What is the theme in City of God by E. L. Doctorow?

Evolution is at the heart of this book. The novel opens with an eloquent scientific description of the Big Bang. The words create a mental image of a vast mixture of darkness and light containing isolated elements whipping around in chaotic order. The description encompasses the massive space of cosmic existence, the vast amount of years accorded to its development, and the wondrous beauty of its elements. The description combines poetic description and scientific language in an image of breathtaking beauty and staggering complication.