the person who introduced democracy in athens 507 bce was cleisthenes. until then, athens had been ruled by aristocrats and tyrants. cleisthenes had the support of the people and completely changed the structure of power.

How would the previous sentence be correctly capitalized?
The person who introduced democracy in Athens 507 BCE was Cleisthenes. Until then, Athens had been ruled by aristocrats and tyrants. Cleisthenes had the support of the people and completely changed the structure of power.