It's been speculated that psychedelic mushrooms and/or entheogenic plants may have played a role in developing language and that these may have at least been part of the 'missing link' between early hominids and homo sapiens. But I would agree with what's already been said about assigning certain letters and symbols to certain sounds. There's apparently about only 40 sounds that the human race can vocalize and these 40 sounds are what created the thousands of known languages.
I guess they associated a symbol of some sort to a sound, and figured out how those symbols and sounds combined would be able to represent different things... It gets very complicated from that point on, but that's just a general idea.
We still invent words and change the meanings of words. It never stops. Separated peoples put a variance or accent on a word. Over time, it becomes unrecognizable to its original pronunciation. Winners of wars force their language onto the conquered. I was told, Native Americans called the wild geese ka-honk. That's a more descriptive sound/name to call them. Human nature is to put a name to something new. Or invent a name to confuse everyone else i.e. call the next teenage girl a "sprink" and see what kind of a response you get. I just invented the word and have no idea what it might mean.
First, it started with prehistoric humans, also known as cavemen, they would talk which sounded like gibberish. Then, the ancient time took over creating writings on walls, kind of like cave wall writings. Next, we get into the 1440's where a professor decides to make lettering and numbers, and that is what we use today.