As the Father of gods and men, Zeus is a very difficult god to understand. Although Gaia [Mother Earth] is the actual progenitor of the Immortals, Zeus was proclaimed to be the Father of the gods after he led his brothers and sisters, along with other disenfranchised Immortals, against the Titans in a long and bitter war. Zeus became the literal Father of men when he created men and gave them homes on the face of the earth. As a punishment for the men he had created, Zeus created the first women.
Zeus was born when the earth and the heavens were almost complete. The Immortals who preceded Zeus were in existence when the earth was still in chaos and there were no human inhabitants. The primal gods and goddesses formed the foundations for a habitable environment on the surface of the earth but their creation was populated only by what we would consider to be lower life forms and monsters.
Gaia and Ouranos [Heavens] began the cycle of life that eventually gave rise to a race of formidable Immortals know as the Titans who in turn mated with one another to create Zeus and the other more benevolent gods and goddesses.