So, I'm sitting here thinking about Japanese and Korean, and a random thought strikes me. It's been hypothesized that Korean and Japanese are sister languages (though the divergence is fairly old). From what I've seen, I can imagine that. The basic mechanics of the two show similarity, although I've also seen some differences.
Korean has 14 consonants, roughly corresponding to h, n, k/g, l/r, b/p, m, ng, s, d/t, ch/j, kh, t, cha, p. It has 8 vowels, corresponding to ah, ou, oh, oh (don't ask me the difference, I don't know), oo, ee, ea, e. Note that there are additional characters (28 total, IIRC), but they appear to not be discrete sounds. Korean syllables can be very complex, like ours in English, having as many as 3 or 4 consonants and a couple vowels in a single syllable.
Now, Japanese is unusual (at least for us westerners) for having a remarkably simple syllable structure. A syllable is formed one of three ways: a vowel alone, a consonant followed by a vowel, and a consonant followed by two vowels (a diphthong). There are 16 consonants, roughly corresponding to k, s, t, n, m, y, r/l (something of a cross between those two; can also sound similar to 'd'), w, g, z, d, b, p, ch, j, sh; n is the only consonant that need not be followed by a vowel (although the vowel can sometimes be slurred/silent in some syllables). There are 5 vowels, which sound something like ah, eh, ee, oh, oo.
Native Japanese speakers have an inherent deficiency in the ability to pronounce more complex syllables. This is due to the fact that the brain develops based on the language spoken in very early childhood. Japanese children use very simple syllables (due to the nature of the Japanese language), and eventually their brains mature, and their old tongues lose the potential to learn new tricks (thus forming the stereotypical Japanese accent; actually, this is what forms basically all accents). You could call this a form of epigenetic inheritance (although that's not exactly what that term is typically used for) - in this case it's something which is inherited by culture, rather than by biology.
Now, a bit of evolutionary biology. The founder effect is a process of evolution where a small number of animals (I'm being general here; don't send me hate speech allegation) separate from the population and move to a new area where there isn't free exchange (breeding) with the original population. They thus form a new population that can evolve separately from the original.
My hypothesis is that this is what happened between Korean and Japanese (actually, it could also have been due to a bottleneck effect); but there's a twist. I think that the population where Japanese arose was in fact founded by members that broke off from the previously common population. Now, what if one of these founders had an unnatural (specifically, nongenetic) speech defect? A defect either in the language center of the brain, or the physical components of the voice system, such that they were unable to pronounce the more complex syllables? In theory, if this was taught to the children, it could produce the drastic simplification in syllable form seen in Japanese today.
Now that I think about it, there was actually a movie that had this idea in it - an isolated mother with an unnatural speech defect gave birth to two daughters and raised them alone. The daughters "inherited" the same slurred speech, even though they did not have the same physical defect.