Where have you postulated this? When? I have not seen it. I have not seen anyone arguing against that idea. It could very well be that such changes have occurred. I have not seen the point raised until now and have seen no one contesting it.
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Check out the Ancient Reality thread. It is not directly relevant here. I postulate that the natural human language became too complex and everybody had to switch to our language which requires growing a new speech center at the back of the forebrain creating a new way to think and a new species.
Like all change in species this happened very suddenly. Indeed, this happened to millions of individuals so people born
homo sapiens sometimes became
homo omnisciencis. An individual; might literally change species overnight in many cases. Most individuals would have lived and died the same species as they were born. It might be impossible to have made the opposite transition. At the very least it was to almost nobody's advantage and was a far more difficult and time consuming "trick".
This is part of the problem here; there is no such thing as "species" just as there is no such thing as
homo sapiens or
homo omnisciencis. Every individual is different and will thrive under different conditions. Before the tower of babel being
homo omnisciencis was highly maladaptive and it would exclude you from every position of power and importance. Afterward it was the reverse. Two things that look alike are not necessarily alike and without an understanding of consciousness you can't even define "life" far less its changes or nuances. We worry way too much about definitions and words and way too little about understanding what we already know. This has become our nature. It is the reason we misinterpret change in species.