Harrison Holt of The Generalist Papers explains why the English language uses both upper and lower case letters in such a structured manner. Holt goes back into the history of English letters, noting how the scripting of letters evolved over time beginning with the original Latin alphabet.
When Romans wrote on paper or papyrus or whatever, they used Roman cursive. This script is more rounded as it was faster to write when writing with a quill as opposed to a hammer and chisel. As you can see some of our lowercase letters are starting to show themselves but this is only the beginning of the story.
He then moves to the Carolingian Renaissance period in the 8th and 9th centuries, during which majuscule and minuscule letters became more defined. The majuscule letters were notable in that they were all of the same height, while minuscule letters consisted of varying heights and widths.
So we should properly define the terms majuscule and minuscule. They don’t just mean uppercase and lowercase majuscule letters are defined by being of a uniform height. For instance, a capital A is the same as A, B, or C and so on. Minuscule letters however have variable heights the lowercase q or p has a descender… and an h has an ascender here while letters like e or a have no such protrusions
As time moved on, the two forms of letters were used more regularly. The terms upper and lower case came from the printing presses beginning in the 15th century.
The printing press worked through movable type. There were two pieces of type for each letter. The minuscule letters were kept in a drawer closer to the printer as these sorts were used more frequently. The capital letters were kept in a case above the main working station so the terms lower case for minuscules and uppercase for capitals were established.
While the use of upper case and lower case letters became prevalent, rules regarding their specific usage didn’t arise until the late 18th and early 19th century when vociferous grammarians decided they’d had enough and put together a manual that guides the use of letters to this very day..
Actual formalized rules in english didn’t exist for centuries with capitalization being quite haphazard. For instance, in documents like the U.S. Constitution and the Declaration of Independence you see seemingly random words capitalized. Capitalization in this time was used basically to add emphasis. But in the late 18th and early 19th centuries, grammarians not amused by this disorderly use of uppercase letters created manuals defining when capitalization should be used rules that we basically follow today.