The only way to be a guardian is to be a human. The inhabitants of Ferrum are a humanoid race but they are not human and are not able to access the powers that a guardian can. Anyone on Ferrum that has a possibility of becoming a senator is sent to Earth to create a bond with a compatible human that has the qualifications to be a guardian.
Once the bonding is complete, they are taken to Ferrum where they will live for the rest of their lives as the guardian of their bonded.
Once the bonding is complete, they are taken to Ferrum where they will live for the rest of their lives as the guardian of their bonded.