There are orante figures in the very earliest Christian catacombs of Rome. The orantes are praying females or males.
Century and # of images
I: 0, II: 5, III: 57, IV: 92, Later: 3
(Lamberton, Clark D. “The Development of Christian Symbolism as Illustrated in Roman Catacomb Painting.” American Journal of Archaeology 15, no. 4 (1911): 507-22. DOI: 10.2307/497187.)
Personally, I think the simplest explanation is this: the orantes represent people who were devoted to a religious life. They have chosen to be represented to the living as people praying. I suggest that the orante images designated the “consecrated widows” or “deacons” of the congregations. (There are not yet monastic orders, so their images cannot be identified easily by a symbol or a clothing uniform.) It is also possible that people known to be pious who had not made their wishes for representation known were depicted as orantes.