Originally Posted by
Lyth
I think the correct term for what the writers are describing in that translated letter (as well as the March 2022 live letter) is evolutionary divergence. Contrary to what is often suggested in popular culture, a mutation refers to a singular statistical event that causes that divergence as it is accumulated over time (these events are happening in us every second due to replication errors and strand breaks, but most get caught). It's not quite the same as dramatically morphing yourself a pair of helmet destroying rabbit ears (although this is still possible with the power of Fantasia.)
Genes will naturally show gradual changes over time in response to selection pressures from the environment. Amaurotines had significantly longer lifespans, so you would expect their generation time would be slower on average than current populations. In addition, as people dispersed and migrated to remote areas following the Sundering, this rate of divergence would have become more rapid, leading to the modern peoples that we see in Etheirys now.
Language is acquired, and the meaning of those sounds is established through social convention and our collective memory. Like our genes, our languages evolve over time as well. Even if you look at English in isolation, you'll see how dramatically the language has changed on its own the span of a thousand years. In the wake of the Sundering, you would expect a dramatic linguistic divergence as people dispersed and formed new cultures. In short, Babel.