Historical survey of the Iranian languages
The Iranian protolanguage and its development
By the time Iranian begins to be attested in the 6th century bc, the language is already found differentiated into several distinct languages. Scholars have reconstructed the sound system and some of the grammatical features of Common Old Iranian, the protolanguage that preceded these dialects.
The phonological system that underlay Common Old Iranian was by and large maintained everywhere throughout the Iranian-speaking world. It consisted of the following distinctive consonant sounds:
Unfamiliar symbols are taken from the International Phonetic Alphabet, or are conventional transcriptions (e.g., š for the sh sound in “ship,” ž for the zh sound in “azure,” č for ch in “church,” and ǰ for j in “jam”). The voiced fricatives (i.e., the first three consonants represented in the fourth column—ɣ, β, and [eth]), which are produced with vibrating vocal cords and local friction, may be regarded as variants of the voiced stops (e.g., g, b, d); but they are characteristic of Iranian languages generally and especially of the eastern Iranian languages. In addition to these sounds Old Persian had another sibilant sound, often transcribed as ç or ss, which developed from the cluster θr (pronounced as the thr in “three”). In Middle Persian it fell together with the s sound. The most noticeable alteration of the old sound system is the introduction in some languages of additional series of consonants under the influence of neighbouring languages. Thus, Ossetic has a series of ejective sounds (uttered with a simultaneous glottal stop) on the pattern of the unrelated Caucasian languages; and a number of Iranian languages have a retroflex series (produced with the tongue tip curled up toward the roof of the mouth) as a result of contact with Indo-Aryan languages.
Some of the differences between Iranian languages arose as a result of different developments of the earlier sounds. Thus, the Indo-European sounds ḱ, ǵ, and ǵh resulted in Indo-Iranian ś, ź, and źh, which in turn became s, z, and z, respectively, in Avestan but θ, d, and d in Old Persian. Hence, Indo-European *ḱṃtó- “hundred” became Indo-Iranian *śatá-, attested by Old Indo-Aryan śatá-, and then Avestan sata-, but Old Persian θata-. Nevertheless, θ and d as well as s and z belong to the basic pattern, the difference being merely distributional.
The main source of differentiation is in the variation of consonant cluster development and that of groups of consonants and semivowels. Here again it is mainly a question of distributional differences. Thus, the Indo-European group *ḱuˆ became Indo-Iranian *śuˆ, retained in Old Indo-Aryan in the spelling śv of the standard transcription. Indo-Iranian *śuˆ developed variously in Iranian: s in Old Persian, sp in Avestan and Median, ś (written śś) in Khotanese, and š in Wakhī. These developments can be seen in the following forms of the Indo-European word *eḱuˆo- “horse”: Old Indo-Aryan áśva-, Avestan and Median aspa-, Old Persian asa-, Khotanese aśśa-, and Wakhī yaš. Yet another development can be seen in Ossetic, in which the word for “mare,” Avestan aspā-, appears as Digor äfsä and Iron yäfs.
The vowel system of Common Old Iranian consisted of short and long varieties of a, i, and u, and a neutral vowel ə (similar to the a in “sofa”). This analysis assumes that the Indo-Iranian vocalic r (r̥) had already developed to ər in Proto-Iranian, just as its long counterpart became ar. An early and general monophthongization of the diphthongs ai and au to ē and ō, respectively, also must be considered characteristic, although it should not be ascribed to Common Old Iranian as is sometimes done. This basic system was almost everywhere maintained, sometimes with the addition of one or two distinctive vowel sounds (phonemes).