Castiel heavily implied that human beings were developed after billions of years of evolution. God gave them a spiritual essence, known as a soul. He created a paradise on Earth called the Garden of Eden. He placed the humans Adam, Lilith and Eve in the Garden, and declared them his favorite creations.
God told all the angels in Heaven to bow down before these humans, and serve them. The Archangel Lucifer refused to bow down before a species that was younger, and inferior to him. He claimed that they were "flawed, and murderous." He declared war against God and was eventually cast down to Earth.
God placed Gadreel (his most trusted angel) to guard Eden from the fallen Lucifer. Lucifer somehow managed to break past Gadreel's defense and snuck into the Garden. As payback for being cast out of Heaven, he corrupted Lilith's soul and made her into the first demon.
Still, in the Garden, Lucifer tricked Adam and Eve into eating the forbidden fruit. God cast them out of the Garden for disobeying him and subsequently created a realm called Hell, placing Lucifer, and Lilith there. He would later create a personal cage for Lucifer, as Hell could not contain him.
It is unknown where the Garden resides now if it still exists.
- Some people view the garden in Heaven as The Garden of Eden.