From Latin -a, from Proto-Italic *-ā, from late Proto-Indo-European *-(e)h2, from Proto-Indo-European *-h2, a suffix indicating collective and abstract animate nouns, from Pre-Proto-Indo-European *-h2, a suffix indicating plurale tantum.
Romance: Western Vulgar Latin: Asturian -a (rarely -e), Portuguese -a, Galician -a, Catalan -a, French x, Italian x Eastern Vulgar Latin: Romanian xIn Pre-Proto-Indo-European, a suffix *-h2 was used to form plurale tantum nouns out of collective nouns (e.g., "waters" from "water"). This was attached to the vowel stem *-e-, *-i-, *-u- and *-0- to form *-eh2, *-ih2, *-uh2, and *-h2. The four different endings all to express a single idea of plurality would not last. But now we must discuss the historic Proto-Indo-European method for feminizing words.In Proto-Indo-European *sor "woman" was added as a suffix to animate nouns (preserved in Anatolian but also as a fossil in a smattering of non-Anatolian words, like Spanish sor. Linguists do not precisely agree as to why *-sor as a feminine marker was gradually replaced by the *-(e)h2 collective suffix. Most recent theories involve the suffix playing an "individualizing" role. As a collective/abstract suffix *-(e)h2 referred to a subset of larger mass nouns (returning to the example "waters" (collective) versus "water" (mass), the "waters" is plural but refers to a subset of all "water"). Over time, the dependence on *-(e)h2 to refer to a subset of a broader category began to individualize the subject. By the late Proto-Indo-European period *-(e)h2 was heavily used, and stood in contrast to the typical animate ending *-s. *-(e)h2 acquired a new sense of "the other."In compelling support of this theory, a word that looks to be a relic from the transition period from collective/abstract suffix (as preserved in Anatolian and in some Core Indo-European fossils) to a feminine suffix (the familiar -a in Core Indo-European). The Proto-Indo-European word *h2u̯idhéu̯eh2- "widow" (animate in Anatolian and feminine in late Proto-Indo-European), looks built off of *h2u̯idhéu̯o- "belonging to the one fatally struck." Thus the original sense of a widow would have been "the one of whom was fatally struck," then later "the bereaved," and finally "the female bereaved." See Luraghi (2011), Tichy (1993), and Melchert (2014b) for a broader discussion.