What is the origin of the '.' (Full stop/period) in written English? Is it known when this sign appeared the first time in written English?
A full set of punctuation marks began to appear only after the invention of printing. Manuscripts were almost totally unpunctuated. The term punctuation itself is first recorded in 1539. Full stop appears about 60 years later, so it’s probably safe to say that the mark itself only became anything like widespread sometime in the sixteenth century.
The full stop was used a long time before the sixteenth century, but didn't have the same purpose - the Anglo-Saxon Corpus 140 manuscript of the Gospel According to Saint John dates from the 11th century and uses the punctus (modern full stop), but has a broad variety of functions rather than simply marking the end of a sentence and was mainly rhetorical. The sign must therefore pre-date the 11th century, but maybe your question is more along the lines of when the full stop was first used in the way we use it today?