The Aztec god who journeyed to the underworld to retrieve the bones used to create humans was Quetzalcoatl. In the creation story, he descended into Mictlan, the realm of the dead, to recover the remains of earlier generations.
Mictlantecuhtli, ruler of Mictlan, did not surrender the bones easily. In one version, he demanded that Quetzalcoatl circle the underworld four times while sounding a conch shell that had no holes. Quetzalcoatl made the shell audible with the help of insects, completed the challenge, and escaped with the bones.
During the escape, he fell and scattered or broke the bones. Cihuacoatl, a mother and fertility goddess, ground them like corn. Quetzalcoatl then mixed them with his own blood, giving the material life and producing the people of the Fifth Sun. The broken bones explain why humans are different sizes.
Xolotl is sometimes confused with the answer because he accompanied Quetzalcoatl in versions of the myth and served as his canine companion or twin. However, Quetzalcoatl is the central figure credited with retrieving and revitalizing the bones.