Around 100,000 B.C. a parasite named the Flood invaded Forerunner space. After a three-hundred year long year was fought between the two races. The flood eventually infected much of the Forerunner civilization and the Forerunners built the Ark and the Halo Array (including Installation 04) and activated them and destroyed the flood, most life in the galaxy, and the forerunners themselves.
According to the records of 343 Guilty Spark another non-human/Covenant race might have visited the ring.