"Vowel dispersion theory", is the reason that a lot of different languages around the world that have nothing to do with each other still share a lot of the same vowel sounds ("tomato" versus "potato") is because when languages are in their infancy, the vowels that people tend to use are the ones that are the most different from each other, because those are the easiest sounds to tell apart.
If you plot them on an acoustic chart, the "ah" in "father", the "ay" in "rain", the "ee" in "sleek", the "oh" in "snow", and the "oo" in "noodles" all get clustered into the corners of the chart - in other words, they're really far away from one another, so it's easy to tell them apart.
So the idea is that because they're "good" / "useful" vowels to have, they show up in a lot of languages, and you get random rhymes / similar words in languages with nothing to do with each other.
