Men have known writing for at least six thousand years.
The oldest kind of writing is picture writing. If you wanted to write 'man' you just drew a picture of a man. If you wanted to write 'the man is eating' you drew a picture of a man eating. A modern example of picture writing is Chinese. The Chinese characters do not look much like pictures because they have changed over the centuries, but that's how they began.
Picture writing isn't very convenient. You have to draw too many pictures. It's very hard to learn to read and write Chinese because you have to remember thousands of characters and what they stand for (each character or group of characters stands for a word).
But gradually people developed marks that stand not for words or ideas but for sounds. This was a great thing because it was much simpler. There's no limit to the number of ideas that a language can express. But the number of phonemes in a language is small - twenty or thirty or forty. So man could now write with only twenty or forty marks. The ancient Phoenicians and ancient Greeks developed this new system of writing.
The Phoenicians wrote their words from right to left. The Greeks, too, wrote from right to left for quite a time, but then they changed and wrote from left to right, just as we do.
The Greeks wrote all in large letters, with no intervals between words or sentences, like this:
THISISTOSHOWYOUHOWDIFFICULTITWASTOREADGREEK
The Romans, whose civilization was very much like the Greek civilization, slowly changed the Greek alphabet (the word 'alphabet' came from the names of the first two Greek letters - alpha and beta) and the writing system. They began to use small letters and large letters differently. They used large letters for the first letter of important words.
In the Middle Ages people began to make the first letter in a manuscript a large and beautiful letter - a real picture - and all the others were small. The first letter was called a 'capital', from the Latin word for 'head', caput, because it came at the head of the manuscript.
There was a time in the history of the English language when they used a capital letter not only at the beginning of each sentence but also for the first letter of each noun. Now, as you know, it is not so.
The use of capitals in different countries is not the same. In all countries they begin a sentence with a capital and also use capitals for proper names. But where in English we find 'the Arabic language', the German write 'die arabische Sprache' and the French 'la langue arabe'.