The Greek area - Greece proper, plus the surrounding waterways and islands - was originally inhabited by other nations. A group known as the Minoans, who seem to have their origins on the island of Crete, planted themselves on the Greek mainland and became known as the Mycenaeans.
But they were not the Greeks.
Starting around 1600 B.C., several waves of invasions brought Indo-European settlers into Greece. These tribes were, in order, Achaeans, Aeolians, Ionians, and finally the Dorians.
Although the Dorians were in some ways technologically advanced, as is seen by their use of iron, they were part of a pattern of instability in area, and so civilization is often considered to have been in decline during the Dorian era.
The distinction between being civilized and being technologically advanced is worth considering.
As the social and political structures gradually stabilized, the new tribes absorbed some aspects of the Mycenaean cultures and blended them with their own Indo-European culture. The result is what might be called the beginning of Greek civilization. As historians Ralph Magoffin and Frederic Duncalf write,
During the three centuries which followed the period of invasion and settlement in Greece, the Greeks laid the foundations for their particular form of civilized life. Being self-reliant folk, they established a form of society in which the individual person had the freedom of self-expression. Wherever Greeks lived they preferred small, independent city-states. They never became cooperative enough to unite in large states.
Already at the beginning of Greek history, the characteristically Greek trait of independence appears. Greece as unified nation-state will not exist until many centuries later.
It is misleading to speak of the ‘ancient Greeks’ - it is more accurate to speak of Spartans, Athenians, Corinthians, Eretrians, etc.
The ‘Greeks’ were a handful of fiercely independent city-states, who occasionally cooperated with each other for mutual military defence, but who were as likely to attack each other.
Why did both independence and individual self-expression emerge as typical Greek qualities? There is no clear answer, but perhaps the origin of the Greeks as a sort of “melting-pot” of various Indo-European and Mycenaean influences was a contributing factor.