Homo Magi were a species who once belonged to a magic-using empire that resided in the continent that would become Atlantis. The Homo Magi evolved in a parallel but separate line, alongside Humans. The continent was a focal point for unharnessed magical energies and the local Homo Sapiens evolved into Homo Magi as a result of their exposure to these energies.
Those humans who gained these nascent mystic abilities interbred for thousands of years, concentrating and strengthening the genetic predisposition. Upon the fall of Atlantis, people who carried the predisposition scattered to the four winds. Today, almost every human being capable of casting spells is a descendant of the Homo Magi.
Homo Magi are genetically compatible with normal humans, and are genetically identical to normal humans. The mysterious property that makes them Homo Magi is somehow "dominant" like a gene: the child of a normal human and a Homo Magi will almost always be Homo Magi.