Redbird,
Interesting question.
As a hobbyist philosopher, I can say it is nearly certain that, people made up words as they went along. And the meanings of words can change over time, the word "gay" for example. There is a discipline... I forget the name, it calculates divergence of cultures using "language drift" as a measure of time, German and Dutch languages being examples there, as languages tend to diverge from their common root at a certain pace. The German and Dutch languages are similar, but not quite the same. For example, in German, the words Father and Mother are: Vater und Mutter ; in Dutch they are: Vader en Moeder. As you see they are very similar, and even bear similarities to English.
I would also direct you to onomatopoeias. I love onomatopoeias. And you will notice how the various functions of the body, to which all our early ancestors would have had frequent access to hearing on a daily basis, typically have onomatopoeic origins, "Burp" for example (I will leave you free to explore the various functions of the human body on your own, in this regard). Also notice how simple actions that make sounds also appear to have these same origins. Words like: Smack, Whack, Whip, Flip, Swish, Swirl, Clap, Scratch, Buzz, etc., etc..
Further, when your include writing in the concept of "words", then you will notice how early writing was in the form of pictograms (cave paintings), and hieroglyphics. The Chinese language, which is very old, can then be seen as a bridge from a pictogram based language to a more advanced and purely phonetically based language, like English. About 13% of the Chinese language is pictographic in nature. The word for "man", for example, is a stick figure of a man. The word for "tree", is like the outline of a pine tree. The word for "rest" then is the pictograms of "man" + "tree" drawn together.
So, as beings of conceptual consciousness (it has been said), we require a series of phonetic sounds to represent various existents in reality, and also our concepts of consciousness itself. In other words, we make up words when we need them. And then we change them when we want to. You know, there's only been a really good and functional dictionary of the English language for a little over 250 years (!)
I'll limit myself to that. Like I said, "Interesting question".
Todd in Tenn.