Greek is first attested by approximately 3000 clay tablets found at Knossos on the island of Crete, at Pylos, at Mycenae and other mainland sites.

These tablets contain texts relating to the management economies of the palaces. The syllabic Linear B script from Crete is used which is singularly unsuitable for writing Greek. This circumstance shows that the script was not invented by Greek speakers. It was almost certainly borrowed and adapted from a Cretan syllabic script similar to Linear A. There are, however, no tablets using linear B and the Cretan language (whatever it may have been).

The Linear A tablets cannot be read and it is generally assumed that they are written in a Cretan language which is definitely not Indo-European.

The extremely archaic Greek revealed by the decipherment of the Linear B texts is usually referred to as Ancient Mycenaean. This language is certainly intrusive in Crete where its us is related to the political domination of the Cretan palace economies by Mycenaeans around 1400 BCE.

The people who developed the Mycenaean palace, or citadel, economies on the mainland also appear to be intrusive from about 2000 BCE.

Their roots are to be looked for to the north, the north-east or to the east in Anatolia. Ancient Mycenaean is therefore probably intrusive, an assumption that is supported by the presence of numerous loan words in Greek that appear to have been acquired from a non-Indo-European substrate language or languages. Of the three possible routes into Greece the north eastern route is the most probable.

aeolic
attic-ionic
greek
doric
mycenean

Modern Greek developed from the Koine the common dialect of the Hellenistic world. This in turn had arisen from Attic.

The decipherment of the Linear B script pushed the origins of Greek back to Mycenean times (~2000 BCE) and it is thought that the known dialects arose from the disintegration of Mycenean.