Was the American or the British accent the original English accent?

The British accent was the original although accents slowly change with time.

It is interesting to note, however, that the original British accent was more like the modern-day American accent, and research has shown that it is the British accent which has evolved more since colonisation of North America. Originally, both the British and Americans spoke with a rhotic accent. Rhotic essentially means an accent where the letter 'r' is pronounced strongly after a vowel. Rhotic accents are strong in both the US and Scotland, but seem to have disappeared from British English and its derivatives, such as Australian and New Zealand English.