Where did they come from? That's the big unknown. Some say it could have been Atlantis. No one has a firm date of their arrival but one can see their influence in many places and in many times.
Archeologists have found evidence of manufacturing sites going back twenty-seven thousand years. The "Grooved Ware People" in ancient England may have been responsible for building the many astronomical engines during the brief reign of this mysterious race seven thousand years ago. After the uniting of Egypt a building culture erupted at its height as though fully formed.
They are mentioned in the Bible and in the non-Biblical "Book of Enoch". Echoes of them resound in stories of The Seven Sages. Legends and rumors.
But great events, throughout history, have pivoted around mysterious personages. The Crusades seem to have originated with the mechanisms of one Ursus.