus. Lots of opportunity for many different isolated language communities to exist.
Sedentary agriculture leads to larger units of society and to more centralized power, which by its very nature erodes linguistic diversity. The existence of trade networks, tax collection, organized religion and the like force people to learn one another's language, and create the concepts of "trade" and "prestige" languages.
You can see how this played out in waves in Europe, the Near East, India, China, etc.
The remaining places with a high degree of linguistic diversity (in terms of more language families or language isolates) until very recently are those where more traditional human societies lasted longer. For example, Siberia, New Guinea, or Australia and North America before European contact.
How fast might a specialty language evolve if changes are no longer a barrier to understanding and propagate to the whole network in real time? Changes that used to take generations could happen in days or hours, meme by meme.
Pretty fast, actually. At least if you leave it up to kids.†
Just look at unit systems. People go crazy over metric versus imperial units or old outdated units being used. And yet converting between units has never been easier. Many industries invented their own units to fit their use case, but once their products spread to national or global distribution everything gets standardized to the most common unit.
> Great loss of linguistic diversity did not begin with recent colonial expansion but as multinational empires first spread along with their languages, pathogens, and cultures.
Insert "they're the same picture" meme here.
Still, the guns (or in general, weapons) part was important, centuries of in practice isolation eventually gives you a different language, until an invader wipes your population, culture and language because it wants more resources and new territories for expansion over slaves that can't understand them (or germs finishing the work anyway).
Low granularity on data over long periods doesn't help there, with history, written word and then mass printing not only you get better records, but also you get a pressure for normalization and loss of language diversity.