“Where Do Personal Pronouns Come From?”, Pierre Bancel, Alain Matthey De L’etang2010-04 (mind, language)⁠:

The stunning preservation of 1^st [“I”] and 2^nd person [“you”] pronouns and possessives in low-level language families [almost all families have some form of ni for “I”] turns into a relative diversity within and between macrofamilies and phyla. However, the global stock of ancestral pronoun stems exhibit particularities hardly compatible with a completely independent origin.

A tentative evolutionary explanation of these apparently contradictory facts is proposed here. In the evolution of language, pronouns may have appeared only with syntactic articulation, often linked to the acceleration of cultural evolution seen in Homo sapiens from around 100 kyBP on. Syntax itself must have evolved over a long timespan, and the emergence of pronouns from preexisting words—nominals that were the most frequent subjects and objects of verbs referring to the speaker and the hearer, though this reference indirectly depended from their original meaning—must have taken time as well.

The multiple stems reconstructed for each person in macrofamilies (and, to a lesser degree, low-level families) might be a trace of a final stage of this evolution.