From what did God create humans?

There are actually two creation stories in the Book of Genesis. Beginning with the second story (from the second sentence of Genesis 2:4), God created Adam out of the dirt and created Eve out of Adam's rib. He even created all the other living creatures out of dirt.

During the Babylonian Exile, the Priestly Source gave us a creation story (Genesis 1:1-2:4a) in which, although God might not have created the earth itself (an issue that some still debate), he made the firmament, the sun, moon and stars. He created all living things simply by talking and willing them into existence. The Priestly Source gave us a God who was all-powerful, even preferring the name El Shaddai ('God Almighty') to earlier divine names such as Yahweh. In this story, humans were created out of nothing.
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