Christopher Ehret has argued that this prefix is a later development that was not present in Proto-Afro-Asiatic, but rather derived from a PAA indefinite pronoun *m-.
kɹˈɪstəfɚɹ ˈeɪɹɪt hɐz ˈɑːɹɡjuːd ðæt ðɪs pɹˈiːfɪks ɪz ɐ lˈeɪɾɚ dɪvˈɛləpmənt ðæt wʌz nˌɑːt pɹˈɛzənt ɪn pɹˈoʊɾoʊˈæfɹoʊˌeɪʒɪˈæɾɪk, bˌʌt ɹˈæðɚ dᵻɹˈaɪvd fɹʌm ɐ pˌiːˌeɪˈeɪ ɪndˈɛfɪnət pɹˈoʊnaʊn ˈæstɚɹˌɪsk ˈɛm.