Why is monotheism an advancement? Here's one take:

American physical anthropologist, Eugenie Scott, provides an explanation for this question in, Evolution vs. Creationism. According to Scott, the creation story in Genesis derives from other "Middle Eastern traditions from Babylonia and Persia" (54). As the ancient Hebrews were often conquered by various polytheistic/pagan tribes, they avoided acculturation through their distinct belief in one god.
Theologian Conrad Hyers purports "the religious meaning of Genesis." He says that it signals to the Hebrew people and surrounding tribes that "one God of Abraham was superior to the false gods of their neighbors: sky gods [...], earth gods, nature gods, light and darkness, rivers and animals." In the seven days of God's creation he declares superiority over these pagan deities. "Each day [God] dismisses an additional cluster of deities, arranged in a cosmological and symmetrical order" (with regards to chapter one especially). For example, when God says "Let there be light," he is in effect overriding any clout that the "pagan gods of light and darkness" might have.
At the same time, the careful phrasing of the creation serves to show that the Hebrew God is "ever present." Hence, "[I]n the beginning," when "God created heaven and earth," one could say he was not born of a "preexisting force." Also, the story establishes God as separate from nature and when he creates human beings "in his image," he endows human beings with dominion over his creation. But they must first answer to him at all costs. Thus, Scott explains, "Genesis reflects the character of a classic origin myth: it presents in symbolic form the values ancient Hebrew felt were most important: the nature of God, the nature of human beings, and the relationship of God to humankind." They "distinguished their God from those of their neighbors and presented His deeds in their oral traditions and, eventually, in written form" (55).
Is monotheism an advancement? Yes or no answers are clearly relative to what an individual considers an advancement or improvement. At least for the sake of Hebrew tradition, only one God can yoke one people.

Lia