Etymologically speaking, it's not a contraction, but a preservation of an old 3rd person plural pronoun form (from Middle English "hem"): see https://en.oxfordd

Etymologically speaking, it's not a contraction, but a preservation of an old 3rd person plural pronoun form (from Middle English "hem"): see https://en.oxforddictionaries.com/definition/'em https://en.oxforddictionaries.com/definition/'em