Short answers, pulled from the story.
The oldest known written form of Greek is Mycenaean Greek, recorded using a syllabary called Linear B. Linear B was not deciphered until 1952. After the collapse of the Mycenaean civilization, there was a gap of around five hundred years during which writing disappeared from Greece entirely.
Scholarly estimates range widely. Most place the arrival somewhere between 3200 BCE and 1600 BCE. A 2023 study by Paul Heggarty and colleagues using Bayesian statistics suggested Greek may have diverged as an already distinct Indo-European branch even earlier, around 5000 BCE.
Koine Greek spread through the conquests of Alexander the Great, who lived from 356 to 323 BCE. As Greek culture expanded from Asia Minor to Egypt and the frontiers of India, the Attic dialect became the basis of a common tongue adopted by the populations of conquered regions. Greek then continued as the lingua franca of the eastern Roman Empire for centuries.
Katharevousa was an archaizing, purified form of Greek sanctioned as the official state language after Greek independence in 1829. Dimotiki was the informal spoken dialect used in everyday life. The two forms coexisted in a state of diglossia until 1974-1976, when democratic reforms made Demotic Greek the official language of government.
The Byzantine emperor Heraclius introduced administrative reforms that made Greek the sole language of government and administration in the Byzantine Empire from 620 onwards, replacing Latin in official use.
Approximately 12 to 15 million people speak Greek today. The majority live in Greece and Cyprus, with additional speakers in minority and immigrant communities across many other countries.