Armenian is an isolate among Indo-European languages; its sole genetic relations among known languages are at the reconstructed Proto-Indo-European (PIE) level. Some have attempted to connect Armenian more closely with other Indo-European (IE) language stock, such as Greek, but the outstanding hypotheses are inconclusive. Historical and linguistic evidence suggests that the proto-language ancestral to Armenian was spoken in the Armenian Highland by 600 BC, but there is little agreement on the post quem of Armenian's presence in the region. A singularly opaque problem lies in determining the rate at which this early Armenian had changed from PIE, if we are to assign an absolute chronology to a reconstructed Proto-Armenian.
Armenian reflects a distinct conservatism-in-change from PIE, paralleled by Grimm's Law in the Germanic languages: Armenian and Germanic each preserve a diverse sound inventory, while the actual phonemic values underlying the inventory have changed. Thus in Armenian, voiceless unaspirated stops became aspirated (*t > *t'); voiced unaspirated stops lost voicing (*d > *t); and voiced aspirated stops lost aspiration (*dh > *d). It is notable, though probably incidental, that the latter two changes have exact parallels among the Germanic languages.
In most other respects, however, Armenian has suffered radical change from PIE. In particular, the extent and character of lexical loans from non-IE languages such as Urartian, and later from the IE Greek and Iranian, speak to ancient and abiding language contacts which, while supplementing Armenian with new vocabulary, slowly came to erode the IE-inherited Armenian lexicon to a stock of fewer than five hundred base words.
An Armenian translation of the Bible dates the beginning of Armenian literature to the early 5th century AD; this was accompanied by Saint Mesrop's invention of the Armenian alphabet, perhaps modeled after the Greek. There followed prolific translation into Armenian of Greek and Syriac texts, while authors such as Agathangelos began to compose original, mostly religious works in Armenian. As the written language of the 10th century would hardly be distinguishable from that of the 5th, the language of the entire period came to be styled as Grabar or Classical Armenian.
But by the 10th century, spoken Armenian had changed drastically from the Classical standard, growing increasingly agglutinative. Middle Armenian literature typically sought continuity with the Classical period, but with mixed results as it came to be influenced by the spoken language -- much as [spoken] Vulgar Latin came to influence the written language (Classical Latin) in Europe. The few luminaries in this period included the poet & prosewriter Nerses Lambronatsi, whose letters rank with the best of the Grabar period.
Modern Armenian stems from the early 19th century, when a dialectal split had become apparent between the Western Armenian spoken around Istanbul and the Eastern Armenian spoken in the Armenian Highland and its hinterland. The 19th and early 20th centuries saw the migration of W. Armenian speakers to the Middle East and to the West, and many E. Armenian speakers would follow in the last decades of the 20th century; diaspora communities often speak Armenian, amongst themselves, for generations.