I'm not sure that Paul Burke's story about possessive apostrophes is more than folklore.
The apostrophe in English is used to indicate a missing letter - it was originally used for possessives because in most Middle English nouns the remnants of the Anglo-Saxon genitive was   "es" - and the e was often omitted...hence the apostrophe.
Example: boy;possessive = boyes;modern _boy's

Great fun!