BUT THEN IF ALL-UPPERCASE WERE THE HISTORICAL DEFAULT, WHY DID MOST OF THE WESTERN WORLD SWITCH TO USING NEARLY ALL LOWERCASE? AND WHY DOES IT FEEL SO JARRING - EVEN RUDE - TO USE ALL CAPS?
It's easier to handwrite than with all Latin capitals. Also, the greater variation in size of characters makes it easier to read at higher text densities, like on the printed or written page.
And from there, it spread mostly via imitation of style. If you look at most of the scripts in the world that don't have upper/lowercase distinction, they tend to be far less blocky than uppercase Latin script, and more like lowercase.
> AND WHY DOES IT FEEL SO JARRING - EVEN RUDE - TO USE ALL CAPS?
Because we are trained to interpret all-uppercase today as form of emphasis. Emphasizing everything, especially when it's not necessary, indicates a lack of social awareness. It would be similarly socially awkward to end every sentence in an exclamation mark.
ALL CAPS IS A BIT LESS JARRING IF YOU
USE 40 COLUMNS AND A FIXED WIDTH FONT.
HOW ABOUT A NICE GAME OF CHESS?In Roman and pre Gutenberg eras, text was inscribed, eg on monuments. The reader might be a casual gawker, or someone at the base of a tower, or generally someone who could not be assumed to be in a position to engage closely with the text.
Once print became widespread, that changed and the superior readability of mixed case won out- it was faster and easier to read. All caps was reserved for things that had to be consumable at speed, at distance and in short chunks, like newspaper headlines and mass media posters. Over time the use of upper case became synonymous with IMPORTANT and since shouting people inherently think their message is important, they used upper case too.
I had heard/read that it was created as a unification of handwriting under the Carolingian empire, but that may be untrue[2]:
“The use of the new script makes an experimental impression,” says the Latin scholar. “They were trying it out. In the Middle Ages a script like this was not just invented, as is the case today. It was developed as part of the living tradition of a scriptorium. In the 8th century Corbie was something akin to a laboratory for new scripts.” This is another point that militates against the idea propounded by many popular history books that in the framework of his cultural and educational policies Charlemagne more or less commissioned the devising of the minuscule with a view to creating a uniform, readily legible script.
Prior to that, people wrote in uncial and half-uncial. The distinction between majuscule and miniscule letters wasn't there yet, but already then, they were enlarging and decorating letters at the beginning of the sentence back then. See this image on Wikipedia for an example[3]: https://en.wikipedia.org/wiki/Uncial_script#/media/File%3AKe...
[1]: https://en.wikipedia.org/wiki/Carolingian_minuscule
[2]: https://www.uni-heidelberg.de/presse/news2013/pm20130109_min...
[3]: https://en.wikipedia.org/wiki/Uncial_script#/media/File%3AKe...
Also in older computers, fonts were designed for all caps text. And it was fine:
https://www.leucht.com/photos-new/var/resizes/appleiie/30.JP...
Because you learned that, and it’s relatively new. Like full stops in text messages.