Ages of Conversion

The Age of Conversion: also known as the Age of Expansion, was when the Elites and the Prophets started to conscript other races (such as when they "tamed" the Hunters) into the Covenant through religious conversion or force.

As the Covenant would have only picked 'Worthy' species. It would be likely that these beings, would have, at some time in their history, either met the Forerunners, or evolved on a forerunner planet.