They began to evolve separately as soon as the Mayflower arrived. And American accents have been influenced not only by a broad variety of English immigrants o

They began to evolve separately as soon as the Mayflower arrived. And American accents have been influenced not only by a broad variety of English immigrants over time by also by immigrants from other parts of the world. For instance, the "standard" (TV broadcaster) accent originated in areas settled largely by German and Scandinavian immigrants. The "New York" and "New Jersey" accents have elements of Irish and Italian and Yiddish accents, and New York accents were influential due to being common among entertainers through the middle of the 20th century. And accents can evolve quickly. The New York accent you'll hear in films from the 1950s is very different from the one you'll hear today if you go deep enough into Queens to find strong accents. That said, IMO moderate Scottish accents sound extremely similar to the "standard" American accent. I think it's actually that the cadences are very similar, which for me at least is what makes other strong British accents (other than RP) often difficult to understand.