To summarize the evidence from
Russian - why I - Sergey.
In ancient attested Indo-European languages like Hittite, Sanskrit and Avestan, the copula was often missing in the present tense for all persons (
#7 and
#4), and in Greek, for the 3rd person (
#4 and
#7). In particular, consider the beginning of the
first official inscription of Persian kings in Old Persian:
“I am Darius, the great king, king of kings, the king of Persia, the king of countries”.
Adam Dārayavauš “I am Darius” (literally “I Darius”, “Я — Дарий”).
In later Indic, Iranic and Greek this absence of “to be” was eliminated.
In the only attested East Slavic vernacular, “to be” is originally often missing in the 3rd person, and this usage gradually spread to other persons towards the end of the Middle Ages (
#2).
In modern Lithuanian, all the three constructions are possible: with the pronoun and “to be” (“I am glad”), with “to be” alone (“am glad”) and with the pronoun alone (“I glad”).
P. S. Regarding the potential substrate influence in Russian. 1500 years ago Slavic was spoken nowhere within the present-day Russian boundaries. The future core Russian ethnic area
was Iranic-speaking in the far south (the steppe around Kursk, Voronezh and Belgorod), Baltic-speaking in the center and Finnic-speaking in the north and east (Baltic-Finnic in the north and “Volgaic” in the east). No Japanese influence is detectable for that period…