They were looking for a stable frontier. When they took over Asia Minor, they thought they had western border in the Aegean Sea. But it included over a hundred Greek city-states, which had been established by the Greek cities of READ MORE
The rule of law
By aadopting superior strategy, superior tactics, and out-fighting the Persian army and fleet.
Around 1700 BCE
Invention of art in the form of cave paintings 40,000 years ago. Or, if you want to go even earlier, invention of controlled fire a couple million years ago.