Forerunner (Halo)

The forerunners are the ones who created the seven Halos that exist all over the Milky Way Galaxy, in order to study and contain  the Flood, and in case of an outbreak, activate Halo´s defenses and kill all  sentient life in the galaxy to starve them to death. They´re believed to be extinct because they were forced to activate Halo to stop an outbreak, much earlier in history; of course, it is unknown whether or not if they in fact perished with the halos' activation or not.

Location of Installation 00
While some believed that Installation 00, also known as "the Ark", was buried under New Mombasa in Africa, this turned out to be false; instead, a Keyship lay there, which in fact was built to open up a portal leading to the Ark. The existence of this Keyship on Earth and Master Chief being called a "reclaimer" indicates some connection between  Humans and Forerunners. While it is known that one Forerunner did discover Earth and its not-yet-sapient species, it is unknown at this time what, if anything, that Forerunner might have done to affect the Human race's evolution. As for the Covenant, the Prophets and their lackey Tartarus know something, which they chose to keep from both the Humans as well as the other Covenant races:

It is believed that Humans are evolved forerunners, or their selected species to "reclaim" their technology. That would explain why the Ark is on Earth. When the prophets and the  elites found the forerunner technology, they started forming the Covenant, as a religious society worshiping forerunners. Later, somehow, the prophets find out about humanity and their right to the forerunner tech, so they started the war with them to prevent the reclamation that they didn't quite understand.